From 6159d5298647234d764fad7268d9bb47e547d36d Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 14:46:40 -0800
Subject: [PATCH 01/15] Refactor

---
 internal/compiler/fileloader.go  | 120 +-----------------------------
 internal/compiler/filesparser.go | 122 +++++++++++++++++++++++++++++++
 2 files changed, 124 insertions(+), 118 deletions(-)

diff --git a/internal/compiler/fileloader.go b/internal/compiler/fileloader.go
index ef866d1921..b77c030c65 100644
--- a/internal/compiler/fileloader.go
+++ b/internal/compiler/fileloader.go
@@ -129,128 +129,12 @@ func processAllProgramFiles(
 	}
 
 	loader.filesParser.parse(&loader, loader.rootTasks)
+
 	// Clear out loader and host to ensure its not used post program creation
 	loader.projectReferenceFileMapper.loader = nil
 	loader.projectReferenceFileMapper.host = nil
 
-	totalFileCount := int(loader.totalFileCount.Load())
-	libFileCount := int(loader.libFileCount.Load())
-
-	var missingFiles []string
-	files := make([]*ast.SourceFile, 0, totalFileCount-libFileCount)
-	libFiles := make([]*ast.SourceFile, 0, totalFileCount) // totalFileCount here since we append files to it later to construct the final list
-
-	filesByPath := make(map[tspath.Path]*ast.SourceFile, totalFileCount)
-	loader.includeProcessor.fileIncludeReasons = make(map[tspath.Path][]*FileIncludeReason, totalFileCount)
-	var outputFileToProjectReferenceSource map[tspath.Path]string
-	if !opts.canUseProjectReferenceSource() {
-		outputFileToProjectReferenceSource = make(map[tspath.Path]string, totalFileCount)
-	}
-	resolvedModules := make(map[tspath.Path]module.ModeAwareCache[*module.ResolvedModule], totalFileCount+1)
-	typeResolutionsInFile := make(map[tspath.Path]module.ModeAwareCache[*module.ResolvedTypeReferenceDirective], totalFileCount)
-	sourceFileMetaDatas := make(map[tspath.Path]ast.SourceFileMetaData, totalFileCount)
-	var jsxRuntimeImportSpecifiers map[tspath.Path]*jsxRuntimeImportSpecifier
-	var importHelpersImportSpecifiers map[tspath.Path]*ast.Node
-	var sourceFilesFoundSearchingNodeModules collections.Set[tspath.Path]
-	libFilesMap := make(map[tspath.Path]*LibFile, libFileCount)
-
-	loader.filesParser.collect(&loader, loader.rootTasks, func(task *parseTask) {
-		if task.redirectedParseTask != nil {
-			if !opts.canUseProjectReferenceSource() {
-				outputFileToProjectReferenceSource[task.redirectedParseTask.path] = task.FileName()
-			}
-			return
-		}
-
-		if task.isForAutomaticTypeDirective {
-			typeResolutionsInFile[task.path] = task.typeResolutionsInFile
-			return
-		}
-		file := task.file
-		path := task.path
-		if file == nil {
-			// !!! sheetal file preprocessing diagnostic explaining getSourceFileFromReferenceWorker
-			missingFiles = append(missingFiles, task.normalizedFilePath)
-			return
-		}
-
-		// !!! sheetal todo porting file case errors
-		// if _, ok := filesByPath[path]; ok {
-		// 	Check if it differs only in drive letters its ok to ignore that error:
-		// 	const checkedAbsolutePath = getNormalizedAbsolutePathWithoutRoot(checkedName, currentDirectory);
-		// 	const inputAbsolutePath = getNormalizedAbsolutePathWithoutRoot(fileName, currentDirectory);
-		// 	if (checkedAbsolutePath !== inputAbsolutePath) {
-		// 	    reportFileNamesDifferOnlyInCasingError(fileName, file, reason);
-		// 	}
-		// } else if loader.comparePathsOptions.UseCaseSensitiveFileNames {
-		// 	pathIgnoreCase := tspath.ToPath(file.FileName(), loader.comparePathsOptions.CurrentDirectory, false)
-		// 	// for case-sensitsive file systems check if we've already seen some file with similar filename ignoring case
-		// 	if _, ok := filesByNameIgnoreCase[pathIgnoreCase]; ok {
-		// 		reportFileNamesDifferOnlyInCasingError(fileName, existingFile, reason);
-		// 	} else {
-		// 		filesByNameIgnoreCase[pathIgnoreCase] = file
-		// 	}
-		// }
-
-		if task.libFile != nil {
-			libFiles = append(libFiles, file)
-			libFilesMap[path] = task.libFile
-		} else {
-			files = append(files, file)
-		}
-		filesByPath[path] = file
-		resolvedModules[path] = task.resolutionsInFile
-		typeResolutionsInFile[path] = task.typeResolutionsInFile
-		sourceFileMetaDatas[path] = task.metadata
-
-		if task.jsxRuntimeImportSpecifier != nil {
-			if jsxRuntimeImportSpecifiers == nil {
-				jsxRuntimeImportSpecifiers = make(map[tspath.Path]*jsxRuntimeImportSpecifier, totalFileCount)
-			}
-			jsxRuntimeImportSpecifiers[path] = task.jsxRuntimeImportSpecifier
-		}
-		if task.importHelpersImportSpecifier != nil {
-			if importHelpersImportSpecifiers == nil {
-				importHelpersImportSpecifiers = make(map[tspath.Path]*ast.Node, totalFileCount)
-			}
-			importHelpersImportSpecifiers[path] = task.importHelpersImportSpecifier
-		}
-		if task.fromExternalLibrary {
-			sourceFilesFoundSearchingNodeModules.Add(path)
-		}
-	})
-	loader.sortLibs(libFiles)
-
-	allFiles := append(libFiles, files...)
-
-	keys := slices.Collect(loader.pathForLibFileResolutions.Keys())
-	slices.Sort(keys)
-	for _, key := range keys {
-		value, _ := loader.pathForLibFileResolutions.Load(key)
-		resolvedModules[key] = module.ModeAwareCache[*module.ResolvedModule]{
-			module.ModeAwareCacheKey{Name: value.libraryName, Mode: core.ModuleKindCommonJS}: value.resolution,
-		}
-		for _, trace := range value.trace {
-			opts.Host.Trace(trace.Message, trace.Args...)
-		}
-	}
-
-	return processedFiles{
-		resolver:                             loader.resolver,
-		files:                                allFiles,
-		filesByPath:                          filesByPath,
-		projectReferenceFileMapper:           loader.projectReferenceFileMapper,
-		resolvedModules:                      resolvedModules,
-		typeResolutionsInFile:                typeResolutionsInFile,
-		sourceFileMetaDatas:                  sourceFileMetaDatas,
-		jsxRuntimeImportSpecifiers:           jsxRuntimeImportSpecifiers,
-		importHelpersImportSpecifiers:        importHelpersImportSpecifiers,
-		sourceFilesFoundSearchingNodeModules: sourceFilesFoundSearchingNodeModules,
-		libFiles:                             libFilesMap,
-		missingFiles:                         missingFiles,
-		includeProcessor:                     loader.includeProcessor,
-		outputFileToProjectReferenceSource:   outputFileToProjectReferenceSource,
-	}
+	return loader.filesParser.getProcessedFiles(&loader)
 }
 
 func (p *fileLoader) toPath(file string) tspath.Path {
diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index 554ca47327..96976d2b42 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -2,6 +2,7 @@ package compiler
 
 import (
 	"math"
+	"slices"
 	"sync"
 
 	"github.com/microsoft/typescript-go/internal/ast"
@@ -227,6 +228,127 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 	}
 }
 
+func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
+	totalFileCount := int(loader.totalFileCount.Load())
+	libFileCount := int(loader.libFileCount.Load())
+
+	var missingFiles []string
+	files := make([]*ast.SourceFile, 0, totalFileCount-libFileCount)
+	libFiles := make([]*ast.SourceFile, 0, totalFileCount) // totalFileCount here since we append files to it later to construct the final list
+
+	filesByPath := make(map[tspath.Path]*ast.SourceFile, totalFileCount)
+	loader.includeProcessor.fileIncludeReasons = make(map[tspath.Path][]*FileIncludeReason, totalFileCount)
+	var outputFileToProjectReferenceSource map[tspath.Path]string
+	if !loader.opts.canUseProjectReferenceSource() {
+		outputFileToProjectReferenceSource = make(map[tspath.Path]string, totalFileCount)
+	}
+	resolvedModules := make(map[tspath.Path]module.ModeAwareCache[*module.ResolvedModule], totalFileCount+1)
+	typeResolutionsInFile := make(map[tspath.Path]module.ModeAwareCache[*module.ResolvedTypeReferenceDirective], totalFileCount)
+	sourceFileMetaDatas := make(map[tspath.Path]ast.SourceFileMetaData, totalFileCount)
+	var jsxRuntimeImportSpecifiers map[tspath.Path]*jsxRuntimeImportSpecifier
+	var importHelpersImportSpecifiers map[tspath.Path]*ast.Node
+	var sourceFilesFoundSearchingNodeModules collections.Set[tspath.Path]
+	libFilesMap := make(map[tspath.Path]*LibFile, libFileCount)
+
+	loader.filesParser.collect(loader, loader.rootTasks, func(task *parseTask) {
+		if task.redirectedParseTask != nil {
+			if !loader.opts.canUseProjectReferenceSource() {
+				outputFileToProjectReferenceSource[task.redirectedParseTask.path] = task.FileName()
+			}
+			return
+		}
+
+		if task.isForAutomaticTypeDirective {
+			typeResolutionsInFile[task.path] = task.typeResolutionsInFile
+			return
+		}
+		file := task.file
+		path := task.path
+		if file == nil {
+			// !!! sheetal file preprocessing diagnostic explaining getSourceFileFromReferenceWorker
+			missingFiles = append(missingFiles, task.normalizedFilePath)
+			return
+		}
+
+		// !!! sheetal todo porting file case errors
+		// if _, ok := filesByPath[path]; ok {
+		// 	Check if it differs only in drive letters its ok to ignore that error:
+		// 	const checkedAbsolutePath = getNormalizedAbsolutePathWithoutRoot(checkedName, currentDirectory);
+		// 	const inputAbsolutePath = getNormalizedAbsolutePathWithoutRoot(fileName, currentDirectory);
+		// 	if (checkedAbsolutePath !== inputAbsolutePath) {
+		// 	    reportFileNamesDifferOnlyInCasingError(fileName, file, reason);
+		// 	}
+		// } else if loader.comparePathsOptions.UseCaseSensitiveFileNames {
+		// 	pathIgnoreCase := tspath.ToPath(file.FileName(), loader.comparePathsOptions.CurrentDirectory, false)
+		// 	// for case-sensitsive file systems check if we've already seen some file with similar filename ignoring case
+		// 	if _, ok := filesByNameIgnoreCase[pathIgnoreCase]; ok {
+		// 		reportFileNamesDifferOnlyInCasingError(fileName, existingFile, reason);
+		// 	} else {
+		// 		filesByNameIgnoreCase[pathIgnoreCase] = file
+		// 	}
+		// }
+
+		if task.libFile != nil {
+			libFiles = append(libFiles, file)
+			libFilesMap[path] = task.libFile
+		} else {
+			files = append(files, file)
+		}
+		filesByPath[path] = file
+		resolvedModules[path] = task.resolutionsInFile
+		typeResolutionsInFile[path] = task.typeResolutionsInFile
+		sourceFileMetaDatas[path] = task.metadata
+
+		if task.jsxRuntimeImportSpecifier != nil {
+			if jsxRuntimeImportSpecifiers == nil {
+				jsxRuntimeImportSpecifiers = make(map[tspath.Path]*jsxRuntimeImportSpecifier, totalFileCount)
+			}
+			jsxRuntimeImportSpecifiers[path] = task.jsxRuntimeImportSpecifier
+		}
+		if task.importHelpersImportSpecifier != nil {
+			if importHelpersImportSpecifiers == nil {
+				importHelpersImportSpecifiers = make(map[tspath.Path]*ast.Node, totalFileCount)
+			}
+			importHelpersImportSpecifiers[path] = task.importHelpersImportSpecifier
+		}
+		if task.fromExternalLibrary {
+			sourceFilesFoundSearchingNodeModules.Add(path)
+		}
+	})
+	loader.sortLibs(libFiles)
+
+	allFiles := append(libFiles, files...)
+
+	keys := slices.Collect(loader.pathForLibFileResolutions.Keys())
+	slices.Sort(keys)
+	for _, key := range keys {
+		value, _ := loader.pathForLibFileResolutions.Load(key)
+		resolvedModules[key] = module.ModeAwareCache[*module.ResolvedModule]{
+			module.ModeAwareCacheKey{Name: value.libraryName, Mode: core.ModuleKindCommonJS}: value.resolution,
+		}
+		for _, trace := range value.trace {
+			loader.opts.Host.Trace(trace.Message, trace.Args...)
+		}
+	}
+
+	return processedFiles{
+		resolver:                             loader.resolver,
+		files:                                allFiles,
+		filesByPath:                          filesByPath,
+		projectReferenceFileMapper:           loader.projectReferenceFileMapper,
+		resolvedModules:                      resolvedModules,
+		typeResolutionsInFile:                typeResolutionsInFile,
+		sourceFileMetaDatas:                  sourceFileMetaDatas,
+		jsxRuntimeImportSpecifiers:           jsxRuntimeImportSpecifiers,
+		importHelpersImportSpecifiers:        importHelpersImportSpecifiers,
+		sourceFilesFoundSearchingNodeModules: sourceFilesFoundSearchingNodeModules,
+		libFiles:                             libFilesMap,
+		missingFiles:                         missingFiles,
+		includeProcessor:                     loader.includeProcessor,
+		outputFileToProjectReferenceSource:   outputFileToProjectReferenceSource,
+	}
+}
+
 func (w *filesParser) collect(loader *fileLoader, tasks []*parseTask, iterate func(*parseTask)) {
 	// Mark all tasks we saw as external after the fact.
 	w.tasksByFileName.Range(func(key string, value *queuedParseTask) bool {

From 00d37dd96401ed6d9b9742da7f04c2f8cf6a299e Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 14:57:04 -0800
Subject: [PATCH 02/15] Refactor 2

---
 internal/compiler/filesparser.go | 195 ++++++++++++++++---------------
 1 file changed, 98 insertions(+), 97 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index 96976d2b42..7646222ab0 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -250,71 +250,112 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 	var sourceFilesFoundSearchingNodeModules collections.Set[tspath.Path]
 	libFilesMap := make(map[tspath.Path]*LibFile, libFileCount)
 
-	loader.filesParser.collect(loader, loader.rootTasks, func(task *parseTask) {
-		if task.redirectedParseTask != nil {
-			if !loader.opts.canUseProjectReferenceSource() {
-				outputFileToProjectReferenceSource[task.redirectedParseTask.path] = task.FileName()
+	var collectFiles func(tasks []*parseTask, seen collections.Set[*queuedParseTask])
+	collectFiles = func(tasks []*parseTask, seen collections.Set[*queuedParseTask]) {
+		for _, task := range tasks {
+			// Exclude automatic type directive tasks from include reason processing,
+			// as these are internal implementation details and should not contribute
+			// to the reasons for including files.
+			if task.redirectedParseTask == nil && !task.isForAutomaticTypeDirective {
+				includeReason := task.includeReason
+				if task.loadedTask != nil {
+					task = task.loadedTask
+				}
+				w.addIncludeReason(loader, task, includeReason)
+			}
+			queued, _ := w.tasksByFileName.Load(task.normalizedFilePath)
+			// ensure we only walk each task once
+			if !task.loaded || !seen.AddIfAbsent(queued) {
+				continue
+			}
+			for _, trace := range task.typeResolutionsTrace {
+				loader.opts.Host.Trace(trace.Message, trace.Args...)
+			}
+			for _, trace := range task.resolutionsTrace {
+				loader.opts.Host.Trace(trace.Message, trace.Args...)
+			}
+			if subTasks := task.subTasks; len(subTasks) > 0 {
+				collectFiles(subTasks, seen)
 			}
-			return
-		}
 
-		if task.isForAutomaticTypeDirective {
-			typeResolutionsInFile[task.path] = task.typeResolutionsInFile
-			return
-		}
-		file := task.file
-		path := task.path
-		if file == nil {
-			// !!! sheetal file preprocessing diagnostic explaining getSourceFileFromReferenceWorker
-			missingFiles = append(missingFiles, task.normalizedFilePath)
-			return
-		}
+			// Exclude automatic type directive tasks from include reason processing,
+			// as these are internal implementation details and should not contribute
+			// to the reasons for including files.
+			if task.redirectedParseTask != nil {
+				if !loader.opts.canUseProjectReferenceSource() {
+					outputFileToProjectReferenceSource[task.redirectedParseTask.path] = task.FileName()
+				}
+				continue
+			}
 
-		// !!! sheetal todo porting file case errors
-		// if _, ok := filesByPath[path]; ok {
-		// 	Check if it differs only in drive letters its ok to ignore that error:
-		// 	const checkedAbsolutePath = getNormalizedAbsolutePathWithoutRoot(checkedName, currentDirectory);
-		// 	const inputAbsolutePath = getNormalizedAbsolutePathWithoutRoot(fileName, currentDirectory);
-		// 	if (checkedAbsolutePath !== inputAbsolutePath) {
-		// 	    reportFileNamesDifferOnlyInCasingError(fileName, file, reason);
-		// 	}
-		// } else if loader.comparePathsOptions.UseCaseSensitiveFileNames {
-		// 	pathIgnoreCase := tspath.ToPath(file.FileName(), loader.comparePathsOptions.CurrentDirectory, false)
-		// 	// for case-sensitsive file systems check if we've already seen some file with similar filename ignoring case
-		// 	if _, ok := filesByNameIgnoreCase[pathIgnoreCase]; ok {
-		// 		reportFileNamesDifferOnlyInCasingError(fileName, existingFile, reason);
-		// 	} else {
-		// 		filesByNameIgnoreCase[pathIgnoreCase] = file
-		// 	}
-		// }
-
-		if task.libFile != nil {
-			libFiles = append(libFiles, file)
-			libFilesMap[path] = task.libFile
-		} else {
-			files = append(files, file)
-		}
-		filesByPath[path] = file
-		resolvedModules[path] = task.resolutionsInFile
-		typeResolutionsInFile[path] = task.typeResolutionsInFile
-		sourceFileMetaDatas[path] = task.metadata
-
-		if task.jsxRuntimeImportSpecifier != nil {
-			if jsxRuntimeImportSpecifiers == nil {
-				jsxRuntimeImportSpecifiers = make(map[tspath.Path]*jsxRuntimeImportSpecifier, totalFileCount)
+			if task.isForAutomaticTypeDirective {
+				typeResolutionsInFile[task.path] = task.typeResolutionsInFile
+				continue
 			}
-			jsxRuntimeImportSpecifiers[path] = task.jsxRuntimeImportSpecifier
-		}
-		if task.importHelpersImportSpecifier != nil {
-			if importHelpersImportSpecifiers == nil {
-				importHelpersImportSpecifiers = make(map[tspath.Path]*ast.Node, totalFileCount)
+			file := task.file
+			path := task.path
+			if file == nil {
+				// !!! sheetal file preprocessing diagnostic explaining getSourceFileFromReferenceWorker
+				missingFiles = append(missingFiles, task.normalizedFilePath)
+				continue
+			}
+
+			// !!! sheetal todo porting file case errors
+			// if _, ok := filesByPath[path]; ok {
+			// 	Check if it differs only in drive letters its ok to ignore that error:
+			// 	const checkedAbsolutePath = getNormalizedAbsolutePathWithoutRoot(checkedName, currentDirectory);
+			// 	const inputAbsolutePath = getNormalizedAbsolutePathWithoutRoot(fileName, currentDirectory);
+			// 	if (checkedAbsolutePath !== inputAbsolutePath) {
+			// 	    reportFileNamesDifferOnlyInCasingError(fileName, file, reason);
+			// 	}
+			// } else if loader.comparePathsOptions.UseCaseSensitiveFileNames {
+			// 	pathIgnoreCase := tspath.ToPath(file.FileName(), loader.comparePathsOptions.CurrentDirectory, false)
+			// 	// for case-sensitsive file systems check if we've already seen some file with similar filename ignoring case
+			// 	if _, ok := filesByNameIgnoreCase[pathIgnoreCase]; ok {
+			// 		reportFileNamesDifferOnlyInCasingError(fileName, existingFile, reason);
+			// 	} else {
+			// 		filesByNameIgnoreCase[pathIgnoreCase] = file
+			// 	}
+			// }
+
+			if task.libFile != nil {
+				libFiles = append(libFiles, file)
+				libFilesMap[path] = task.libFile
+			} else {
+				files = append(files, file)
+			}
+			filesByPath[path] = file
+			resolvedModules[path] = task.resolutionsInFile
+			typeResolutionsInFile[path] = task.typeResolutionsInFile
+			sourceFileMetaDatas[path] = task.metadata
+
+			if task.jsxRuntimeImportSpecifier != nil {
+				if jsxRuntimeImportSpecifiers == nil {
+					jsxRuntimeImportSpecifiers = make(map[tspath.Path]*jsxRuntimeImportSpecifier, totalFileCount)
+				}
+				jsxRuntimeImportSpecifiers[path] = task.jsxRuntimeImportSpecifier
+			}
+			if task.importHelpersImportSpecifier != nil {
+				if importHelpersImportSpecifiers == nil {
+					importHelpersImportSpecifiers = make(map[tspath.Path]*ast.Node, totalFileCount)
+				}
+				importHelpersImportSpecifiers[path] = task.importHelpersImportSpecifier
+			}
+			if task.fromExternalLibrary {
+				sourceFilesFoundSearchingNodeModules.Add(path)
 			}
-			importHelpersImportSpecifiers[path] = task.importHelpersImportSpecifier
 		}
-		if task.fromExternalLibrary {
-			sourceFilesFoundSearchingNodeModules.Add(path)
+	}
+
+	// Mark all tasks we saw as external after the fact.
+	w.tasksByFileName.Range(func(key string, value *queuedParseTask) bool {
+		if value.fromExternalLibrary {
+			value.task.fromExternalLibrary = true
 		}
+		return true
 	})
+
+	collectFiles(loader.rootTasks, collections.Set[*queuedParseTask]{})
 	loader.sortLibs(libFiles)
 
 	allFiles := append(libFiles, files...)
@@ -349,46 +390,6 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 	}
 }
 
-func (w *filesParser) collect(loader *fileLoader, tasks []*parseTask, iterate func(*parseTask)) {
-	// Mark all tasks we saw as external after the fact.
-	w.tasksByFileName.Range(func(key string, value *queuedParseTask) bool {
-		if value.fromExternalLibrary {
-			value.task.fromExternalLibrary = true
-		}
-		return true
-	})
-	w.collectWorker(loader, tasks, iterate, collections.Set[*parseTask]{})
-}
-
-func (w *filesParser) collectWorker(loader *fileLoader, tasks []*parseTask, iterate func(*parseTask), seen collections.Set[*parseTask]) {
-	for _, task := range tasks {
-		// Exclude automatic type directive tasks from include reason processing,
-		// as these are internal implementation details and should not contribute
-		// to the reasons for including files.
-		if task.redirectedParseTask == nil && !task.isForAutomaticTypeDirective {
-			includeReason := task.includeReason
-			if task.loadedTask != nil {
-				task = task.loadedTask
-			}
-			w.addIncludeReason(loader, task, includeReason)
-		}
-		// ensure we only walk each task once
-		if !task.loaded || !seen.AddIfAbsent(task) {
-			continue
-		}
-		for _, trace := range task.typeResolutionsTrace {
-			loader.opts.Host.Trace(trace.Message, trace.Args...)
-		}
-		for _, trace := range task.resolutionsTrace {
-			loader.opts.Host.Trace(trace.Message, trace.Args...)
-		}
-		if subTasks := task.subTasks; len(subTasks) > 0 {
-			w.collectWorker(loader, subTasks, iterate, seen)
-		}
-		iterate(task)
-	}
-}
-
 func (w *filesParser) addIncludeReason(loader *fileLoader, task *parseTask, reason *FileIncludeReason) {
 	if task.redirectedParseTask != nil {
 		w.addIncludeReason(loader, task.redirectedParseTask, reason)

From ea8ca90c323c7905149b0dcf1b3b124a14cada73 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 15:07:09 -0800
Subject: [PATCH 03/15] Refactor 3

---
 internal/compiler/filesparser.go | 37 ++++++++++++++++----------------
 1 file changed, 18 insertions(+), 19 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index 7646222ab0..158b8c1d5b 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -161,11 +161,11 @@ func (t *parseTask) addSubTask(ref resolvedRef, libFile *LibFile) {
 
 type filesParser struct {
 	wg              core.WorkGroup
-	tasksByFileName collections.SyncMap[string, *queuedParseTask]
+	tasksByFileName collections.SyncMap[string, *parseTaskData]
 	maxDepth        int
 }
 
-type queuedParseTask struct {
+type parseTaskData struct {
 	task                *parseTask
 	mu                  sync.Mutex
 	lowestDepth         int
@@ -180,9 +180,8 @@ func (w *filesParser) parse(loader *fileLoader, tasks []*parseTask) {
 func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, isFromExternalLibrary bool) {
 	for i, task := range tasks {
 		taskIsFromExternalLibrary := isFromExternalLibrary || task.fromExternalLibrary
-		newTask := &queuedParseTask{task: task, lowestDepth: math.MaxInt}
-		loadedTask, loaded := w.tasksByFileName.LoadOrStore(task.FileName(), newTask)
-		task = loadedTask.task
+		data, loaded := w.tasksByFileName.LoadOrStore(task.FileName(), &parseTaskData{task: task, lowestDepth: math.MaxInt})
+		task = data.task
 		if loaded {
 			tasks[i].loadedTask = task
 			// Add in the loaded task's external-ness.
@@ -190,8 +189,8 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 		}
 
 		w.wg.Queue(func() {
-			loadedTask.mu.Lock()
-			defer loadedTask.mu.Unlock()
+			data.mu.Lock()
+			defer data.mu.Unlock()
 
 			startSubtasks := false
 
@@ -199,17 +198,17 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 			if task.increaseDepth {
 				currentDepth++
 			}
-			if currentDepth < loadedTask.lowestDepth {
+			if currentDepth < data.lowestDepth {
 				// If we're seeing this task at a lower depth than before,
 				// reprocess its subtasks to ensure they are loaded.
-				loadedTask.lowestDepth = currentDepth
+				data.lowestDepth = currentDepth
 				startSubtasks = true
 			}
 
-			if !task.isRoot() && taskIsFromExternalLibrary && !loadedTask.fromExternalLibrary {
+			if !task.isRoot() && taskIsFromExternalLibrary && !data.fromExternalLibrary {
 				// If we're seeing this task now as an external library,
 				// reprocess its subtasks to ensure they are also marked as external.
-				loadedTask.fromExternalLibrary = true
+				data.fromExternalLibrary = true
 				startSubtasks = true
 			}
 
@@ -222,7 +221,7 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 			}
 
 			if startSubtasks {
-				w.start(loader, task.subTasks, loadedTask.lowestDepth, loadedTask.fromExternalLibrary)
+				w.start(loader, task.subTasks, data.lowestDepth, data.fromExternalLibrary)
 			}
 		})
 	}
@@ -250,8 +249,8 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 	var sourceFilesFoundSearchingNodeModules collections.Set[tspath.Path]
 	libFilesMap := make(map[tspath.Path]*LibFile, libFileCount)
 
-	var collectFiles func(tasks []*parseTask, seen collections.Set[*queuedParseTask])
-	collectFiles = func(tasks []*parseTask, seen collections.Set[*queuedParseTask]) {
+	var collectFiles func(tasks []*parseTask, seen collections.Set[*parseTaskData])
+	collectFiles = func(tasks []*parseTask, seen collections.Set[*parseTaskData]) {
 		for _, task := range tasks {
 			// Exclude automatic type directive tasks from include reason processing,
 			// as these are internal implementation details and should not contribute
@@ -263,9 +262,9 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 				}
 				w.addIncludeReason(loader, task, includeReason)
 			}
-			queued, _ := w.tasksByFileName.Load(task.normalizedFilePath)
+			data, _ := w.tasksByFileName.Load(task.normalizedFilePath)
 			// ensure we only walk each task once
-			if !task.loaded || !seen.AddIfAbsent(queued) {
+			if !task.loaded || !seen.AddIfAbsent(data) {
 				continue
 			}
 			for _, trace := range task.typeResolutionsTrace {
@@ -341,21 +340,21 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 				}
 				importHelpersImportSpecifiers[path] = task.importHelpersImportSpecifier
 			}
-			if task.fromExternalLibrary {
+			if data.fromExternalLibrary {
 				sourceFilesFoundSearchingNodeModules.Add(path)
 			}
 		}
 	}
 
 	// Mark all tasks we saw as external after the fact.
-	w.tasksByFileName.Range(func(key string, value *queuedParseTask) bool {
+	w.tasksByFileName.Range(func(key string, value *parseTaskData) bool {
 		if value.fromExternalLibrary {
 			value.task.fromExternalLibrary = true
 		}
 		return true
 	})
 
-	collectFiles(loader.rootTasks, collections.Set[*queuedParseTask]{})
+	collectFiles(loader.rootTasks, collections.Set[*parseTaskData]{})
 	loader.sortLibs(libFiles)
 
 	allFiles := append(libFiles, files...)

From 16b4c46a65087ea57ceea4c63839670659c5243e Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 15:07:31 -0800
Subject: [PATCH 04/15] Remove unnecessary loop

---
 internal/compiler/filesparser.go | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index 158b8c1d5b..35a0fa323c 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -346,14 +346,6 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 		}
 	}
 
-	// Mark all tasks we saw as external after the fact.
-	w.tasksByFileName.Range(func(key string, value *parseTaskData) bool {
-		if value.fromExternalLibrary {
-			value.task.fromExternalLibrary = true
-		}
-		return true
-	})
-
 	collectFiles(loader.rootTasks, collections.Set[*parseTaskData]{})
 	loader.sortLibs(libFiles)
 

From 55c0c988c11d78d6f1154ed89e45f12cc7edb8aa Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 15:26:06 -0800
Subject: [PATCH 05/15] Use data instead of task for starting tasks

---
 internal/compiler/filesparser.go | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index 35a0fa323c..ac3e9aff83 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -58,7 +58,6 @@ func (t *parseTask) isRoot() bool {
 
 func (t *parseTask) load(loader *fileLoader) {
 	t.loaded = true
-	t.path = loader.toPath(t.normalizedFilePath)
 	if t.isForAutomaticTypeDirective {
 		t.loadAutomaticTypeDirectives(loader)
 		return
@@ -168,6 +167,7 @@ type filesParser struct {
 type parseTaskData struct {
 	task                *parseTask
 	mu                  sync.Mutex
+	isRoot              bool
 	lowestDepth         int
 	fromExternalLibrary bool
 }
@@ -179,13 +179,18 @@ func (w *filesParser) parse(loader *fileLoader, tasks []*parseTask) {
 
 func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, isFromExternalLibrary bool) {
 	for i, task := range tasks {
+		task.path = loader.toPath(task.normalizedFilePath)
 		taskIsFromExternalLibrary := isFromExternalLibrary || task.fromExternalLibrary
-		data, loaded := w.tasksByFileName.LoadOrStore(task.FileName(), &parseTaskData{task: task, lowestDepth: math.MaxInt})
-		task = data.task
+		data, loaded := w.tasksByFileName.LoadOrStore(task.FileName(), &parseTaskData{
+			task:        task,
+			isRoot:      task.isRoot(),
+			lowestDepth: math.MaxInt,
+		})
+		// task = data.task
 		if loaded {
-			tasks[i].loadedTask = task
+			tasks[i].loadedTask = data.task
 			// Add in the loaded task's external-ness.
-			taskIsFromExternalLibrary = taskIsFromExternalLibrary || task.fromExternalLibrary
+			taskIsFromExternalLibrary = taskIsFromExternalLibrary || data.fromExternalLibrary
 		}
 
 		w.wg.Queue(func() {
@@ -194,10 +199,7 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 
 			startSubtasks := false
 
-			currentDepth := depth
-			if task.increaseDepth {
-				currentDepth++
-			}
+			currentDepth := core.IfElse(task.increaseDepth, depth+1, depth)
 			if currentDepth < data.lowestDepth {
 				// If we're seeing this task at a lower depth than before,
 				// reprocess its subtasks to ensure they are loaded.
@@ -205,7 +207,7 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 				startSubtasks = true
 			}
 
-			if !task.isRoot() && taskIsFromExternalLibrary && !data.fromExternalLibrary {
+			if !data.isRoot && taskIsFromExternalLibrary && !data.fromExternalLibrary {
 				// If we're seeing this task now as an external library,
 				// reprocess its subtasks to ensure they are also marked as external.
 				data.fromExternalLibrary = true
@@ -216,12 +218,12 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 				return
 			}
 
-			if !task.loaded {
-				task.load(loader)
+			if !data.task.loaded {
+				data.task.load(loader)
 			}
 
 			if startSubtasks {
-				w.start(loader, task.subTasks, data.lowestDepth, data.fromExternalLibrary)
+				w.start(loader, data.task.subTasks, data.lowestDepth, data.fromExternalLibrary)
 			}
 		})
 	}

From 34edd58b78ea576e04f6206d7d56c99fe478b377 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 15:43:35 -0800
Subject: [PATCH 06/15] Remove unnecessary fileFromExternalLibrary and handle
 depth instead

---
 internal/compiler/fileloader.go  | 16 ++++---
 internal/compiler/filesparser.go | 71 +++++++++++---------------------
 2 files changed, 32 insertions(+), 55 deletions(-)

diff --git a/internal/compiler/fileloader.go b/internal/compiler/fileloader.go
index b77c030c65..c1122f44ce 100644
--- a/internal/compiler/fileloader.go
+++ b/internal/compiler/fileloader.go
@@ -350,11 +350,10 @@ func (p *fileLoader) resolveTypeReferenceDirectives(t *parseTask) {
 
 		if resolved.IsResolved() {
 			t.addSubTask(resolvedRef{
-				fileName:              resolved.ResolvedFileName,
-				increaseDepth:         resolved.IsExternalLibraryImport,
-				elideOnDepth:          false,
-				isFromExternalLibrary: resolved.IsExternalLibraryImport,
-				includeReason:         includeReason,
+				fileName:      resolved.ResolvedFileName,
+				increaseDepth: resolved.IsExternalLibraryImport,
+				elideOnDepth:  false,
+				includeReason: includeReason,
 			}, nil)
 		} else {
 			p.includeProcessor.addProcessingDiagnostic(&processingDiagnostic{
@@ -450,10 +449,9 @@ func (p *fileLoader) resolveImportsAndModuleAugmentations(t *parseTask) {
 
 			if shouldAddFile {
 				t.addSubTask(resolvedRef{
-					fileName:              resolvedFileName,
-					increaseDepth:         resolvedModule.IsExternalLibraryImport,
-					elideOnDepth:          isJsFileFromNodeModules,
-					isFromExternalLibrary: resolvedModule.IsExternalLibraryImport,
+					fileName:      resolvedFileName,
+					increaseDepth: resolvedModule.IsExternalLibraryImport,
+					elideOnDepth:  isJsFileFromNodeModules,
 					includeReason: &FileIncludeReason{
 						kind: fileIncludeKindImport,
 						data: &referencedFileData{
diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index ac3e9aff83..e364c98795 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -32,12 +32,9 @@ type parseTask struct {
 	resolutionDiagnostics        []*ast.Diagnostic
 	importHelpersImportSpecifier *ast.Node
 	jsxRuntimeImportSpecifier    *jsxRuntimeImportSpecifier
-	increaseDepth                bool
-	elideOnDepth                 bool
 
-	// Track if this file is from an external library (node_modules)
-	// This mirrors the TypeScript currentNodeModulesDepth > 0 check
-	fromExternalLibrary bool
+	increaseDepth bool
+	elideOnDepth  bool
 
 	loadedTask        *parseTask
 	allIncludeReasons []*FileIncludeReason
@@ -51,11 +48,6 @@ func (t *parseTask) Path() tspath.Path {
 	return t.path
 }
 
-func (t *parseTask) isRoot() bool {
-	// Intentionally not checking t.includeReason != nil to ensure we can catch cases for missing include reason
-	return !t.isForAutomaticTypeDirective && (t.includeReason.kind == fileIncludeKindRootFile || t.includeReason.kind == fileIncludeKindLibFile)
-}
-
 func (t *parseTask) load(loader *fileLoader) {
 	t.loaded = true
 	if t.isForAutomaticTypeDirective {
@@ -119,10 +111,9 @@ func (t *parseTask) load(loader *fileLoader) {
 
 func (t *parseTask) redirect(loader *fileLoader, fileName string) {
 	t.redirectedParseTask = &parseTask{
-		normalizedFilePath:  tspath.NormalizePath(fileName),
-		libFile:             t.libFile,
-		fromExternalLibrary: t.fromExternalLibrary,
-		includeReason:       t.includeReason,
+		normalizedFilePath: tspath.NormalizePath(fileName),
+		libFile:            t.libFile,
+		includeReason:      t.includeReason,
 	}
 	// increaseDepth and elideOnDepth are not copied to redirects, otherwise their depth would be double counted.
 	t.subTasks = []*parseTask{t.redirectedParseTask}
@@ -138,22 +129,20 @@ func (t *parseTask) loadAutomaticTypeDirectives(loader *fileLoader) {
 }
 
 type resolvedRef struct {
-	fileName              string
-	increaseDepth         bool
-	elideOnDepth          bool
-	isFromExternalLibrary bool
-	includeReason         *FileIncludeReason
+	fileName      string
+	increaseDepth bool
+	elideOnDepth  bool
+	includeReason *FileIncludeReason
 }
 
 func (t *parseTask) addSubTask(ref resolvedRef, libFile *LibFile) {
 	normalizedFilePath := tspath.NormalizePath(ref.fileName)
 	subTask := &parseTask{
-		normalizedFilePath:  normalizedFilePath,
-		libFile:             libFile,
-		increaseDepth:       ref.increaseDepth,
-		elideOnDepth:        ref.elideOnDepth,
-		fromExternalLibrary: ref.isFromExternalLibrary,
-		includeReason:       ref.includeReason,
+		normalizedFilePath: normalizedFilePath,
+		libFile:            libFile,
+		increaseDepth:      ref.increaseDepth,
+		elideOnDepth:       ref.elideOnDepth,
+		includeReason:      ref.includeReason,
 	}
 	t.subTasks = append(t.subTasks, subTask)
 }
@@ -165,32 +154,25 @@ type filesParser struct {
 }
 
 type parseTaskData struct {
-	task                *parseTask
-	mu                  sync.Mutex
-	isRoot              bool
-	lowestDepth         int
-	fromExternalLibrary bool
+	task        *parseTask
+	mu          sync.Mutex
+	lowestDepth int
 }
 
 func (w *filesParser) parse(loader *fileLoader, tasks []*parseTask) {
-	w.start(loader, tasks, 0, false)
+	w.start(loader, tasks, 0)
 	w.wg.RunAndWait()
 }
 
-func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, isFromExternalLibrary bool) {
+func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int) {
 	for i, task := range tasks {
 		task.path = loader.toPath(task.normalizedFilePath)
-		taskIsFromExternalLibrary := isFromExternalLibrary || task.fromExternalLibrary
 		data, loaded := w.tasksByFileName.LoadOrStore(task.FileName(), &parseTaskData{
 			task:        task,
-			isRoot:      task.isRoot(),
 			lowestDepth: math.MaxInt,
 		})
-		// task = data.task
 		if loaded {
 			tasks[i].loadedTask = data.task
-			// Add in the loaded task's external-ness.
-			taskIsFromExternalLibrary = taskIsFromExternalLibrary || data.fromExternalLibrary
 		}
 
 		w.wg.Queue(func() {
@@ -207,23 +189,20 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int, i
 				startSubtasks = true
 			}
 
-			if !data.isRoot && taskIsFromExternalLibrary && !data.fromExternalLibrary {
-				// If we're seeing this task now as an external library,
-				// reprocess its subtasks to ensure they are also marked as external.
-				data.fromExternalLibrary = true
-				startSubtasks = true
-			}
-
 			if task.elideOnDepth && currentDepth > w.maxDepth {
 				return
 			}
 
 			if !data.task.loaded {
 				data.task.load(loader)
+				if data.task.redirectedParseTask != nil {
+					// Always load redirected task
+					startSubtasks = true
+				}
 			}
 
 			if startSubtasks {
-				w.start(loader, data.task.subTasks, data.lowestDepth, data.fromExternalLibrary)
+				w.start(loader, data.task.subTasks, data.lowestDepth)
 			}
 		})
 	}
@@ -342,7 +321,7 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 				}
 				importHelpersImportSpecifiers[path] = task.importHelpersImportSpecifier
 			}
-			if data.fromExternalLibrary {
+			if data.lowestDepth > 0 {
 				sourceFilesFoundSearchingNodeModules.Add(path)
 			}
 		}

From 30b8730dd5fa1ad24aa5a16c277001c5d92e1459 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 15:52:43 -0800
Subject: [PATCH 07/15] Make stub for casing checks

---
 internal/compiler/filesparser.go | 62 ++++++++++++++++++++------------
 1 file changed, 39 insertions(+), 23 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index e364c98795..17bc2d6bab 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -148,13 +148,14 @@ func (t *parseTask) addSubTask(ref resolvedRef, libFile *LibFile) {
 }
 
 type filesParser struct {
-	wg              core.WorkGroup
-	tasksByFileName collections.SyncMap[string, *parseTaskData]
-	maxDepth        int
+	wg             core.WorkGroup
+	taskDataByPath collections.SyncMap[tspath.Path, *parseTaskData]
+	maxDepth       int
 }
 
 type parseTaskData struct {
-	task        *parseTask
+	// map of tasks by file casing
+	tasks       map[string]*parseTask
 	mu          sync.Mutex
 	lowestDepth int
 }
@@ -167,18 +168,23 @@ func (w *filesParser) parse(loader *fileLoader, tasks []*parseTask) {
 func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int) {
 	for i, task := range tasks {
 		task.path = loader.toPath(task.normalizedFilePath)
-		data, loaded := w.tasksByFileName.LoadOrStore(task.FileName(), &parseTaskData{
-			task:        task,
+		data, loaded := w.taskDataByPath.LoadOrStore(task.path, &parseTaskData{
+			tasks:       map[string]*parseTask{task.normalizedFilePath: task},
 			lowestDepth: math.MaxInt,
 		})
-		if loaded {
-			tasks[i].loadedTask = data.task
-		}
 
 		w.wg.Queue(func() {
 			data.mu.Lock()
 			defer data.mu.Unlock()
 
+			if loaded {
+				if existingTask, ok := data.tasks[task.normalizedFilePath]; ok {
+					tasks[i].loadedTask = existingTask
+				} else {
+					data.tasks[task.normalizedFilePath] = task
+				}
+			}
+
 			startSubtasks := false
 
 			currentDepth := core.IfElse(task.increaseDepth, depth+1, depth)
@@ -193,16 +199,18 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int) {
 				return
 			}
 
-			if !data.task.loaded {
-				data.task.load(loader)
-				if data.task.redirectedParseTask != nil {
-					// Always load redirected task
-					startSubtasks = true
+			for _, taskByFileName := range data.tasks {
+				loadSubTasks := startSubtasks
+				if !taskByFileName.loaded {
+					taskByFileName.load(loader)
+					if taskByFileName.redirectedParseTask != nil {
+						// Always load redirected task
+						loadSubTasks = true
+					}
+				}
+				if loadSubTasks {
+					w.start(loader, taskByFileName.subTasks, data.lowestDepth)
 				}
-			}
-
-			if startSubtasks {
-				w.start(loader, data.task.subTasks, data.lowestDepth)
 			}
 		})
 	}
@@ -230,8 +238,8 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 	var sourceFilesFoundSearchingNodeModules collections.Set[tspath.Path]
 	libFilesMap := make(map[tspath.Path]*LibFile, libFileCount)
 
-	var collectFiles func(tasks []*parseTask, seen collections.Set[*parseTaskData])
-	collectFiles = func(tasks []*parseTask, seen collections.Set[*parseTaskData]) {
+	var collectFiles func(tasks []*parseTask, seen map[*parseTaskData]string)
+	collectFiles = func(tasks []*parseTask, seen map[*parseTaskData]string) {
 		for _, task := range tasks {
 			// Exclude automatic type directive tasks from include reason processing,
 			// as these are internal implementation details and should not contribute
@@ -243,11 +251,19 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 				}
 				w.addIncludeReason(loader, task, includeReason)
 			}
-			data, _ := w.tasksByFileName.Load(task.normalizedFilePath)
+			data, _ := w.taskDataByPath.Load(task.path)
+			if !task.loaded {
+				continue
+			}
+
 			// ensure we only walk each task once
-			if !task.loaded || !seen.AddIfAbsent(data) {
+			if _, ok := seen[data]; ok {
+				// !!! already seen verify casing
 				continue
+			} else {
+				seen[data] = task.normalizedFilePath
 			}
+
 			for _, trace := range task.typeResolutionsTrace {
 				loader.opts.Host.Trace(trace.Message, trace.Args...)
 			}
@@ -327,7 +343,7 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 		}
 	}
 
-	collectFiles(loader.rootTasks, collections.Set[*parseTaskData]{})
+	collectFiles(loader.rootTasks, make(map[*parseTaskData]string, totalFileCount))
 	loader.sortLibs(libFiles)
 
 	allFiles := append(libFiles, files...)

From 50f7178e1b89345ff73cc63c98e43546af54c374 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 16:03:54 -0800
Subject: [PATCH 08/15] Actual file casing checks

---
 internal/compiler/filesparser.go      | 45 +++++++++++++++------------
 internal/compiler/includeprocessor.go | 27 ++++++++++++++++
 internal/tspath/path.go               |  6 ++++
 3 files changed, 58 insertions(+), 20 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index 17bc2d6bab..ad470ff934 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -225,6 +225,13 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 	libFiles := make([]*ast.SourceFile, 0, totalFileCount) // totalFileCount here since we append files to it later to construct the final list
 
 	filesByPath := make(map[tspath.Path]*ast.SourceFile, totalFileCount)
+	// stores 'filename -> file association' ignoring case
+	// used to track cases when two file names differ only in casing
+	var filesByNameIgnoreCase map[string]*ast.SourceFile
+	if loader.comparePathsOptions.UseCaseSensitiveFileNames {
+		filesByNameIgnoreCase = make(map[string]*ast.SourceFile, totalFileCount)
+	}
+
 	loader.includeProcessor.fileIncludeReasons = make(map[tspath.Path][]*FileIncludeReason, totalFileCount)
 	var outputFileToProjectReferenceSource map[tspath.Path]string
 	if !loader.opts.canUseProjectReferenceSource() {
@@ -241,11 +248,11 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 	var collectFiles func(tasks []*parseTask, seen map[*parseTaskData]string)
 	collectFiles = func(tasks []*parseTask, seen map[*parseTaskData]string) {
 		for _, task := range tasks {
+			includeReason := task.includeReason
 			// Exclude automatic type directive tasks from include reason processing,
 			// as these are internal implementation details and should not contribute
 			// to the reasons for including files.
 			if task.redirectedParseTask == nil && !task.isForAutomaticTypeDirective {
-				includeReason := task.includeReason
 				if task.loadedTask != nil {
 					task = task.loadedTask
 				}
@@ -257,8 +264,15 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 			}
 
 			// ensure we only walk each task once
-			if _, ok := seen[data]; ok {
-				// !!! already seen verify casing
+			if checkedName, ok := seen[data]; ok {
+				if !loader.opts.Config.CompilerOptions().ForceConsistentCasingInFileNames.IsFalse() {
+					// Check if it differs only in drive letters its ok to ignore that error:
+					checkedAbsolutePath := tspath.GetNormalizedAbsolutePathWithoutRoot(checkedName, loader.comparePathsOptions.CurrentDirectory)
+					inputAbsolutePath := tspath.GetNormalizedAbsolutePathWithoutRoot(task.normalizedFilePath, loader.comparePathsOptions.CurrentDirectory)
+					if checkedAbsolutePath != inputAbsolutePath {
+						loader.includeProcessor.addProcessingDiagnosticsForFileCasing(task.path, checkedName, task.normalizedFilePath, includeReason)
+					}
+				}
 				continue
 			} else {
 				seen[data] = task.normalizedFilePath
@@ -296,23 +310,14 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 				continue
 			}
 
-			// !!! sheetal todo porting file case errors
-			// if _, ok := filesByPath[path]; ok {
-			// 	Check if it differs only in drive letters its ok to ignore that error:
-			// 	const checkedAbsolutePath = getNormalizedAbsolutePathWithoutRoot(checkedName, currentDirectory);
-			// 	const inputAbsolutePath = getNormalizedAbsolutePathWithoutRoot(fileName, currentDirectory);
-			// 	if (checkedAbsolutePath !== inputAbsolutePath) {
-			// 	    reportFileNamesDifferOnlyInCasingError(fileName, file, reason);
-			// 	}
-			// } else if loader.comparePathsOptions.UseCaseSensitiveFileNames {
-			// 	pathIgnoreCase := tspath.ToPath(file.FileName(), loader.comparePathsOptions.CurrentDirectory, false)
-			// 	// for case-sensitsive file systems check if we've already seen some file with similar filename ignoring case
-			// 	if _, ok := filesByNameIgnoreCase[pathIgnoreCase]; ok {
-			// 		reportFileNamesDifferOnlyInCasingError(fileName, existingFile, reason);
-			// 	} else {
-			// 		filesByNameIgnoreCase[pathIgnoreCase] = file
-			// 	}
-			// }
+			if filesByNameIgnoreCase != nil {
+				pathLowerCase := tspath.ToFileNameLowerCase(string(path))
+				if existingFile, ok := filesByNameIgnoreCase[pathLowerCase]; ok {
+					loader.includeProcessor.addProcessingDiagnosticsForFileCasing(path, existingFile.FileName(), file.FileName(), includeReason)
+				} else {
+					filesByNameIgnoreCase[pathLowerCase] = file
+				}
+			}
 
 			if task.libFile != nil {
 				libFiles = append(libFiles, file)
diff --git a/internal/compiler/includeprocessor.go b/internal/compiler/includeprocessor.go
index 83691f45a1..a40534bbde 100644
--- a/internal/compiler/includeprocessor.go
+++ b/internal/compiler/includeprocessor.go
@@ -1,6 +1,7 @@
 package compiler
 
 import (
+	"slices"
 	"sync"
 
 	"github.com/microsoft/typescript-go/internal/ast"
@@ -59,6 +60,32 @@ func (i *includeProcessor) addProcessingDiagnostic(d ...*processingDiagnostic) {
 	i.processingDiagnostics = append(i.processingDiagnostics, d...)
 }
 
+func (i *includeProcessor) addProcessingDiagnosticsForFileCasing(file tspath.Path, existingCasing string, currentCasing string, reason *FileIncludeReason) {
+	if !reason.isReferencedFile() && slices.ContainsFunc(i.fileIncludeReasons[file], func(r *FileIncludeReason) bool {
+		return r.isReferencedFile()
+	}) {
+		i.addProcessingDiagnostic(&processingDiagnostic{
+			kind: processingDiagnosticKindExplainingFileInclude,
+			data: &includeExplainingDiagnostic{
+				file:             file,
+				diagnosticReason: reason,
+				message:          diagnostics.Already_included_file_name_0_differs_from_file_name_1_only_in_casing,
+				args:             []any{existingCasing, currentCasing},
+			},
+		})
+	} else {
+		i.addProcessingDiagnostic(&processingDiagnostic{
+			kind: processingDiagnosticKindExplainingFileInclude,
+			data: &includeExplainingDiagnostic{
+				file:             file,
+				diagnosticReason: reason,
+				message:          diagnostics.File_name_0_differs_from_already_included_file_name_1_only_in_casing,
+				args:             []any{currentCasing, existingCasing},
+			},
+		})
+	}
+}
+
 func (i *includeProcessor) getReferenceLocation(r *FileIncludeReason, program *Program) *referenceFileLocation {
 	if existing, ok := i.reasonToReferenceLocation.Load(r); ok {
 		return existing
diff --git a/internal/tspath/path.go b/internal/tspath/path.go
index de09221507..26abe6778d 100644
--- a/internal/tspath/path.go
+++ b/internal/tspath/path.go
@@ -334,6 +334,12 @@ func GetNormalizedPathComponents(path string, currentDirectory string) []string
 	return reducePathComponents(GetPathComponents(path, currentDirectory))
 }
 
+func GetNormalizedAbsolutePathWithoutRoot(fileName string, currentDirectory string) string {
+	absolutePath := GetNormalizedAbsolutePath(fileName, currentDirectory)
+	rootLength := GetRootLength(absolutePath)
+	return absolutePath[rootLength:]
+}
+
 func GetNormalizedAbsolutePath(fileName string, currentDirectory string) string {
 	rootLength := GetRootLength(fileName)
 	if rootLength == 0 && currentDirectory != "" {

From 73aa765cde7b758892346b235f216bc81b6cf75a Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Tue, 25 Nov 2025 16:33:48 -0800
Subject: [PATCH 09/15] Tests TODO: some formatting issue with reason repeating

---
 internal/execute/tsctests/tsc_test.go         |  66 ++++++
 ...m-multiple-places-with-different-casing.js | 211 ++++++++++++++++++
 ...ative-and-non-relative-file-resolutions.js |  83 +++++++
 .../with-type-ref-from-file.js                |  58 +++++
 4 files changed, 418 insertions(+)
 create mode 100644 testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
 create mode 100644 testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js
 create mode 100644 testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-type-ref-from-file.js

diff --git a/internal/execute/tsctests/tsc_test.go b/internal/execute/tsctests/tsc_test.go
index e618ce7829..e0a2263bc6 100644
--- a/internal/execute/tsctests/tsc_test.go
+++ b/internal/execute/tsctests/tsc_test.go
@@ -981,6 +981,72 @@ func TestTscExtends(t *testing.T) {
 	}
 }
 
+func TestForceConsistentCasingInFileNames(t *testing.T) {
+	t.Parallel()
+	testCases := []*tscInput{
+		{
+			subScenario: "with relative and non relative file resolutions",
+			files: FileMap{
+				"/user/username/projects/myproject/src/struct.d.ts": stringtestutil.Dedent(`
+                    import * as xs1 from "fp-ts/lib/Struct";
+                    import * as xs2 from "fp-ts/lib/struct";
+                    import * as xs3 from "./Struct";
+                    import * as xs4 from "./struct";
+                `),
+				"/user/username/projects/myproject/node_modules/fp-ts/lib/struct.d.ts": `export function foo(): void`,
+			},
+			cwd:             "/user/username/projects/myproject",
+			commandLineArgs: []string{"/user/username/projects/myproject/src/struct.d.ts", "--forceConsistentCasingInFileNames", "--explainFiles"},
+			ignoreCase:      true,
+		},
+		{
+			subScenario: "when file is included from multiple places with different casing",
+			files: FileMap{
+				"/home/src/projects/project/src/struct.d.ts": stringtestutil.Dedent(`
+					import * as xs1 from "fp-ts/lib/Struct";
+					import * as xs2 from "fp-ts/lib/struct";
+					import * as xs3 from "./Struct";
+					import * as xs4 from "./struct";
+				`),
+				"/home/src/projects/project/src/anotherFile.ts": stringtestutil.Dedent(`
+					import * as xs1 from "fp-ts/lib/Struct";
+					import * as xs2 from "fp-ts/lib/struct";
+					import * as xs3 from "./Struct";
+					import * as xs4 from "./struct";
+				`),
+				"/home/src/projects/project/src/oneMore.ts": stringtestutil.Dedent(`
+					import * as xs1 from "fp-ts/lib/Struct";
+					import * as xs2 from "fp-ts/lib/struct";
+					import * as xs3 from "./Struct";
+					import * as xs4 from "./struct";
+				`),
+				"/home/src/projects/project/tsconfig.json":                      `{}`,
+				"/home/src/projects/project/node_modules/fp-ts/lib/struct.d.ts": `export function foo(): void`,
+			},
+			cwd:             "/home/src/projects/project",
+			commandLineArgs: []string{"--explainFiles"},
+			ignoreCase:      true,
+		},
+		{
+			subScenario: "with type ref from file",
+			files: FileMap{
+				"/user/username/projects/myproject/src/fileOne.d.ts": `declare class c { }`,
+				"/user/username/projects/myproject/src/file2.d.ts": stringtestutil.Dedent(`
+                    /// <reference types="./fileOne.d.ts"/>
+                    declare const y: c;
+                `),
+				"/user/username/projects/myproject/tsconfig.json": "{ }",
+			},
+			cwd:             "/user/username/projects/myproject",
+			commandLineArgs: []string{"-p", "/user/username/projects/myproject", "--explainFiles", "--traceResolution"},
+			ignoreCase:      true,
+		},
+	}
+	for _, test := range testCases {
+		test.run(t, "forceConsistentCasingInFileNames")
+	}
+}
+
 func TestTscIgnoreConfig(t *testing.T) {
 	t.Parallel()
 	filesWithoutConfig := func() FileMap {
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
new file mode 100644
index 0000000000..d35c87fc41
--- /dev/null
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
@@ -0,0 +1,211 @@
+currentDirectory::/home/src/projects/project
+useCaseSensitiveFileNames::false
+Input::
+//// [/home/src/projects/project/node_modules/fp-ts/lib/struct.d.ts] *new* 
+export function foo(): void
+//// [/home/src/projects/project/src/anotherFile.ts] *new* 
+import * as xs1 from "fp-ts/lib/Struct";
+import * as xs2 from "fp-ts/lib/struct";
+import * as xs3 from "./Struct";
+import * as xs4 from "./struct";
+//// [/home/src/projects/project/src/oneMore.ts] *new* 
+import * as xs1 from "fp-ts/lib/Struct";
+import * as xs2 from "fp-ts/lib/struct";
+import * as xs3 from "./Struct";
+import * as xs4 from "./struct";
+//// [/home/src/projects/project/src/struct.d.ts] *new* 
+import * as xs1 from "fp-ts/lib/Struct";
+import * as xs2 from "fp-ts/lib/struct";
+import * as xs3 from "./Struct";
+import * as xs4 from "./struct";
+//// [/home/src/projects/project/tsconfig.json] *new* 
+{}
+
+tsgo --explainFiles
+ExitStatus:: DiagnosticsPresent_OutputsGenerated
+Output::
+src/anotherFile.ts:2:22 - error TS1149: File name '/home/src/projects/project/node_modules/fp-ts/lib/struct.d.ts' differs from already included file name '/home/src/projects/project/node_modules/fp-ts/lib/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/oneMore.ts'
+
+2 import * as xs2 from "fp-ts/lib/struct";
+                       ~~~~~~~~~~~~~~~~~~
+
+  src/anotherFile.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/anotherFile.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/oneMore.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/oneMore.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+src/anotherFile.ts:3:22 - error TS1261: Already included file name '/home/src/projects/project/src/Struct.d.ts' differs from file name '/home/src/projects/project/src/struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Matched by default include pattern '**/*'
+
+3 import * as xs3 from "./Struct";
+                       ~~~~~~~~~~
+
+  src/anotherFile.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+src/anotherFile.ts:4:22 - error TS1149: File name '/home/src/projects/project/src/struct.d.ts' differs from already included file name '/home/src/projects/project/src/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Matched by default include pattern '**/*'
+
+4 import * as xs4 from "./struct";
+                       ~~~~~~~~~~
+
+  src/anotherFile.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/anotherFile.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+src/oneMore.ts:2:22 - error TS1149: File name '/home/src/projects/project/node_modules/fp-ts/lib/struct.d.ts' differs from already included file name '/home/src/projects/project/node_modules/fp-ts/lib/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/oneMore.ts'
+
+2 import * as xs2 from "fp-ts/lib/struct";
+                       ~~~~~~~~~~~~~~~~~~
+
+  src/anotherFile.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/anotherFile.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/oneMore.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/oneMore.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+src/oneMore.ts:4:22 - error TS1149: File name '/home/src/projects/project/src/struct.d.ts' differs from already included file name '/home/src/projects/project/src/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Matched by default include pattern '**/*'
+
+4 import * as xs4 from "./struct";
+                       ~~~~~~~~~~
+
+  src/anotherFile.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/anotherFile.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+../../tslibs/TS/Lib/lib.d.ts
+   Default library for target 'ES5'
+node_modules/fp-ts/lib/Struct.d.ts
+   Imported via "fp-ts/lib/Struct" from file 'src/anotherFile.ts'
+   Imported via "fp-ts/lib/struct" from file 'src/anotherFile.ts'
+   Imported via "fp-ts/lib/Struct" from file 'src/oneMore.ts'
+   Imported via "fp-ts/lib/struct" from file 'src/oneMore.ts'
+src/Struct.d.ts
+   Imported via "./Struct" from file 'src/anotherFile.ts'
+   Imported via "./struct" from file 'src/anotherFile.ts'
+   Imported via "./Struct" from file 'src/oneMore.ts'
+   Imported via "./struct" from file 'src/oneMore.ts'
+   Matched by default include pattern '**/*'
+src/anotherFile.ts
+   Matched by default include pattern '**/*'
+src/oneMore.ts
+   Matched by default include pattern '**/*'
+
+Found 5 errors in 2 files.
+
+Errors  Files
+     3  src/anotherFile.ts:2
+     2  src/oneMore.ts:2
+
+//// [/home/src/projects/project/src/anotherFile.js] *new* 
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+
+//// [/home/src/projects/project/src/oneMore.js] *new* 
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+
+//// [/home/src/tslibs/TS/Lib/lib.d.ts] *Lib*
+/// <reference no-default-lib="true"/>
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+interface SymbolConstructor {
+    (desc?: string | number): symbol;
+    for(name: string): symbol;
+    readonly toStringTag: symbol;
+}
+declare var Symbol: SymbolConstructor;
+interface Symbol {
+    readonly [Symbol.toStringTag]: string;
+}
+declare const console: { log(msg: any): void; };
+
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js
new file mode 100644
index 0000000000..e39a374e3a
--- /dev/null
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js
@@ -0,0 +1,83 @@
+currentDirectory::/user/username/projects/myproject
+useCaseSensitiveFileNames::false
+Input::
+//// [/user/username/projects/myproject/node_modules/fp-ts/lib/struct.d.ts] *new* 
+export function foo(): void
+//// [/user/username/projects/myproject/src/struct.d.ts] *new* 
+import * as xs1 from "fp-ts/lib/Struct";
+import * as xs2 from "fp-ts/lib/struct";
+import * as xs3 from "./Struct";
+import * as xs4 from "./struct";
+
+tsgo /user/username/projects/myproject/src/struct.d.ts --forceConsistentCasingInFileNames --explainFiles
+ExitStatus:: DiagnosticsPresent_OutputsGenerated
+Output::
+src/struct.d.ts:2:22 - error TS1149: File name '/user/username/projects/myproject/node_modules/fp-ts/lib/struct.d.ts' differs from already included file name '/user/username/projects/myproject/node_modules/fp-ts/lib/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "fp-ts/lib/Struct" from file '/user/username/projects/myproject/src/struct.d.ts'
+    Imported via "fp-ts/lib/struct" from file '/user/username/projects/myproject/src/struct.d.ts'
+
+2 import * as xs2 from "fp-ts/lib/struct";
+                       ~~~~~~~~~~~~~~~~~~
+
+  src/struct.d.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/struct.d.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+src/struct.d.ts:3:22 - error TS1149: File name '/user/username/projects/myproject/src/Struct.d.ts' differs from already included file name '/user/username/projects/myproject/src/struct.d.ts' only in casing.
+  The file is in the program because:
+    Root file specified for compilation
+    Imported via "./Struct" from file '/user/username/projects/myproject/src/struct.d.ts'
+    Imported via "./struct" from file '/user/username/projects/myproject/src/struct.d.ts'
+
+3 import * as xs3 from "./Struct";
+                       ~~~~~~~~~~
+
+  src/struct.d.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/struct.d.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+../../../../home/src/tslibs/TS/Lib/lib.d.ts
+   Default library for target 'ES5'
+node_modules/fp-ts/lib/Struct.d.ts
+   Imported via "fp-ts/lib/Struct" from file 'src/struct.d.ts'
+   Imported via "fp-ts/lib/struct" from file 'src/struct.d.ts'
+src/struct.d.ts
+   Root file specified for compilation
+   Imported via "./Struct" from file 'src/struct.d.ts'
+   Imported via "./struct" from file 'src/struct.d.ts'
+
+Found 2 errors in the same file, starting at: src/struct.d.ts:2
+
+//// [/home/src/tslibs/TS/Lib/lib.d.ts] *Lib*
+/// <reference no-default-lib="true"/>
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+interface SymbolConstructor {
+    (desc?: string | number): symbol;
+    for(name: string): symbol;
+    readonly toStringTag: symbol;
+}
+declare var Symbol: SymbolConstructor;
+interface Symbol {
+    readonly [Symbol.toStringTag]: string;
+}
+declare const console: { log(msg: any): void; };
+
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-type-ref-from-file.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-type-ref-from-file.js
new file mode 100644
index 0000000000..2f55514b08
--- /dev/null
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-type-ref-from-file.js
@@ -0,0 +1,58 @@
+currentDirectory::/user/username/projects/myproject
+useCaseSensitiveFileNames::false
+Input::
+//// [/user/username/projects/myproject/src/file2.d.ts] *new* 
+/// <reference types="./fileOne.d.ts"/>
+declare const y: c;
+//// [/user/username/projects/myproject/src/fileOne.d.ts] *new* 
+declare class c { }
+//// [/user/username/projects/myproject/tsconfig.json] *new* 
+{ }
+
+tsgo -p /user/username/projects/myproject --explainFiles --traceResolution
+ExitStatus:: Success
+Output::
+======== Resolving type reference directive './fileOne.d.ts', containing file '/user/username/projects/myproject/src/file2.d.ts', root directory '/user/username/projects/myproject/node_modules/@types,/user/username/projects/node_modules/@types,/user/username/node_modules/@types,/user/node_modules/@types,/node_modules/@types'. ========
+Resolving with primary search path '/user/username/projects/myproject/node_modules/@types, /user/username/projects/node_modules/@types, /user/username/node_modules/@types, /user/node_modules/@types, /node_modules/@types'.
+Directory '/user/username/projects/myproject/node_modules/@types' does not exist, skipping all lookups in it.
+Directory '/user/username/projects/node_modules/@types' does not exist, skipping all lookups in it.
+Directory '/user/username/node_modules/@types' does not exist, skipping all lookups in it.
+Directory '/user/node_modules/@types' does not exist, skipping all lookups in it.
+Directory '/node_modules/@types' does not exist, skipping all lookups in it.
+Looking up in 'node_modules' folder, initial location '/user/username/projects/myproject/src'.
+Loading module as file / folder, candidate module location '/user/username/projects/myproject/src/fileOne.d.ts', target file types: Declaration.
+File name '/user/username/projects/myproject/src/fileOne.d.ts' has a '.d.ts' extension - stripping it.
+File '/user/username/projects/myproject/src/fileOne.d.ts' exists - use it as a name resolution result.
+Resolving real path for '/user/username/projects/myproject/src/fileOne.d.ts', result '/user/username/projects/myproject/src/fileOne.d.ts'.
+======== Type reference directive './fileOne.d.ts' was successfully resolved to '/user/username/projects/myproject/src/fileOne.d.ts', primary: false. ========
+../../../../home/src/tslibs/TS/Lib/lib.d.ts
+   Default library for target 'ES5'
+src/fileOne.d.ts
+   Type library referenced via './fileOne.d.ts' from file 'src/file2.d.ts'
+   Matched by default include pattern '**/*'
+src/file2.d.ts
+   Matched by default include pattern '**/*'
+//// [/home/src/tslibs/TS/Lib/lib.d.ts] *Lib*
+/// <reference no-default-lib="true"/>
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+interface SymbolConstructor {
+    (desc?: string | number): symbol;
+    for(name: string): symbol;
+    readonly toStringTag: symbol;
+}
+declare var Symbol: SymbolConstructor;
+interface Symbol {
+    readonly [Symbol.toStringTag]: string;
+}
+declare const console: { log(msg: any): void; };
+

From b288016a04e085cb467b41c319ccf488915f34d8 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Wed, 26 Nov 2025 13:04:43 -0800
Subject: [PATCH 10/15] Fix related info when related info is added for reason
 where the diagnostics is reported

---
 internal/compiler/processingDiagnostic.go        |  2 +-
 ...from-multiple-places-with-different-casing.js | 16 ----------------
 ...relative-and-non-relative-file-resolutions.js |  8 --------
 3 files changed, 1 insertion(+), 25 deletions(-)

diff --git a/internal/compiler/processingDiagnostic.go b/internal/compiler/processingDiagnostic.go
index 20e5c8d18c..a2f7d89e90 100644
--- a/internal/compiler/processingDiagnostic.go
+++ b/internal/compiler/processingDiagnostic.go
@@ -79,7 +79,7 @@ func (d *processingDiagnostic) createDiagnosticExplainingFile(program *Program)
 	processRelatedInfo := func(includeReason *FileIncludeReason) {
 		if preferredLocation == nil && includeReason.isReferencedFile() && !program.includeProcessor.getReferenceLocation(includeReason, program).isSynthetic {
 			preferredLocation = includeReason
-		} else {
+		} else if preferredLocation != includeReason {
 			info := program.includeProcessor.getRelatedInfo(includeReason, program)
 			if info != nil {
 				relatedInfo = append(relatedInfo, info)
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
index d35c87fc41..3613e8cbe0 100644
--- a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
@@ -38,10 +38,6 @@ Output::
     1 import * as xs1 from "fp-ts/lib/Struct";
                            ~~~~~~~~~~~~~~~~~~
 
-  src/anotherFile.ts:2:22 - File is included via import here.
-    2 import * as xs2 from "fp-ts/lib/struct";
-                           ~~~~~~~~~~~~~~~~~~
-
   src/oneMore.ts:1:22 - File is included via import here.
     1 import * as xs1 from "fp-ts/lib/Struct";
                            ~~~~~~~~~~~~~~~~~~
@@ -88,10 +84,6 @@ Output::
     3 import * as xs3 from "./Struct";
                            ~~~~~~~~~~
 
-  src/anotherFile.ts:4:22 - File is included via import here.
-    4 import * as xs4 from "./struct";
-                           ~~~~~~~~~~
-
   src/oneMore.ts:3:22 - File is included via import here.
     3 import * as xs3 from "./Struct";
                            ~~~~~~~~~~
@@ -122,10 +114,6 @@ Output::
     1 import * as xs1 from "fp-ts/lib/Struct";
                            ~~~~~~~~~~~~~~~~~~
 
-  src/oneMore.ts:2:22 - File is included via import here.
-    2 import * as xs2 from "fp-ts/lib/struct";
-                           ~~~~~~~~~~~~~~~~~~
-
 src/oneMore.ts:4:22 - error TS1149: File name '/home/src/projects/project/src/struct.d.ts' differs from already included file name '/home/src/projects/project/src/Struct.d.ts' only in casing.
   The file is in the program because:
     Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
@@ -149,10 +137,6 @@ Output::
     3 import * as xs3 from "./Struct";
                            ~~~~~~~~~~
 
-  src/oneMore.ts:4:22 - File is included via import here.
-    4 import * as xs4 from "./struct";
-                           ~~~~~~~~~~
-
 ../../tslibs/TS/Lib/lib.d.ts
    Default library for target 'ES5'
 node_modules/fp-ts/lib/Struct.d.ts
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js
index e39a374e3a..19ce089f68 100644
--- a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-relative-and-non-relative-file-resolutions.js
@@ -24,10 +24,6 @@ Output::
     1 import * as xs1 from "fp-ts/lib/Struct";
                            ~~~~~~~~~~~~~~~~~~
 
-  src/struct.d.ts:2:22 - File is included via import here.
-    2 import * as xs2 from "fp-ts/lib/struct";
-                           ~~~~~~~~~~~~~~~~~~
-
 src/struct.d.ts:3:22 - error TS1149: File name '/user/username/projects/myproject/src/Struct.d.ts' differs from already included file name '/user/username/projects/myproject/src/struct.d.ts' only in casing.
   The file is in the program because:
     Root file specified for compilation
@@ -37,10 +33,6 @@ Output::
 3 import * as xs3 from "./Struct";
                        ~~~~~~~~~~
 
-  src/struct.d.ts:3:22 - File is included via import here.
-    3 import * as xs3 from "./Struct";
-                           ~~~~~~~~~~
-
   src/struct.d.ts:4:22 - File is included via import here.
     4 import * as xs4 from "./struct";
                            ~~~~~~~~~~

From 5743d7aaa394b561073e8b916c057f6504f74f19 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Wed, 26 Nov 2025 14:47:51 -0800
Subject: [PATCH 11/15] Ensure the subtasks for file with different casing are
 started if we already startedSubTasks with earlier seen different casing

---
 internal/compiler/filesparser.go              |  18 ++-
 ...m-multiple-places-with-different-casing.js | 122 +++++++++++++++++-
 2 files changed, 133 insertions(+), 7 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index ad470ff934..f324308669 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -21,6 +21,7 @@ type parseTask struct {
 	redirectedParseTask         *parseTask
 	subTasks                    []*parseTask
 	loaded                      bool
+	startedSubTasks             bool
 	isForAutomaticTypeDirective bool
 	includeReason               *FileIncludeReason
 
@@ -155,9 +156,10 @@ type filesParser struct {
 
 type parseTaskData struct {
 	// map of tasks by file casing
-	tasks       map[string]*parseTask
-	mu          sync.Mutex
-	lowestDepth int
+	tasks           map[string]*parseTask
+	mu              sync.Mutex
+	lowestDepth     int
+	startedSubTasks bool
 }
 
 func (w *filesParser) parse(loader *fileLoader, tasks []*parseTask) {
@@ -177,22 +179,24 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int) {
 			data.mu.Lock()
 			defer data.mu.Unlock()
 
+			startSubtasks := false
 			if loaded {
 				if existingTask, ok := data.tasks[task.normalizedFilePath]; ok {
 					tasks[i].loadedTask = existingTask
 				} else {
 					data.tasks[task.normalizedFilePath] = task
+					// This is new task for file name - so load subtasks if there was loading for any other casing
+					startSubtasks = data.startedSubTasks
 				}
 			}
 
-			startSubtasks := false
-
 			currentDepth := core.IfElse(task.increaseDepth, depth+1, depth)
 			if currentDepth < data.lowestDepth {
 				// If we're seeing this task at a lower depth than before,
 				// reprocess its subtasks to ensure they are loaded.
 				data.lowestDepth = currentDepth
 				startSubtasks = true
+				data.startedSubTasks = true
 			}
 
 			if task.elideOnDepth && currentDepth > w.maxDepth {
@@ -206,9 +210,11 @@ func (w *filesParser) start(loader *fileLoader, tasks []*parseTask, depth int) {
 					if taskByFileName.redirectedParseTask != nil {
 						// Always load redirected task
 						loadSubTasks = true
+						data.startedSubTasks = true
 					}
 				}
-				if loadSubTasks {
+				if !taskByFileName.startedSubTasks && loadSubTasks {
+					taskByFileName.startedSubTasks = true
 					w.start(loader, taskByFileName.subTasks, data.lowestDepth)
 				}
 			}
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
index 3613e8cbe0..2c5f0bfa46 100644
--- a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/when-file-is-included-from-multiple-places-with-different-casing.js
@@ -24,10 +24,77 @@ import * as xs4 from "./struct";
 tsgo --explainFiles
 ExitStatus:: DiagnosticsPresent_OutputsGenerated
 Output::
+src/Struct.d.ts:2:22 - error TS1149: File name '/home/src/projects/project/node_modules/fp-ts/lib/struct.d.ts' differs from already included file name '/home/src/projects/project/node_modules/fp-ts/lib/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/oneMore.ts'
+
+2 import * as xs2 from "fp-ts/lib/struct";
+                       ~~~~~~~~~~~~~~~~~~
+
+  src/anotherFile.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/anotherFile.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/Struct.d.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/oneMore.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/oneMore.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+src/Struct.d.ts:4:22 - error TS1149: File name '/home/src/projects/project/src/struct.d.ts' differs from already included file name '/home/src/projects/project/src/Struct.d.ts' only in casing.
+  The file is in the program because:
+    Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/oneMore.ts'
+    Matched by default include pattern '**/*'
+
+4 import * as xs4 from "./struct";
+                       ~~~~~~~~~~
+
+  src/anotherFile.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/Struct.d.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/anotherFile.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/oneMore.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
 src/anotherFile.ts:2:22 - error TS1149: File name '/home/src/projects/project/node_modules/fp-ts/lib/struct.d.ts' differs from already included file name '/home/src/projects/project/node_modules/fp-ts/lib/Struct.d.ts' only in casing.
   The file is in the program because:
     Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/anotherFile.ts'
     Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/Struct.d.ts'
     Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/oneMore.ts'
     Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/oneMore.ts'
 
@@ -38,6 +105,14 @@ Output::
     1 import * as xs1 from "fp-ts/lib/Struct";
                            ~~~~~~~~~~~~~~~~~~
 
+  src/Struct.d.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/Struct.d.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
   src/oneMore.ts:1:22 - File is included via import here.
     1 import * as xs1 from "fp-ts/lib/Struct";
                            ~~~~~~~~~~~~~~~~~~
@@ -49,6 +124,8 @@ Output::
 src/anotherFile.ts:3:22 - error TS1261: Already included file name '/home/src/projects/project/src/Struct.d.ts' differs from file name '/home/src/projects/project/src/struct.d.ts' only in casing.
   The file is in the program because:
     Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/Struct.d.ts'
     Imported via "./struct" from file '/home/src/projects/project/src/anotherFile.ts'
     Imported via "./Struct" from file '/home/src/projects/project/src/oneMore.ts'
     Imported via "./struct" from file '/home/src/projects/project/src/oneMore.ts'
@@ -57,6 +134,14 @@ Output::
 3 import * as xs3 from "./Struct";
                        ~~~~~~~~~~
 
+  src/Struct.d.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/Struct.d.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
   src/anotherFile.ts:4:22 - File is included via import here.
     4 import * as xs4 from "./struct";
                            ~~~~~~~~~~
@@ -72,6 +157,8 @@ Output::
 src/anotherFile.ts:4:22 - error TS1149: File name '/home/src/projects/project/src/struct.d.ts' differs from already included file name '/home/src/projects/project/src/Struct.d.ts' only in casing.
   The file is in the program because:
     Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/Struct.d.ts'
     Imported via "./struct" from file '/home/src/projects/project/src/anotherFile.ts'
     Imported via "./Struct" from file '/home/src/projects/project/src/oneMore.ts'
     Imported via "./struct" from file '/home/src/projects/project/src/oneMore.ts'
@@ -84,6 +171,14 @@ Output::
     3 import * as xs3 from "./Struct";
                            ~~~~~~~~~~
 
+  src/Struct.d.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/Struct.d.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
   src/oneMore.ts:3:22 - File is included via import here.
     3 import * as xs3 from "./Struct";
                            ~~~~~~~~~~
@@ -96,6 +191,8 @@ Output::
   The file is in the program because:
     Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/anotherFile.ts'
     Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/Struct.d.ts'
     Imported via "fp-ts/lib/Struct" from file '/home/src/projects/project/src/oneMore.ts'
     Imported via "fp-ts/lib/struct" from file '/home/src/projects/project/src/oneMore.ts'
 
@@ -110,6 +207,14 @@ Output::
     2 import * as xs2 from "fp-ts/lib/struct";
                            ~~~~~~~~~~~~~~~~~~
 
+  src/Struct.d.ts:1:22 - File is included via import here.
+    1 import * as xs1 from "fp-ts/lib/Struct";
+                           ~~~~~~~~~~~~~~~~~~
+
+  src/Struct.d.ts:2:22 - File is included via import here.
+    2 import * as xs2 from "fp-ts/lib/struct";
+                           ~~~~~~~~~~~~~~~~~~
+
   src/oneMore.ts:1:22 - File is included via import here.
     1 import * as xs1 from "fp-ts/lib/Struct";
                            ~~~~~~~~~~~~~~~~~~
@@ -117,6 +222,8 @@ Output::
 src/oneMore.ts:4:22 - error TS1149: File name '/home/src/projects/project/src/struct.d.ts' differs from already included file name '/home/src/projects/project/src/Struct.d.ts' only in casing.
   The file is in the program because:
     Imported via "./Struct" from file '/home/src/projects/project/src/anotherFile.ts'
+    Imported via "./Struct" from file '/home/src/projects/project/src/Struct.d.ts'
+    Imported via "./struct" from file '/home/src/projects/project/src/Struct.d.ts'
     Imported via "./struct" from file '/home/src/projects/project/src/anotherFile.ts'
     Imported via "./Struct" from file '/home/src/projects/project/src/oneMore.ts'
     Imported via "./struct" from file '/home/src/projects/project/src/oneMore.ts'
@@ -129,6 +236,14 @@ Output::
     3 import * as xs3 from "./Struct";
                            ~~~~~~~~~~
 
+  src/Struct.d.ts:3:22 - File is included via import here.
+    3 import * as xs3 from "./Struct";
+                           ~~~~~~~~~~
+
+  src/Struct.d.ts:4:22 - File is included via import here.
+    4 import * as xs4 from "./struct";
+                           ~~~~~~~~~~
+
   src/anotherFile.ts:4:22 - File is included via import here.
     4 import * as xs4 from "./struct";
                            ~~~~~~~~~~
@@ -142,10 +257,14 @@ Output::
 node_modules/fp-ts/lib/Struct.d.ts
    Imported via "fp-ts/lib/Struct" from file 'src/anotherFile.ts'
    Imported via "fp-ts/lib/struct" from file 'src/anotherFile.ts'
+   Imported via "fp-ts/lib/Struct" from file 'src/Struct.d.ts'
+   Imported via "fp-ts/lib/struct" from file 'src/Struct.d.ts'
    Imported via "fp-ts/lib/Struct" from file 'src/oneMore.ts'
    Imported via "fp-ts/lib/struct" from file 'src/oneMore.ts'
 src/Struct.d.ts
    Imported via "./Struct" from file 'src/anotherFile.ts'
+   Imported via "./Struct" from file 'src/Struct.d.ts'
+   Imported via "./struct" from file 'src/Struct.d.ts'
    Imported via "./struct" from file 'src/anotherFile.ts'
    Imported via "./Struct" from file 'src/oneMore.ts'
    Imported via "./struct" from file 'src/oneMore.ts'
@@ -155,9 +274,10 @@ src/anotherFile.ts
 src/oneMore.ts
    Matched by default include pattern '**/*'
 
-Found 5 errors in 2 files.
+Found 7 errors in 3 files.
 
 Errors  Files
+     2  src/Struct.d.ts:2
      3  src/anotherFile.ts:2
      2  src/oneMore.ts:2
 

From d09ac1f75f9d916f2d1009278202f2f4328fc99a Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Wed, 26 Nov 2025 15:00:56 -0800
Subject: [PATCH 12/15] Test for GetNormalizedAbsolutePathWithoutRoot

---
 internal/tspath/path_test.go | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/internal/tspath/path_test.go b/internal/tspath/path_test.go
index 85d7829de6..711030a1cd 100644
--- a/internal/tspath/path_test.go
+++ b/internal/tspath/path_test.go
@@ -417,6 +417,14 @@ func TestGetNormalizedAbsolutePath(t *testing.T) {
 	assert.Equal(t, GetNormalizedAbsolutePath("\\\\a\\b\\\\c", ""), "//a/b/c")
 }
 
+func TestGetNormalizedAbsolutePathWithoutRoot(t *testing.T) {
+	t.Parallel()
+
+	assert.Equal(t, GetNormalizedAbsolutePathWithoutRoot("/a/b/c.txt", "/a/b"), "a/b/c.txt")
+	assert.Equal(t, GetNormalizedAbsolutePathWithoutRoot("c:/work/hello.txt", "c:/work"), "work/hello.txt")
+	assert.Equal(t, GetNormalizedAbsolutePathWithoutRoot("c:/work/hello.txt", "d:/worspaces"), "work/hello.txt")
+}
+
 var getNormalizedAbsolutePathTests = map[string][][]string{
 	"non-normalized inputs": {
 		{"/.", ""},

From 06b18b354c07ad2c4c09d5f2819b87939a7b94a0 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Wed, 26 Nov 2025 15:42:51 -0800
Subject: [PATCH 13/15] More tests

---
 internal/execute/tsctests/tsc_test.go         | 21 ++++++
 ...ist-on-disk-that-differs-only-in-casing.js | 67 +++++++++++++++++++
 .../with-triple-slash-ref-from-file.js        | 53 +++++++++++++++
 3 files changed, 141 insertions(+)
 create mode 100644 testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js
 create mode 100644 testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-triple-slash-ref-from-file.js

diff --git a/internal/execute/tsctests/tsc_test.go b/internal/execute/tsctests/tsc_test.go
index e0a2263bc6..52586c4d6d 100644
--- a/internal/execute/tsctests/tsc_test.go
+++ b/internal/execute/tsctests/tsc_test.go
@@ -1041,6 +1041,27 @@ func TestForceConsistentCasingInFileNames(t *testing.T) {
 			commandLineArgs: []string{"-p", "/user/username/projects/myproject", "--explainFiles", "--traceResolution"},
 			ignoreCase:      true,
 		},
+		{
+			subScenario: "with triple slash ref from file",
+			files: FileMap{
+				"/home/src/workspaces/project/src/c.ts":      `/// <reference path="./D.ts"/>`,
+				"/home/src/workspaces/project/src/d.ts":      `declare class c { }`,
+				"/home/src/workspaces/project/tsconfig.json": "{ }",
+			},
+			ignoreCase: true,
+		},
+		{
+			subScenario: "two files exist on disk that differs only in casing",
+			files: FileMap{
+				"/home/src/workspaces/project/c.ts": `import {x} from "./D"`,
+				"/home/src/workspaces/project/D.ts": `export const x = 10;`,
+				"/home/src/workspaces/project/d.ts": `export const y = 20;`,
+				"/home/src/workspaces/project/tsconfig.json": stringtestutil.Dedent(`
+					{
+						"files": ["c.ts", "d.ts"]
+					}`),
+			},
+		},
 	}
 	for _, test := range testCases {
 		test.run(t, "forceConsistentCasingInFileNames")
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js
new file mode 100644
index 0000000000..eb8e5043ee
--- /dev/null
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js
@@ -0,0 +1,67 @@
+currentDirectory::/home/src/workspaces/project
+useCaseSensitiveFileNames::true
+Input::
+//// [/home/src/workspaces/project/D.ts] *new* 
+export const x = 10;
+//// [/home/src/workspaces/project/c.ts] *new* 
+import {x} from "./D"
+//// [/home/src/workspaces/project/d.ts] *new* 
+export const y = 20;
+//// [/home/src/workspaces/project/tsconfig.json] *new* 
+{
+    "files": ["c.ts", "d.ts"]
+}
+
+tsgo 
+ExitStatus:: DiagnosticsPresent_OutputsGenerated
+Output::
+error TS1149: File name '/home/src/workspaces/project/d.ts' differs from already included file name '/home/src/workspaces/project/D.ts' only in casing.
+  The file is in the program because:
+    Part of 'files' list in tsconfig.json
+  tsconfig.json:2:23 - File is matched by 'files' list specified here.
+    2     "files": ["c.ts", "d.ts"]
+                            ~~~~~~
+
+
+Found 1 error.
+
+//// [/home/src/tslibs/TS/Lib/lib.d.ts] *Lib*
+/// <reference no-default-lib="true"/>
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+interface SymbolConstructor {
+    (desc?: string | number): symbol;
+    for(name: string): symbol;
+    readonly toStringTag: symbol;
+}
+declare var Symbol: SymbolConstructor;
+interface Symbol {
+    readonly [Symbol.toStringTag]: string;
+}
+declare const console: { log(msg: any): void; };
+//// [/home/src/workspaces/project/D.js] *new* 
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.x = void 0;
+exports.x = 10;
+
+//// [/home/src/workspaces/project/c.js] *new* 
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+
+//// [/home/src/workspaces/project/d.js] *new* 
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.y = void 0;
+exports.y = 20;
+
+
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-triple-slash-ref-from-file.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-triple-slash-ref-from-file.js
new file mode 100644
index 0000000000..1d16927567
--- /dev/null
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/with-triple-slash-ref-from-file.js
@@ -0,0 +1,53 @@
+currentDirectory::/home/src/workspaces/project
+useCaseSensitiveFileNames::false
+Input::
+//// [/home/src/workspaces/project/src/c.ts] *new* 
+/// <reference path="./D.ts"/>
+//// [/home/src/workspaces/project/src/d.ts] *new* 
+declare class c { }
+//// [/home/src/workspaces/project/tsconfig.json] *new* 
+{ }
+
+tsgo 
+ExitStatus:: DiagnosticsPresent_OutputsGenerated
+Output::
+src/c.ts:1:22 - error TS1261: Already included file name '/home/src/workspaces/project/src/D.ts' differs from file name '/home/src/workspaces/project/src/d.ts' only in casing.
+  The file is in the program because:
+    Referenced via './D.ts' from file '/home/src/workspaces/project/src/c.ts'
+    Matched by default include pattern '**/*'
+
+1 /// <reference path="./D.ts"/>
+                       ~~~~~~
+
+
+Found 1 error in src/c.ts:1
+
+//// [/home/src/tslibs/TS/Lib/lib.d.ts] *Lib*
+/// <reference no-default-lib="true"/>
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+interface SymbolConstructor {
+    (desc?: string | number): symbol;
+    for(name: string): symbol;
+    readonly toStringTag: symbol;
+}
+declare var Symbol: SymbolConstructor;
+interface Symbol {
+    readonly [Symbol.toStringTag]: string;
+}
+declare const console: { log(msg: any): void; };
+//// [/home/src/workspaces/project/src/D.js] *new* 
+
+//// [/home/src/workspaces/project/src/c.js] *new* 
+/// <reference path="./D.ts"/>
+
+

From 6742e55c7ce1d8bc00910945a3372f20828ca52d Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Wed, 26 Nov 2025 15:56:09 -0800
Subject: [PATCH 14/15] Fix error reporting to match with strada for case
 sensitive file system

---
 internal/compiler/filesparser.go              | 22 +++++++++----------
 ...ist-on-disk-that-differs-only-in-casing.js |  9 ++++++--
 2 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/internal/compiler/filesparser.go b/internal/compiler/filesparser.go
index f324308669..0aa19b024a 100644
--- a/internal/compiler/filesparser.go
+++ b/internal/compiler/filesparser.go
@@ -233,9 +233,9 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 	filesByPath := make(map[tspath.Path]*ast.SourceFile, totalFileCount)
 	// stores 'filename -> file association' ignoring case
 	// used to track cases when two file names differ only in casing
-	var filesByNameIgnoreCase map[string]*ast.SourceFile
+	var tasksSeenByNameIgnoreCase map[string]*parseTask
 	if loader.comparePathsOptions.UseCaseSensitiveFileNames {
-		filesByNameIgnoreCase = make(map[string]*ast.SourceFile, totalFileCount)
+		tasksSeenByNameIgnoreCase = make(map[string]*parseTask, totalFileCount)
 	}
 
 	loader.includeProcessor.fileIncludeReasons = make(map[tspath.Path][]*FileIncludeReason, totalFileCount)
@@ -284,6 +284,15 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 				seen[data] = task.normalizedFilePath
 			}
 
+			if tasksSeenByNameIgnoreCase != nil {
+				pathLowerCase := tspath.ToFileNameLowerCase(string(task.path))
+				if taskByIgnoreCase, ok := tasksSeenByNameIgnoreCase[pathLowerCase]; ok {
+					loader.includeProcessor.addProcessingDiagnosticsForFileCasing(taskByIgnoreCase.path, taskByIgnoreCase.normalizedFilePath, task.normalizedFilePath, includeReason)
+				} else {
+					tasksSeenByNameIgnoreCase[pathLowerCase] = task
+				}
+			}
+
 			for _, trace := range task.typeResolutionsTrace {
 				loader.opts.Host.Trace(trace.Message, trace.Args...)
 			}
@@ -316,15 +325,6 @@ func (w *filesParser) getProcessedFiles(loader *fileLoader) processedFiles {
 				continue
 			}
 
-			if filesByNameIgnoreCase != nil {
-				pathLowerCase := tspath.ToFileNameLowerCase(string(path))
-				if existingFile, ok := filesByNameIgnoreCase[pathLowerCase]; ok {
-					loader.includeProcessor.addProcessingDiagnosticsForFileCasing(path, existingFile.FileName(), file.FileName(), includeReason)
-				} else {
-					filesByNameIgnoreCase[pathLowerCase] = file
-				}
-			}
-
 			if task.libFile != nil {
 				libFiles = append(libFiles, file)
 				libFilesMap[path] = task.libFile
diff --git a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js
index eb8e5043ee..cf19f856c1 100644
--- a/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js
+++ b/testdata/baselines/reference/tsc/forceConsistentCasingInFileNames/two-files-exist-on-disk-that-differs-only-in-casing.js
@@ -15,15 +15,20 @@ export const y = 20;
 tsgo 
 ExitStatus:: DiagnosticsPresent_OutputsGenerated
 Output::
-error TS1149: File name '/home/src/workspaces/project/d.ts' differs from already included file name '/home/src/workspaces/project/D.ts' only in casing.
+c.ts:1:17 - error TS1261: Already included file name '/home/src/workspaces/project/D.ts' differs from file name '/home/src/workspaces/project/d.ts' only in casing.
   The file is in the program because:
+    Imported via "./D" from file '/home/src/workspaces/project/c.ts'
     Part of 'files' list in tsconfig.json
+
+1 import {x} from "./D"
+                  ~~~~~
+
   tsconfig.json:2:23 - File is matched by 'files' list specified here.
     2     "files": ["c.ts", "d.ts"]
                             ~~~~~~
 
 
-Found 1 error.
+Found 1 error in c.ts:1
 
 //// [/home/src/tslibs/TS/Lib/lib.d.ts] *Lib*
 /// <reference no-default-lib="true"/>

From 7ef9a158069ae583bc1598f58b1e6c8b6cedf9c6 Mon Sep 17 00:00:00 2001
From: Sheetal Nandi <shkamat@microsoft.com>
Date: Wed, 26 Nov 2025 15:58:52 -0800
Subject: [PATCH 15/15] Revert duplicate file check when writing buildInfo

---
 .../incremental/snapshottobuildinfo.go        |  10 +-
 internal/execute/tsctests/tsc_test.go         |  48 +++++
 ...mental-with-case-insensitive-file-names.js | 196 ++++++++++++++++++
 3 files changed, 245 insertions(+), 9 deletions(-)
 create mode 100644 testdata/baselines/reference/tsc/incremental/Compile-incremental-with-case-insensitive-file-names.js

diff --git a/internal/execute/incremental/snapshottobuildinfo.go b/internal/execute/incremental/snapshottobuildinfo.go
index fc83153362..528097d564 100644
--- a/internal/execute/incremental/snapshottobuildinfo.go
+++ b/internal/execute/incremental/snapshottobuildinfo.go
@@ -197,7 +197,7 @@ func (t *toBuildInfo) collectRootFiles() {
 }
 
 func (t *toBuildInfo) setFileInfoAndEmitSignatures() {
-	t.buildInfo.FileInfos = core.MapNonNil(t.program.GetSourceFiles(), func(file *ast.SourceFile) *BuildInfoFileInfo {
+	t.buildInfo.FileInfos = core.Map(t.program.GetSourceFiles(), func(file *ast.SourceFile) *BuildInfoFileInfo {
 		info, _ := t.snapshot.fileInfos.Load(file.Path())
 		fileId := t.toFileId(file.Path())
 		//  tryAddRoot(key, fileId);
@@ -206,11 +206,6 @@ func (t *toBuildInfo) setFileInfoAndEmitSignatures() {
 				panic(fmt.Sprintf("File name at index %d does not match expected relative path or libName: %s != %s", fileId-1, t.buildInfo.FileNames[fileId-1], t.relativeToBuildInfo(string(file.Path()))))
 			}
 		}
-		if int(fileId) != len(t.buildInfo.FileNames) {
-			// Duplicate - for now ignore
-			return nil
-		}
-
 		if t.snapshot.options.Composite.IsTrue() {
 			if !ast.IsJsonSourceFile(file) && t.program.SourceFileMayBeEmitted(file, false) {
 				if emitSignature, loaded := t.snapshot.emitSignatures.Load(file.Path()); !loaded {
@@ -235,9 +230,6 @@ func (t *toBuildInfo) setFileInfoAndEmitSignatures() {
 		}
 		return newBuildInfoFileInfo(info)
 	})
-	if t.buildInfo.FileInfos == nil {
-		t.buildInfo.FileInfos = []*BuildInfoFileInfo{}
-	}
 }
 
 func (t *toBuildInfo) setRootOfIncrementalProgram() {
diff --git a/internal/execute/tsctests/tsc_test.go b/internal/execute/tsctests/tsc_test.go
index 52586c4d6d..87fca6fdca 100644
--- a/internal/execute/tsctests/tsc_test.go
+++ b/internal/execute/tsctests/tsc_test.go
@@ -1832,6 +1832,54 @@ func TestTscIncremental(t *testing.T) {
 				},
 			},
 		},
+		{
+			subScenario:     "Compile incremental with case insensitive file names",
+			commandLineArgs: []string{"-p", "."},
+			files: FileMap{
+				"/home/project/tsconfig.json": stringtestutil.Dedent(`
+					{
+						"compilerOptions": {
+							"incremental": true
+						},
+					}`),
+				"/home/project/src/index.ts": stringtestutil.Dedent(`
+					import type { Foo1 } from 'lib1';
+					import type { Foo2 } from 'lib2';
+					export const foo1: Foo1 = { foo: "a" };
+					export const foo2: Foo2 = { foo: "b" };`),
+				"/home/node_modules/lib1/index.d.ts": stringtestutil.Dedent(`
+					import type { Foo } from 'someLib';
+					export type { Foo as Foo1 };`),
+				"/home/node_modules/lib1/package.json": stringtestutil.Dedent(`
+					{
+						"name": "lib1"
+					}`),
+				"/home/node_modules/lib2/index.d.ts": stringtestutil.Dedent(`
+					import type { Foo } from 'somelib';
+					export type { Foo as Foo2 };
+					export declare const foo2: Foo;`),
+				"/home/node_modules/lib2/package.json": stringtestutil.Dedent(`
+					{
+						"name": "lib2"
+					}
+					`),
+				"/home/node_modules/someLib/index.d.ts": stringtestutil.Dedent(`
+					import type { Str } from 'otherLib';
+					export type Foo = { foo: Str; };`),
+				"/home/node_modules/someLib/package.json": stringtestutil.Dedent(`
+					{
+						"name": "somelib"
+					}`),
+				"/home/node_modules/otherLib/index.d.ts": stringtestutil.Dedent(`
+					export type Str = string;`),
+				"/home/node_modules/otherLib/package.json": stringtestutil.Dedent(`
+					{
+						"name": "otherlib"
+					}`),
+			},
+			cwd:        "/home/project",
+			ignoreCase: true,
+		},
 	}
 
 	for _, test := range testCases {
diff --git a/testdata/baselines/reference/tsc/incremental/Compile-incremental-with-case-insensitive-file-names.js b/testdata/baselines/reference/tsc/incremental/Compile-incremental-with-case-insensitive-file-names.js
new file mode 100644
index 0000000000..416b8b8923
--- /dev/null
+++ b/testdata/baselines/reference/tsc/incremental/Compile-incremental-with-case-insensitive-file-names.js
@@ -0,0 +1,196 @@
+currentDirectory::/home/project
+useCaseSensitiveFileNames::false
+Input::
+//// [/home/node_modules/lib1/index.d.ts] *new* 
+import type { Foo } from 'someLib';
+export type { Foo as Foo1 };
+//// [/home/node_modules/lib1/package.json] *new* 
+{
+    "name": "lib1"
+}
+//// [/home/node_modules/lib2/index.d.ts] *new* 
+import type { Foo } from 'somelib';
+export type { Foo as Foo2 };
+export declare const foo2: Foo;
+//// [/home/node_modules/lib2/package.json] *new* 
+{
+    "name": "lib2"
+}
+//// [/home/node_modules/otherLib/index.d.ts] *new* 
+export type Str = string;
+//// [/home/node_modules/otherLib/package.json] *new* 
+{
+    "name": "otherlib"
+}
+//// [/home/node_modules/someLib/index.d.ts] *new* 
+import type { Str } from 'otherLib';
+export type Foo = { foo: Str; };
+//// [/home/node_modules/someLib/package.json] *new* 
+{
+    "name": "somelib"
+}
+//// [/home/project/src/index.ts] *new* 
+import type { Foo1 } from 'lib1';
+import type { Foo2 } from 'lib2';
+export const foo1: Foo1 = { foo: "a" };
+export const foo2: Foo2 = { foo: "b" };
+//// [/home/project/tsconfig.json] *new* 
+{
+    "compilerOptions": {
+        "incremental": true
+    },
+}
+
+tsgo -p .
+ExitStatus:: DiagnosticsPresent_OutputsGenerated
+Output::
+../node_modules/lib2/index.d.ts:1:26 - error TS1149: File name '/home/node_modules/somelib/index.d.ts' differs from already included file name '/home/node_modules/someLib/index.d.ts' only in casing.
+  The file is in the program because:
+    Imported via 'someLib' from file '/home/node_modules/lib1/index.d.ts'
+    Imported via 'somelib' from file '/home/node_modules/lib2/index.d.ts'
+
+1 import type { Foo } from 'somelib';
+                           ~~~~~~~~~
+
+  ../node_modules/lib1/index.d.ts:1:26 - File is included via import here.
+    1 import type { Foo } from 'someLib';
+                               ~~~~~~~~~
+
+
+Found 1 error in ../node_modules/lib2/index.d.ts:1
+
+//// [/home/project/src/index.js] *new* 
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.foo2 = exports.foo1 = void 0;
+exports.foo1 = { foo: "a" };
+exports.foo2 = { foo: "b" };
+
+//// [/home/project/tsconfig.tsbuildinfo] *new* 
+{"version":"FakeTSVersion","errors":true,"root":[6],"fileNames":["lib.d.ts","../node_modules/otherlib/index.d.ts","../node_modules/somelib/index.d.ts","../node_modules/lib1/index.d.ts","../node_modules/lib2/index.d.ts","./src/index.ts"],"fileInfos":[{"version":"8859c12c614ce56ba9a18e58384a198f-/// <reference no-default-lib=\"true\"/>\ninterface Boolean {}\ninterface Function {}\ninterface CallableFunction {}\ninterface NewableFunction {}\ninterface IArguments {}\ninterface Number { toExponential: any; }\ninterface Object {}\ninterface RegExp {}\ninterface String { charAt: any; }\ninterface Array<T> { length: number; [n: number]: T; }\ninterface ReadonlyArray<T> {}\ninterface SymbolConstructor {\n    (desc?: string | number): symbol;\n    for(name: string): symbol;\n    readonly toStringTag: symbol;\n}\ndeclare var Symbol: SymbolConstructor;\ninterface Symbol {\n    readonly [Symbol.toStringTag]: string;\n}\ndeclare const console: { log(msg: any): void; };","affectsGlobalScope":true,"impliedNodeFormat":1},"1fe659ed0634bb57b6dc25e9062f1162-export type Str = string;","12e112ff6e2744bb42d8e0b511e44117-import type { Str } from 'otherLib';\nexport type Foo = { foo: Str; };","b6305455d920a6729c435e6acf45eff6-import type { Foo } from 'someLib';\nexport type { Foo as Foo1 };","a5393e550a9c20a242a120bf6410db48-import type { Foo } from 'somelib';\nexport type { Foo as Foo2 };\nexport declare const foo2: Foo;","42aef197ff5f079223e2c29fb2e77cc5-import type { Foo1 } from 'lib1';\nimport type { Foo2 } from 'lib2';\nexport const foo1: Foo1 = { foo: \"a\" };\nexport const foo2: Foo2 = { foo: \"b\" };"],"fileIdsList":[[3],[2],[4,5]],"referencedMap":[[4,1],[5,1],[3,2],[6,3]]}
+//// [/home/project/tsconfig.tsbuildinfo.readable.baseline.txt] *new* 
+{
+  "version": "FakeTSVersion",
+  "errors": true,
+  "root": [
+    {
+      "files": [
+        "./src/index.ts"
+      ],
+      "original": 6
+    }
+  ],
+  "fileNames": [
+    "lib.d.ts",
+    "../node_modules/otherlib/index.d.ts",
+    "../node_modules/somelib/index.d.ts",
+    "../node_modules/lib1/index.d.ts",
+    "../node_modules/lib2/index.d.ts",
+    "./src/index.ts"
+  ],
+  "fileInfos": [
+    {
+      "fileName": "lib.d.ts",
+      "version": "8859c12c614ce56ba9a18e58384a198f-/// <reference no-default-lib=\"true\"/>\ninterface Boolean {}\ninterface Function {}\ninterface CallableFunction {}\ninterface NewableFunction {}\ninterface IArguments {}\ninterface Number { toExponential: any; }\ninterface Object {}\ninterface RegExp {}\ninterface String { charAt: any; }\ninterface Array<T> { length: number; [n: number]: T; }\ninterface ReadonlyArray<T> {}\ninterface SymbolConstructor {\n    (desc?: string | number): symbol;\n    for(name: string): symbol;\n    readonly toStringTag: symbol;\n}\ndeclare var Symbol: SymbolConstructor;\ninterface Symbol {\n    readonly [Symbol.toStringTag]: string;\n}\ndeclare const console: { log(msg: any): void; };",
+      "signature": "8859c12c614ce56ba9a18e58384a198f-/// <reference no-default-lib=\"true\"/>\ninterface Boolean {}\ninterface Function {}\ninterface CallableFunction {}\ninterface NewableFunction {}\ninterface IArguments {}\ninterface Number { toExponential: any; }\ninterface Object {}\ninterface RegExp {}\ninterface String { charAt: any; }\ninterface Array<T> { length: number; [n: number]: T; }\ninterface ReadonlyArray<T> {}\ninterface SymbolConstructor {\n    (desc?: string | number): symbol;\n    for(name: string): symbol;\n    readonly toStringTag: symbol;\n}\ndeclare var Symbol: SymbolConstructor;\ninterface Symbol {\n    readonly [Symbol.toStringTag]: string;\n}\ndeclare const console: { log(msg: any): void; };",
+      "affectsGlobalScope": true,
+      "impliedNodeFormat": "CommonJS",
+      "original": {
+        "version": "8859c12c614ce56ba9a18e58384a198f-/// <reference no-default-lib=\"true\"/>\ninterface Boolean {}\ninterface Function {}\ninterface CallableFunction {}\ninterface NewableFunction {}\ninterface IArguments {}\ninterface Number { toExponential: any; }\ninterface Object {}\ninterface RegExp {}\ninterface String { charAt: any; }\ninterface Array<T> { length: number; [n: number]: T; }\ninterface ReadonlyArray<T> {}\ninterface SymbolConstructor {\n    (desc?: string | number): symbol;\n    for(name: string): symbol;\n    readonly toStringTag: symbol;\n}\ndeclare var Symbol: SymbolConstructor;\ninterface Symbol {\n    readonly [Symbol.toStringTag]: string;\n}\ndeclare const console: { log(msg: any): void; };",
+        "affectsGlobalScope": true,
+        "impliedNodeFormat": 1
+      }
+    },
+    {
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+      "../node_modules/lib1/index.d.ts",
+      "../node_modules/lib2/index.d.ts"
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+    ],
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+      "../node_modules/otherlib/index.d.ts"
+    ],
+    "./src/index.ts": [
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+    ]
+  },
+  "size": 1685
+}
+//// [/home/src/tslibs/TS/Lib/lib.d.ts] *Lib*
+/// <reference no-default-lib="true"/>
+interface Boolean {}
+interface Function {}
+interface CallableFunction {}
+interface NewableFunction {}
+interface IArguments {}
+interface Number { toExponential: any; }
+interface Object {}
+interface RegExp {}
+interface String { charAt: any; }
+interface Array<T> { length: number; [n: number]: T; }
+interface ReadonlyArray<T> {}
+interface SymbolConstructor {
+    (desc?: string | number): symbol;
+    for(name: string): symbol;
+    readonly toStringTag: symbol;
+}
+declare var Symbol: SymbolConstructor;
+interface Symbol {
+    readonly [Symbol.toStringTag]: string;
+}
+declare const console: { log(msg: any): void; };
+
+tsconfig.json::
+SemanticDiagnostics::
+*refresh*    /home/src/tslibs/TS/Lib/lib.d.ts
+*refresh*    /home/node_modules/otherLib/index.d.ts
+*refresh*    /home/node_modules/someLib/index.d.ts
+*refresh*    /home/node_modules/lib1/index.d.ts
+*refresh*    /home/node_modules/lib2/index.d.ts
+*refresh*    /home/project/src/index.ts
+Signatures::