diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index dbee278386036..8cda6b72da572 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -875,6 +875,8 @@ WARNING(sema_import_current_module_with_file,none,
         (StringRef, Identifier))
 ERROR(sema_opening_import,Fatal,
       "opening import file for module %0: %1", (Identifier, StringRef))
+NOTE(sema_module_aliased,none,
+     "module name '%0' is aliased to '%1'", (StringRef, StringRef))
 
 REMARK(serialization_skipped_invalid_decl,none,
        "serialization skipped invalid %kind0",
diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index 60cc7c87fbd32..b7d8c4671c1ef 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -2415,10 +2415,6 @@ ModuleDecl *ClangImporter::Implementation::loadModule(
   ModuleDecl *MD = nullptr;
   ASTContext &ctx = getNameImporter().getContext();
 
-  // `CxxStdlib` is the only accepted spelling of the C++ stdlib module name.
-  if (path.front().Item.is("std") ||
-      path.front().Item.str().starts_with("std_"))
-    return nullptr;
   if (path.front().Item == ctx.Id_CxxStdlib) {
     ImportPath::Builder adjustedPath(ctx.getIdentifier("std"), importLoc);
     adjustedPath.append(path.getSubmodulePath());
@@ -8891,7 +8887,7 @@ bool importer::isCxxStdModule(StringRef moduleName, bool IsSystem) {
 ImportPath::Builder ClangImporter::Implementation::getSwiftModulePath(const clang::Module *M) {
   ImportPath::Builder builder;
   while (M) {
-    if (!M->isSubModule() && isCxxStdModule(M))
+    if (!M->isSubModule() && M->Name == "std")
       builder.push_back(SwiftContext.Id_CxxStdlib);
     else
       builder.push_back(SwiftContext.getIdentifier(M->Name));
diff --git a/lib/Sema/ImportResolution.cpp b/lib/Sema/ImportResolution.cpp
index 3c4f4325f4674..f8de48dc4ef18 100644
--- a/lib/Sema/ImportResolution.cpp
+++ b/lib/Sema/ImportResolution.cpp
@@ -359,6 +359,26 @@ static bool isSubmodule(const ModuleDecl* M) {
 void ImportResolver::visitImportDecl(ImportDecl *ID) {
   assert(unboundImports.empty());
 
+  // `CxxStdlib` is the only accepted spelling of the C++ stdlib module name.
+  ImportPath::Builder builder;
+  const ImportPath path = ID->getRealImportPath(builder);
+  const llvm::StringRef front = path.front().Item.str();
+  if (front == "std" || front.starts_with("std_")) {
+    SmallString<64> modulePathStr;
+    path.getString(modulePathStr);
+    auto diagKind = ctx.LangOpts.DebuggerSupport ? diag::sema_no_import_repl
+                                                 : diag::sema_no_import;
+    const SourceLoc importLoc = ID->getLoc();
+    const ImportPath sourcePath = ID->getImportPath();
+    const llvm::StringRef sourceFront = sourcePath.front().Item.str();
+    ctx.Diags.diagnose(importLoc, diagKind, modulePathStr);
+    ctx.Diags.diagnose(importLoc, diag::did_you_mean_cxxstdlib)
+        .fixItReplaceChars(importLoc, importLoc.getAdvancedLoc(sourceFront.size()), "CxxStdlib");
+    if (front != sourceFront)
+      ctx.Diags.diagnose(importLoc, diag::sema_module_aliased, sourceFront, front);
+    return;
+  }
+
   unboundImports.emplace_back(ID);
   bindPendingImports();
 }
@@ -513,13 +533,15 @@ UnboundImport::getTopLevelModule(ModuleDecl *M, SourceFile &SF) {
 // MARK: Implicit imports
 //===----------------------------------------------------------------------===//
 
-static void tryStdlibFixit(ASTContext &ctx,
-                           StringRef moduleName,
-                           SourceLoc loc) {
-  if (moduleName.starts_with("std")) {
-    ctx.Diags.diagnose(loc, diag::did_you_mean_cxxstdlib)
-      .fixItReplaceChars(loc, loc.getAdvancedLoc(3), "CxxStdlib");
+ImportPath::Module getRealModulePath(ImportPath::Builder &builder, ImportPath::Module path, ASTContext &ctx) {
+  for (size_t i = 0; i < path.size(); i++) {
+    if (i == 0) {
+      builder.push_back(ctx.getRealModuleName(path[i].Item));
+    } else {
+      builder.push_back(path[i]);
+    }
   }
+  return builder.get().getModulePath(false);
 }
 
 static void diagnoseNoSuchModule(ModuleDecl *importingModule,
@@ -534,13 +556,20 @@ static void diagnoseNoSuchModule(ModuleDecl *importingModule,
                        importingModule->getName());
   } else {
     SmallString<64> modulePathStr;
-    modulePath.getString(modulePathStr);
+    ImportPath::Builder builder;
+    ImportPath::Module realModulePath = getRealModulePath(builder, modulePath, ctx);
+    realModulePath.getString(modulePathStr);
 
     auto diagKind = diag::sema_no_import;
     if (nonfatalInREPL && ctx.LangOpts.DebuggerSupport)
       diagKind = diag::sema_no_import_repl;
     ctx.Diags.diagnose(importLoc, diagKind, modulePathStr);
-    tryStdlibFixit(ctx, modulePathStr, importLoc);
+
+    const llvm::StringRef sourceFront = modulePath.front().Item.str();
+    const llvm::StringRef realFront = realModulePath.front().Item.str();
+    if (realFront != sourceFront)
+      ctx.Diags.diagnose(importLoc, diag::sema_module_aliased, sourceFront,
+                         realFront);
   }
 
   if (ctx.SearchPathOpts.getSDKPath().empty() &&
diff --git a/test/ImportResolution/import-alias-fail.swift b/test/ImportResolution/import-alias-fail.swift
new file mode 100644
index 0000000000000..8a033b0292c44
--- /dev/null
+++ b/test/ImportResolution/import-alias-fail.swift
@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift -module-alias Foo=Bar
+
+import Foo // expected-error{{no such module 'Bar'}}
+// expected-note@-1{{module name 'Foo' is aliased to 'Bar'}}
+
diff --git a/test/ImportResolution/import-std-fail.swift b/test/ImportResolution/import-std-fail.swift
new file mode 100644
index 0000000000000..96b5d44a556bc
--- /dev/null
+++ b/test/ImportResolution/import-std-fail.swift
@@ -0,0 +1,42 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// These errors are fatal, so test each one separately
+
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/a.swift
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/b.swift
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/c.swift
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/d.swift
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/e.swift -verify-additional-prefix aliased- -module-alias FooBar=std_foo_bar
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/e.swift -verify-additional-prefix unaliased-
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/f.swift -verify-additional-prefix aliased- -module-alias X=std_x_h
+// RUN: %target-swift-frontend -typecheck -verify -cxx-interoperability-mode=default %t/f.swift -verify-additional-prefix unaliased-
+
+//--- a.swift
+import std // expected-error{{no such module 'std'}}
+           // expected-note@-1{{did you mean 'CxxStdlib'?}}{{8-11=CxxStdlib}} {{none}}
+
+//--- b.swift
+import std_ // expected-error{{no such module 'std_'}}
+            // expected-note@-1{{did you mean 'CxxStdlib'?}}{{8-12=CxxStdlib}} {{none}}
+
+//--- c.swift
+import std_error_h // expected-error{{no such module 'std_error_h'}}
+                   // expected-note@-1{{did you mean 'CxxStdlib'?}}{{8-19=CxxStdlib}} {{none}}
+
+//--- d.swift
+import std_core.math.abs // expected-error{{no such module 'std_core.math.abs'}}
+                         // expected-note@-1{{did you mean 'CxxStdlib'?}}{{8-16=CxxStdlib}} {{none}}
+
+//--- e.swift
+import FooBar // expected-aliased-error{{no such module 'std_foo_bar'}}
+              // expected-aliased-note@-1{{did you mean 'CxxStdlib'?}}{{8-14=CxxStdlib}} {{none}}
+              // expected-aliased-note@-2{{module name 'FooBar' is aliased to 'std_foo_bar'}} {{none}}
+              // expected-unaliased-error@-3{{no such module 'FooBar'}}
+
+//--- f.swift
+import X
+// expected-aliased-error@-1{{no such module 'std_x_h'}}
+// expected-aliased-note@-2{{did you mean 'CxxStdlib'?}}{{8-9=CxxStdlib}} {{none}}
+// expected-aliased-note@-3{{module name 'X' is aliased to 'std_x_h'}} {{none}}
+// expected-unaliased-error@-4{{no such module 'X'}}
diff --git a/test/Interop/Cxx/swiftify-import/transitive-std-core-import.swift b/test/Interop/Cxx/swiftify-import/transitive-std-core-import.swift
new file mode 100644
index 0000000000000..c304506fd07db
--- /dev/null
+++ b/test/Interop/Cxx/swiftify-import/transitive-std-core-import.swift
@@ -0,0 +1,38 @@
+// REQUIRES: swift_feature_SafeInteropWrappers
+// REQUIRES: OS=macosx
+
+// Don't run this test with libc++ versions 17-19, when the top-level std module was split into multiple top-level modules.
+// RUN: %empty-directory(%t)
+// RUN: %target-clangxx %S/../stdlib/Inputs/check-libcxx-version.cpp -o %t/check-libcxx-version
+// RUN: %target-codesign %t/check-libcxx-version
+
+// RUN: %target-run %t/check-libcxx-version || split-file %s %t
+// RUN: %target-run %t/check-libcxx-version || %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/Test.swiftmodule -I %t/Inputs -enable-experimental-feature SafeInteropWrappers -strict-memory-safety -warnings-as-errors -Xcc -Werror %t/test.swift  -cxx-interoperability-mode=default -Xcc -std=c++20
+
+// Test that implicit imports can refer to std_core etc., even though Swift source code may not.
+
+//--- Inputs/module.modulemap
+module TestClang {
+    header "test.h"
+    requires cplusplus
+    export std_core.cstddef.size_t
+    export span
+}
+
+//--- Inputs/test.h
+
+#include <__cstddef/size_t.h>
+#include <span>
+#include <lifetimebound.h>
+
+using FloatSpan = std::span<const float>;
+
+size_t foo(FloatSpan x __noescape);
+
+//--- test.swift
+import TestClang
+  
+func test(x: Span<Float>) {
+  let _ = foo(x)
+}
+