CMake on Windows in Swift 6.2 introduced a new warning: warning LNK4098: defaultlib 'msvcrtd.lib' conflicts with use of other libs; use /NODEFAULTLIB:library

[tothambrus11]

Description

We upgraded from Swift 6.1 to Swift 6.2, and we noticed a new warning in our Windows CMake builds, only in debug mode.

[211/242] Linking Swift executable Tests\HyloTests.exe
   Creating library Tests\HyloTests.lib and object Tests\HyloTests.exp
LINK : warning LNK4098: defaultlib 'msvcrtd.lib' conflicts with use of other libs; use /NODEFAULTLIB:library

Reproduction

• Doesn't appear using Swift 6.1 (neither on debug/release builds) https://github.com/hylo-lang/Swifty-LLVM/actions/runs/17927198031/job/50976142251
• Appears using Swift 6.2 only using debug build https://github.com/hylo-lang/Swifty-LLVM/actions/runs/17924909643/job/50968479482

The commit in which it's reproduced: hylo-lang/Swifty-LLVM@e8aac57
Feel free to fork the repo and run the CI with a pull request.

Expected behavior

No warning (mitigation of the linking issue)

Environment

Swift version 6.2 (swift-6.2-RELEASE)
Target: x86_64-unknown-windows-msvc

Relevant issues / background

This is the only remotely relevant issue I found: #75174

Official description of warning: https://learn.microsoft.com/en-us/cpp/error-messages/tool-errors/linker-tools-warning-lnk4098?view=msvc-170

CMake post suggesting there may be some mix of release/debug built stuff https://discourse.cmake.org/t/lnk4098-use-nodefaultlib-library/5427/2

Some stackoverflow post regarding the warning, non-Swift context: https://stackoverflow.com/questions/3007312/resolving-lnk4098-defaultlib-msvcrt-conflicts-with

Additional information

Note: the link.exe invokation is exactly the same between release and debug build and 6.1/6.2 for the places I looked at with warnings, so the problem must come from earlier in the build.

Details using debug build using Swift 6.2 + CMake:

2025-09-22T18:38:06.0635714Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -c D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\BasicFormat.swift -primary-file D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\InferIndentation.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Syntax+Extensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\SyntaxProtocol+Formatted.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Trivia+FormatExtensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-build\Sources\SwiftBasicFormat\forced-SwiftSyntax-dep.swift -emit-reference-dependencies-path _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\InferIndentation.swift.obj.swiftdeps -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL -parse-as-library -num-threads 4 -o _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\InferIndentation.swift.obj
2025-09-22T18:38:06.0665008Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -c -primary-file D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\BasicFormat.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\InferIndentation.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Syntax+Extensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\SyntaxProtocol+Formatted.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Trivia+FormatExtensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-build\Sources\SwiftBasicFormat\forced-SwiftSyntax-dep.swift -emit-reference-dependencies-path _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\BasicFormat.swift.obj.swiftdeps -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL -parse-as-library -num-threads 4 -o _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\BasicFormat.swift.obj
2025-09-22T18:38:06.0696701Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -emit-module -experimental-skip-non-inlinable-function-bodies-without-types D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\BasicFormat.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\InferIndentation.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Syntax+Extensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\SyntaxProtocol+Formatted.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Trivia+FormatExtensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-build\Sources\SwiftBasicFormat\forced-SwiftSyntax-dep.swift -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -emit-module-doc-path D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.swiftdoc -emit-module-source-info-path D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.swiftsourceinfo -emit-module-interface-path D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.swiftinterface -emit-private-module-interface-path D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.private.swiftinterface -parse-as-library -o D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.swiftmodule -emit-abi-descriptor-path D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.abi.json
2025-09-22T18:38:06.0726546Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -c D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\BasicFormat.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\InferIndentation.swift -primary-file D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Syntax+Extensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\SyntaxProtocol+Formatted.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Trivia+FormatExtensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-build\Sources\SwiftBasicFormat\forced-SwiftSyntax-dep.swift -emit-reference-dependencies-path _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\Syntax+Extensions.swift.obj.swiftdeps -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL -parse-as-library -num-threads 4 -o _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\Syntax+Extensions.swift.obj
2025-09-22T18:38:06.0752951Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -c D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\BasicFormat.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\InferIndentation.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Syntax+Extensions.swift -primary-file D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\SyntaxProtocol+Formatted.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Trivia+FormatExtensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-build\Sources\SwiftBasicFormat\forced-SwiftSyntax-dep.swift -emit-reference-dependencies-path _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\SyntaxProtocol+Formatted.swift.obj.swiftdeps -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL -parse-as-library -num-threads 4 -o _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\SyntaxProtocol+Formatted.swift.obj
2025-09-22T18:38:06.0779722Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -c D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\BasicFormat.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\InferIndentation.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Syntax+Extensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\SyntaxProtocol+Formatted.swift -primary-file D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Trivia+FormatExtensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-build\Sources\SwiftBasicFormat\forced-SwiftSyntax-dep.swift -emit-reference-dependencies-path _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\Trivia+FormatExtensions.swift.obj.swiftdeps -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL -parse-as-library -num-threads 4 -o _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\Trivia+FormatExtensions.swift.obj
2025-09-22T18:38:06.0808351Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -c D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\BasicFormat.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\InferIndentation.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Syntax+Extensions.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\SyntaxProtocol+Formatted.swift D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-src\Sources\SwiftBasicFormat\Trivia+FormatExtensions.swift -primary-file D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\_deps\swiftsyntax-build\Sources\SwiftBasicFormat\forced-SwiftSyntax-dep.swift -emit-reference-dependencies-path _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\forced-SwiftSyntax-dep.swift.obj.swiftdeps -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL -parse-as-library -num-threads 4 -o _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\forced-SwiftSyntax-dep.swift.obj
2025-09-22T18:38:06.0829626Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -typecheck-module-from-interface D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.swiftinterface -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL
2025-09-22T18:38:06.0839841Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -modulewrap D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.swiftmodule -target x86_64-unknown-windows-msvc -o D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.o
2025-09-22T18:38:06.0850082Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\swift-frontend.exe -frontend -typecheck-module-from-interface D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host\SwiftBasicFormat.swiftmodule\x86_64-unknown-windows-msvc.private.swiftinterface -target x86_64-unknown-windows-msvc -disable-objc-interop -sdk C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -I D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -color-diagnostics -Xcc -fcolor-diagnostics -enable-library-evolution -g -debug-info-format=dwarf -dwarf-version=4 -module-link-name SwiftBasicFormat -swift-version 5 -Onone -D SWIFT_SYNTAX_BUILD_USING_CMAKE -D SwiftBasicFormat_EXPORTS -disable-implicit-string-processing-module-import -enable-anonymous-context-mangled-names -file-compilation-dir D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build -no-auto-bridging-header-chaining -module-name SwiftBasicFormat -in-process-plugin-server-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\SwiftInProcPluginServer.dll -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin -plugin-path C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\local\bin -autolink-library oldnames -autolink-library msvcrtd -Xcc -D_MT -Xcc -D_DLL
2025-09-22T18:38:06.4622439Z [14/73] Generating module directory for SwiftBasicFormat; Linking Swift shared library bin\SwiftBasicFormat.dll; Update mtime of library outputs workaround
2025-09-22T18:38:06.4625897Z Swift version 6.2 (swift-6.2-RELEASE)
2025-09-22T18:38:06.4626952Z Target: x86_64-unknown-windows-msvc
2025-09-22T18:38:06.4641170Z C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin\clang.exe --rsp-quoting=windows -target x86_64-unknown-windows-msvc -shared -nostartfiles -L C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\lib\swift\windows -L C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\lib\swift\windows\x86_64 -L C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\usr\lib\swift\windows -L C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\usr\lib\swift\windows\x86_64 C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\usr\lib\swift\windows\x86_64\swiftrt.obj _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\BasicFormat.swift.obj _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\InferIndentation.swift.obj _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\Syntax+Extensions.swift.obj _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\SyntaxProtocol+Formatted.swift.obj _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\Trivia+FormatExtensions.swift.obj _deps\swiftsyntax-build\Sources\SwiftBasicFormat\CMakeFiles\SwiftBasicFormat.dir\forced-SwiftSyntax-dep.swift.obj lib\swift\host\SwiftSyntax.lib -L D:\a\Swifty-LLVM\Swifty-LLVM\Swifty-LLVM\.cmake-build\lib\swift\host -I C:\Users\runneradmin\AppData\Local\Programs\Swift\Platforms\6.2.0\Windows.platform\Developer\SDKs\Windows.sdk\ -O0 -Xlinker -implib:lib\swift\host\SwiftBasicFormat.lib -v -o bin\SwiftBasicFormat.dll
2025-09-22T18:38:06.4656081Z clang version 19.1.5
2025-09-22T18:38:06.4656975Z Target: x86_64-unknown-windows-msvc
2025-09-22T18:38:06.4657869Z Thread model: posix
2025-09-22T18:38:06.4659076Z InstalledDir: C:\Users\runneradmin\AppData\Local\Programs\Swift\Toolchains\6.2.0+Asserts\usr\bin
2025-09-22T18:38:06.4660442Z Build config: +assertions
2025-09-22T18:38:06.4677604Z  "C:\\Program Files\\Microsoft Visual Studio\\2022\\Enterprise\\VC\\Tools\\MSVC\\14.44.35207\\bin\\Hostx64\\x64\\link.exe" "-out:bin\\SwiftBasicFormat.dll" "-libpath:C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Toolchains\\6.2.0+Asserts\\usr\\lib\\swift\\windows" "-libpath:C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Toolchains\\6.2.0+Asserts\\usr\\lib\\swift\\windows\\x86_64" "-libpath:C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Platforms\\6.2.0\\Windows.platform\\Developer\\SDKs\\Windows.sdk\\usr\\lib\\swift\\windows" "-libpath:C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Platforms\\6.2.0\\Windows.platform\\Developer\\SDKs\\Windows.sdk\\usr\\lib\\swift\\windows\\x86_64" "-libpath:D:\\a\\Swifty-LLVM\\Swifty-LLVM\\Swifty-LLVM\\.cmake-build\\lib\\swift\\host" "-libpath:C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Toolchains\\6.2.0+Asserts\\usr\\lib\\clang\\19\\lib\\x86_64-unknown-windows-msvc" -nologo -dll "-implib:bin\\SwiftBasicFormat.lib" "C:\\Users\\runneradmin\\AppData\\Local\\Programs\\Swift\\Platforms\\6.2.0\\Windows.platform\\Developer\\SDKs\\Windows.sdk\\usr\\lib\\swift\\windows\\x86_64\\swiftrt.obj" "_deps\\swiftsyntax-build\\Sources\\SwiftBasicFormat\\CMakeFiles\\SwiftBasicFormat.dir\\BasicFormat.swift.obj" "_deps\\swiftsyntax-build\\Sources\\SwiftBasicFormat\\CMakeFiles\\SwiftBasicFormat.dir\\InferIndentation.swift.obj" "_deps\\swiftsyntax-build\\Sources\\SwiftBasicFormat\\CMakeFiles\\SwiftBasicFormat.dir\\Syntax+Extensions.swift.obj" "_deps\\swiftsyntax-build\\Sources\\SwiftBasicFormat\\CMakeFiles\\SwiftBasicFormat.dir\\SyntaxProtocol+Formatted.swift.obj" "_deps\\swiftsyntax-build\\Sources\\SwiftBasicFormat\\CMakeFiles\\SwiftBasicFormat.dir\\Trivia+FormatExtensions.swift.obj" "_deps\\swiftsyntax-build\\Sources\\SwiftBasicFormat\\CMakeFiles\\SwiftBasicFormat.dir\\forced-SwiftSyntax-dep.swift.obj" "lib\\swift\\host\\SwiftSyntax.lib" "-implib:lib\\swift\\host\\SwiftBasicFormat.lib"
2025-09-22T18:38:06.4692745Z    Creating library lib\swift\host\SwiftBasicFormat.lib and object lib\swift\host\SwiftBasicFormat.exp
2025-09-22T18:38:06.4694759Z LINK : warning LNK4098: defaultlib 'msvcrtd.lib' conflicts with use of other libs; use /NODEFAULTLIB:library
2025-09-22T18:38:06.5969646Z [15/73] Building Swift Module 'SwiftDiagnostics' with 11 sources
2025-09-22T18:38:06.5970919Z Swift version 6.2 (swift-6.2-RELEASE)
2025-09-22T18:38:06.5971598Z Target: x86_64-unknown-windows-msvc

[compnerd]

Please provide a standalone repro for this. The default library is based upon the value being passed to the compiler (i.e. -libc {MD,MDd,MT,MTd}), in release mode, it should be msvcrt and in debug mode it should be msvcrtd. This obviously would collide if you mix and match release and debug builds (in general we do not really support debug mode builds on Windows as there is no ABI stability and debug and release mode builds are incompatible ABI wise).

[tothambrus11]

Thank you very much for the explanation! The problem was on our side. It turns out, Swifty-LLVM was setting compiler flags to always use a specific version of the runtime library because our LLVM builds always get built that way. Swift 6.2 actually helped detecting this issue, we are very thankful of this new warning.

set(CMAKE_Swift_COMPILE_OPTIONS_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)
set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreadedDLL)

For future reference: https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html

The issue was that those flags were set only in Sources/CMakeLists.txt and didn't apply to the tests and other dependencies. Setting the flags on the top level appears to fix the issue and the warnings go away.

This requirement needs to be better documented on our side, since all projects depending on Swifty-LLVM will also need to set those flags on their top-level. Alternatively, we could change our LLVM builds to use msvcrtd in debug builds of LLVM. But that is less ideal, we don't want to couple LLVM's debuggability with our Swift code's debuggability - most of the time we can use MinSizeRel+assertions builds of LLVM and we only need to debug the Swift code.

Thanks again for the help!