[C++20][Modules] Implement include translation.

[iains]

Summary

This addresses [cpp.include]/7

(when encountering #include header-name)

If the header identified by the header-name denotes an importable header, it
is implementation-defined whether the #include preprocessing directive is
instead replaced by an import directive.

In this implementation, include translation is performed _only_ for headers
in the Global Module fragment, so:

module;
 #include "will-be-translated.h" // IFF the header unit is available.

export module M;
 #include "will-not-be-translated.h" // even if the header unit is available

The reasoning is that, in general, includes in the module purview would not
be validly translatable (they would have to immediately follow the module
decl and without any other intervening decls). Otherwise that would violate
the rules on contiguous import directives.

This would be quite complex to track in the preprocessor, and for relatively
little gain (the user can 'import "will-not-be-translated.h";' instead.)

TODO: This is one area where it becomes increasingly difficult to disambiguate
clang modules in C++ from C++ standard modules. That needs to be addressed in
both the driver and the FE.

[iains]

fix a state transition after include translation.

This update fixes an (apparently pre-existing) bug in handling:

module;
 #include "translated-header.h"
 import "header-unit.h";

The translation process pushes a tok::annot_module_include which was being
handled as "misc" in the state machine, leading to the wrong starting state
for processing the 'import' following. The change here treats this token
as if it were the ';' that is notionally injected as part of the translation.

Updated the test case to test cases where header units can be imported.

[ChuanqiXu]

clang/include/clang/Lex/Preprocessor.h

434

What's the meaning of ATLTS?

clang/lib/Lex/PPDirectives.cpp

2226–2227

From what @rsmith said in https://reviews.llvm.org/D113391, it looks like the ideal direction is to use C++ clang modules and C++ standard modules together. So it looks like we couldn't disambiguate them from command line options.

clang/test/Modules/cxx20-include-translation.cpp

11

Maybe we need an example h5.h which is not pre-compiled as a header unit and it would be handled correctly.

[iains]

clang/include/clang/Lex/Preprocessor.h

434

AfterTopLevelTokenSeq I guess it seemed a rather long name (but I do not mind to spell it out if that makes sense).

clang/lib/Lex/PPDirectives.cpp

2226–2227

Well, I think there is some more debate to have around how to solve this problem (i.e. it might be intended that clang++ modules and standard c++ modules converge but as things stand we still need to support the cases that they have different behaviour, or break existing users)
... - but please let us not have that debate in this patch :-)

clang/test/Modules/cxx20-include-translation.cpp

11

I am not sure if you mean that the un-converted header would be included in the GMF or in the module purview - IMO we already have test-cases that cover this, but I do not mind adding something extra if there's a missing case?

[ChuanqiXu]

clang/include/clang/Lex/Preprocessor.h

434

Oh, I feel better to spell it out.

clang/lib/Lex/PPDirectives.cpp

2226–2227

It is not good to break existing users. Generally, a breaking change patch could be reverted directly... We must care about it to avoid unnecessary efforts. And it looks like the current implementation wouldn't break any existing users, right? Since it uses isHeaderUnit(). I remember HeaderUnit is introduced by you so it shouldn't conflict with clang modules.

BTW, may I ask the behavior is consistency with GCC?

2335

nit: It looks like we prefer to use C++20 modules over standard modules, although standard modules must be the right term.

clang/lib/Parse/Parser.cpp

672

I think we could ignore getLangOpts().ModulesTS here.

clang/test/Modules/cxx20-include-translation.cpp

11

I am not sure if you mean that the un-converted header would be included in the GMF or in the module purview

At least in the GMF.

IMO we already have test-cases that cover this, but I do not mind adding something extra if there's a missing case?

Yeah, I feel better to have more coverage.

[iains]

rebased, addressed review comments.

[iains]

clang/lib/Lex/PPDirectives.cpp

2226–2227

It is not good to break existing users. Generally, a breaking change patch could be reverted directly... We must care about it to avoid unnecessary efforts. And it looks like the current implementation wouldn't break any existing users, right? Since it uses isHeaderUnit(). I remember HeaderUnit is introduced by you so it shouldn't conflict with clang modules.

correct, in this case, the fact that Header Units are specific to the C++20 implementation (they are quite different from clang header modules) allows us to tell the difference.

BTW, may I ask the behavior is consistency with GCC?

yes, I discussed this with @urnathan (especially that it is very difficult to get consistent behaviour if we were to include-translate in the module purview).

2335

since we are heading for C++23 ... perhaps now would be a good time to start using "standard modules"? (I can change to C++20 modules if there's objection).

clang/lib/Parse/Parser.cpp

672

well, we agreed that (for the present) we would try to avoid changing the behaviour w.r.t modules-ts (but spend no effort to complete the implementation) - and to remove it from comments; we can then take a pass through to remove the modules-ts behaviour (assuming that no-one is using it!)

[ChuanqiXu]

Then LGTM if all the comments addressed.

clang/lib/Lex/PPDirectives.cpp

2226–2227

Got it. Thanks.

2335

It is OK to me. But I feel better if we use C++ standard modules since modules is a feature lives everywhere.

clang/lib/Parse/Parser.cpp

672

Given the previous status of C++ modules, it is hard to believe there is existing users for it. So I think it should be fine to remove the support for module-ts.

[iains]

addressed review comments.

[jansvoboda11]

Hi @iains, upstream Clang crashes on the attached test case due to an assertion failure. Git bisect pointed me to this commit. Can you please take a look? Previously, the test would result in a warning of incomplete umbrella header.

// RUN: rm -rf %t && mkdir %t
// RUN: split-file %s %t

//--- frameworks/FW.framework/Modules/module.modulemap
framework module FW {
  umbrella header "FW.h"
  module * { export * }
}
//--- frameworks/FW.framework/Headers/FW.h
#include "One.h"
//--- frameworks/FW.framework/Headers/One.h
//--- frameworks/FW.framework/Headers/Two.h

//--- module.modulemap
module Mod { header "Mod.h" }
//--- Mod.h
#include "FW/Two.h"
//--- from_module.m
#include "Mod.h"

// RUN: %clang -fmodules -fmodules-cache-path=%t/cache -iframework %t/frameworks -c %t/from_module.m -o %t/from_module.o

[iains]

In D128981#3728070, @jansvoboda11 wrote:

Hi @iains, upstream Clang crashes on the attached test case due to an assertion failure. Git bisect pointed me to this commit. Can you please take a look? Previously, the test would result in a warning of incomplete umbrella header.

yes, I can repeat this - will take a look.

[iains]

In D128981#3728503, @iains wrote:

In D128981#3728070, @jansvoboda11 wrote:

Hi @iains, upstream Clang crashes on the attached test case due to an assertion failure. Git bisect pointed me to this commit. Can you please take a look? Previously, the test would result in a warning of incomplete umbrella header.

yes, I can repeat this - will take a look.

I landed https://github.com/llvm/llvm-project/commit/a2dd6130d49777d63c2d1b641bd8e56f26fa0822 to fix this (assuming that there is no new fallout) - do you think we should back port to llvm-15?

[jansvoboda11]

Thank you! Yes, I think this would be a good candidate for backporting.

clang/include/clang/Lex/Preprocessor.h

public:
     /// Saw any other token.
     void handleMisc() {
       if (S <= 0)
         S = AtTopLevel;
     }
 
     bool atTopLevel() { return S <= 0; }
     bool afterImportSeq() { return S == AfterImportSeq; }
+    bool afterTopLevelSeq() { return S == AfterTopLevelTokenSeq; }
 
   private:
     State S;
     /// Whether we're in the pp-import-suffix following the header-name in a
     /// pp-import. If so, a close-brace is not sufficient to end the
     /// top-level-token-seq of an import-seq.
     bool AfterHeaderName = false;
   };
 
   /// Our current position within a C++20 import-seq.
   ImportSeq ImportSeqState = ImportSeq::AfterTopLevelTokenSeq;
 
+  /// Track whether we are in a Global Module Fragment
+  class TrackGMF {
+  public:
+    enum GMFState : int {
+      GMFActive = 1,
+      MaybeGMF = 0,
+      BeforeGMFIntroducer = -1,
+      GMFAbsentOrEnded = -2,
+    };
+
+    TrackGMF(GMFState S) : S(S) {}
+
+    /// Saw a semicolon.
+    void handleSemi() {
+      // If it is immediately after the first instance of the module keyword,
+      // then that introduces the GMF.
+      if (S == MaybeGMF)
+        S = GMFActive;
+    }
+
+    /// Saw an 'export' identifier.
+    void handleExport() {
+      // The presence of an 'export' keyword always ends or excludes a GMF.
+      S = GMFAbsentOrEnded;
+    }
+
+    /// Saw an 'import' identifier.
+    void handleImport(bool AfterTopLevelTokenSeq) {
+      // If we see this before any 'module' kw, then we have no GMF.
+      if (AfterTopLevelTokenSeq && S == BeforeGMFIntroducer)
+        S = GMFAbsentOrEnded;
+    }
+
+    /// Saw a 'module' identifier.
+    void handleModule(bool AfterTopLevelTokenSeq) {
+      // This was the first module identifier and not preceded by any token
+      // that would exclude a GMF.  It could begin a GMF, but only if directly
+      // followed by a semicolon.
+      if (AfterTopLevelTokenSeq && S == BeforeGMFIntroducer)
+        S = MaybeGMF;
+      else
+        S = GMFAbsentOrEnded;
+    }
+
+    /// Saw any other token.
+    void handleMisc() {
+      // We saw something other than ; after the 'module' kw, so not a GMF.
+      if (S == MaybeGMF)
+        S = GMFAbsentOrEnded;
+    }
+
+    bool inGMF() { return S == GMFActive; }
+
+  private:
+    /// Track the transitions into and out of a Global Module Fragment,
+    /// if one is present.
+    GMFState S;
+  };
+
+  TrackGMF TrackGMFState = TrackGMF::BeforeGMFIntroducer;
+
   /// Whether the module import expects an identifier next. Otherwise,
   /// it expects a '.' or ';'.
   bool ModuleImportExpectsIdentifier = false;
 
   /// The identifier and source location of the currently-active
   /// \#pragma clang arc_cf_code_audited begin.
   std::pair<IdentifierInfo *, SourceLocation> PragmaARCCFCodeAuditedInfo;
 
private:
 
   /// An additional notification that can be produced by a header inclusion or
   /// import to tell the parser what happened.
   struct ImportAction {
     enum ActionKind {
       None,
       ModuleBegin,
       ModuleImport,
+      HeaderUnitImport,
       SkippedModuleImport,
       Failure,
     } Kind;
     Module *ModuleForHeader = nullptr;
 
     ImportAction(ActionKind AK, Module *Mod = nullptr)
         : Kind(AK), ModuleForHeader(Mod) {
       assert((AK == None || Mod || AK == Failure) &&

clang/lib/Lex/PPDirectives.cpp

void Preprocessor::HandleIncludeDirective(SourceLocation HashLoc,
   switch (Action.Kind) {
   case ImportAction::None:
   case ImportAction::SkippedModuleImport:
     break;
   case ImportAction::ModuleBegin:
     EnterAnnotationToken(SourceRange(HashLoc, EndLoc),
                          tok::annot_module_begin, Action.ModuleForHeader);
     break;
+  case ImportAction::HeaderUnitImport:
+    EnterAnnotationToken(SourceRange(HashLoc, EndLoc), tok::annot_header_unit,
+                         Action.ModuleForHeader);
+    break;
   case ImportAction::ModuleImport:
     EnterAnnotationToken(SourceRange(HashLoc, EndLoc),
                          tok::annot_module_include, Action.ModuleForHeader);
     break;
   case ImportAction::Failure:
     assert(TheModuleLoader.HadFatalFailure &&
            "This should be an early exit only to a fatal error");
     TheModuleLoader.HadFatalFailure = true;
if (File && isPCHThroughHeader(&File->getFileEntry()))
       SkippingUntilPCHThroughHeader = false;
     return {ImportAction::None};
   }
 
   // Should we enter the source file? Set to Skip if either the source file is
   // known to have no effect beyond its effect on module visibility -- that is,
   // if it's got an include guard that is already defined, set to Import if it
   // is a modular header we've already built and should import.
+
+  // For C++20 Modules
+  // [cpp.include]/7 If the header identified by the header-name denotes an
+  // importable header, it is implementation-defined whether the #include
+  // preprocessing directive is instead replaced by an import directive.
+  // For this implementation, the translation is permitted when we are parsing
+  // the Global Module Fragment, and not otherwise (the cases where it would be
+  // valid to replace an include with an import are highly constrained once in
+  // named module purview; this choice avoids considerable complexity in
+  // determining valid cases).
+
   enum { Enter, Import, Skip, IncludeLimitReached } Action = Enter;
 
   if (PPOpts->SingleFileParseMode)
     Action = IncludeLimitReached;
 
   // If we've reached the max allowed include depth, it is usually due to an
   // include cycle. Don't enter already processed files again as it can lead to
   // reaching the max allowed include depth again.
   if (Action == Enter && HasReachedMaxIncludeDepth && File &&
       alreadyIncluded(*File))
     Action = IncludeLimitReached;
 
+  bool MaybeTranslateInclude = Action == Enter && File && SuggestedModule &&
+                               !isForModuleBuilding(SuggestedModule.getModule(),
+                                                    getLangOpts().CurrentModule,
+                                                    getLangOpts().ModuleName);
+
+  // FIXME: We do not have a good way to disambiguate C++ clang modules from
+  // C++ standard modules (other than use/non-use of Header Units).
+  Module *SM = SuggestedModule.getModule();
+  // Maybe a usable Header Unit
+  bool UsableHeaderUnit = false;
+  if (getLangOpts().CPlusPlusModules && SM && SM->isHeaderUnit()) {
+    if (TrackGMFState.inGMF() || IsImportDecl)
+      UsableHeaderUnit = true;
+    else if (!IsImportDecl) {
+      // This is a Header Unit that we do not include-translate
+      SuggestedModule = ModuleMap::KnownHeader();
+      SM = nullptr;
+    }
+  }
+  // Maybe a usable clang header module.
+  bool UsableHeaderModule =
+      (getLangOpts().CPlusPlusModules || getLangOpts().Modules) && SM &&
+      !SM->isHeaderUnit();
+
   // Determine whether we should try to import the module for this #include, if
   // there is one. Don't do so if precompiled module support is disabled or we
   // are processing this module textually (because we're building the module).
-  if (Action == Enter && File && SuggestedModule && getLangOpts().Modules &&
-      !isForModuleBuilding(SuggestedModule.getModule(),
-                           getLangOpts().CurrentModule,
-                           getLangOpts().ModuleName)) {
+  if (MaybeTranslateInclude && (UsableHeaderUnit || UsableHeaderModule)) {
     // If this include corresponds to a module but that module is
     // unavailable, diagnose the situation and bail out.
     // FIXME: Remove this; loadModule does the same check (but produces
     // slightly worse diagnostics).
     if (checkModuleIsAvailable(getLangOpts(), getTargetInfo(), getDiagnostics(),
                                SuggestedModule.getModule())) {
       Diag(FilenameTok.getLocation(),
            diag::note_implicit_top_level_module_import_here)
           << SuggestedModule.getModule()->getTopLevelModuleName();
       return {ImportAction::None};
     }
 
     // Compute the module access path corresponding to this module.
     // FIXME: Should we have a second loadModule() overload to avoid this
     // extra lookup step?
     SmallVector<std::pair<IdentifierInfo *, SourceLocation>, 2> Path;
-    for (Module *Mod = SuggestedModule.getModule(); Mod; Mod = Mod->Parent)
+    for (Module *Mod = SM; Mod; Mod = Mod->Parent)
       Path.push_back(std::make_pair(getIdentifierInfo(Mod->Name),
                                     FilenameTok.getLocation()));
     std::reverse(Path.begin(), Path.end());
 
     // Warn that we're replacing the include/import with a module import.
     if (!IsImportDecl)
       diagnoseAutoModuleImport(*this, StartLoc, IncludeTok, Path, CharEnd);
 
bool EnterOnce =
       IsImportDecl ||
       IncludeTok.getIdentifierInfo()->getPPKeywordID() == tok::pp_import;
 
   bool IsFirstIncludeOfFile = false;
 
   // Ask HeaderInfo if we should enter this #include file.  If not, #including
   // this file will have no effect.
   if (Action == Enter && File &&
-      !HeaderInfo.ShouldEnterIncludeFile(
-          *this, &File->getFileEntry(), EnterOnce, getLangOpts().Modules,
-          SuggestedModule.getModule(), IsFirstIncludeOfFile)) {
+      !HeaderInfo.ShouldEnterIncludeFile(*this, &File->getFileEntry(),
+                                         EnterOnce, getLangOpts().Modules, SM,
+                                         IsFirstIncludeOfFile)) {
+    // C++ standard modules:
+    // If we are not in the GMF, then we textually include only
+    // clang modules:
     // Even if we've already preprocessed this header once and know that we
     // don't need to see its contents again, we still need to import it if it's
     // modular because we might not have imported it from this submodule before.
     //
     // FIXME: We don't do this when compiling a PCH because the AST
     // serialization layer can't cope with it. This means we get local
     // submodule visibility semantics wrong in that case.
+    if (UsableHeaderUnit && !getLangOpts().CompilingPCH)
+      Action = TrackGMFState.inGMF() ? Import : Skip;
+    else
       Action = (SuggestedModule && !getLangOpts().CompilingPCH) ? Import : Skip;
   }
 
   // Check for circular inclusion of the main file.
   // We can't generate a consistent preamble with regard to the conditional
   // stack if the main file is included again as due to the preamble bounds
   // some directives (e.g. #endif of a header guard) will never be seen.
   // Since this will lead to confusing errors, avoid the inclusion.
   if (Action == Enter && File && PreambleConditionalStack.isRecording() &&
#endif
       Diag(FilenameTok, DiagId) << Path <<
         FixItHint::CreateReplacement(FilenameRange, Path);
     }
   }
 
   switch (Action) {
   case Skip:
     // If we don't need to enter the file, stop now.
-    if (Module *M = SuggestedModule.getModule())
-      return {ImportAction::SkippedModuleImport, M};
+    if (SM)
+      return {ImportAction::SkippedModuleImport, SM};
     return {ImportAction::None};
 
   case IncludeLimitReached:
     // If we reached our include limit and don't want to enter any more files,
     // don't go any further.
     return {ImportAction::None};
 
   case Import: {
     // If this is a module import, make it visible if needed.
-    Module *M = SuggestedModule.getModule();
-    assert(M && "no module to import");
+    assert(SM && "no module to import");
 
-    makeModuleVisible(M, EndLoc);
+    makeModuleVisible(SM, EndLoc);
 
     if (IncludeTok.getIdentifierInfo()->getPPKeywordID() ==
         tok::pp___include_macros)
       return {ImportAction::None};
 
-    return {ImportAction::ModuleImport, M};
+    return {ImportAction::ModuleImport, SM};
   }
 
   case Enter:
     break;
   }
 
   // Check that we don't have infinite #include recursion.
   if (IncludeMacroStack.size() == MaxAllowedIncludeStackDepth-1) {
#endif
   }
 
   // If all is good, enter the new file!
   if (EnterSourceFile(FID, CurDir, FilenameTok.getLocation(),
                       IsFirstIncludeOfFile))
     return {ImportAction::None};
 
   // Determine if we're switching to building a new submodule, and which one.
-  if (auto *M = SuggestedModule.getModule()) {
-    if (M->getTopLevelModule()->ShadowingModule) {
+  // This does not apply for C++20 modules header units.
+  if (SM && !SM->isHeaderUnit()) {
+    if (SM->getTopLevelModule()->ShadowingModule) {
       // We are building a submodule that belongs to a shadowed module. This
       // means we find header files in the shadowed module.
-      Diag(M->DefinitionLoc, diag::err_module_build_shadowed_submodule)
-        << M->getFullModuleName();
-      Diag(M->getTopLevelModule()->ShadowingModule->DefinitionLoc,
+      Diag(SM->DefinitionLoc, diag::err_module_build_shadowed_submodule)
+          << SM->getFullModuleName();
+      Diag(SM->getTopLevelModule()->ShadowingModule->DefinitionLoc,
            diag::note_previous_definition);
       return {ImportAction::None};
     }
     // When building a pch, -fmodule-name tells the compiler to textually
     // include headers in the specified module. We are not building the
     // specified module.
     //
     // FIXME: This is the wrong way to handle this. We should produce a PCH
     // that behaves the same as the header would behave in a compilation using
     // that PCH, which means we should enter the submodule. We need to teach
     // the AST serialization layer to deal with the resulting AST.
     if (getLangOpts().CompilingPCH &&
-        isForModuleBuilding(M, getLangOpts().CurrentModule,
+        isForModuleBuilding(SM, getLangOpts().CurrentModule,
                             getLangOpts().ModuleName))
       return {ImportAction::None};
 
     assert(!CurLexerSubmodule && "should not have marked this as a module yet");
-    CurLexerSubmodule = M;
+    CurLexerSubmodule = SM;
 
     // Let the macro handling code know that any future macros are within
     // the new submodule.
-    EnterSubmodule(M, EndLoc, /*ForPragma*/false);
+    EnterSubmodule(SM, EndLoc, /*ForPragma*/ false);
 
     // Let the parser know that any future declarations are within the new
     // submodule.
     // FIXME: There's no point doing this if we're handling a #__include_macros
     // directive.
-    return {ImportAction::ModuleBegin, M};
+    return {ImportAction::ModuleBegin, SM};
   }
 
   assert(!IsImportDecl && "failed to diagnose missing module for import decl");
   return {ImportAction::None};
 }
 
 /// HandleIncludeNextDirective - Implements \#include_next.
 ///

clang/lib/Lex/Preprocessor.cpp

if (Result.is(tok::code_completion) && Result.getIdentifierInfo()) {
     setCodeCompletionTokenRange(Result.getLocation(), Result.getEndLoc());
     // Set IdenfitierInfo to null to avoid confusing code that handles both
     // identifiers and completion tokens.
     Result.setIdentifierInfo(nullptr);
   }
 
   // Update ImportSeqState to track our position within a C++20 import-seq
   // if this token is being produced as a result of phase 4 of translation.
+  // Update TrackGMFState to decide if we are currently in a Global Module
+  // Fragment. GMF state updates should precede ImportSeq ones, since GMF state
+  // depends on the prevailing ImportSeq state in two cases.
   if (getLangOpts().CPlusPlusModules && LexLevel == 1 &&
       !Result.getFlag(Token::IsReinjected)) {
     switch (Result.getKind()) {
     case tok::l_paren: case tok::l_square: case tok::l_brace:
       ImportSeqState.handleOpenBracket();
       break;
     case tok::r_paren: case tok::r_square:
       ImportSeqState.handleCloseBracket();
       break;
     case tok::r_brace:
       ImportSeqState.handleCloseBrace();
       break;
+    // This token is injected to represent the translation of '#include "a.h"'
+    // into "import a.h;". Mimic the notional ';'.
+    case tok::annot_module_include:
     case tok::semi:
+      TrackGMFState.handleSemi();
       ImportSeqState.handleSemi();
       break;
     case tok::header_name:
     case tok::annot_header_unit:
       ImportSeqState.handleHeaderName();
       break;
     case tok::kw_export:
+      TrackGMFState.handleExport();
       ImportSeqState.handleExport();
       break;
     case tok::identifier:
       if (Result.getIdentifierInfo()->isModulesImport()) {
+        TrackGMFState.handleImport(ImportSeqState.afterTopLevelSeq());
         ImportSeqState.handleImport();
         if (ImportSeqState.afterImportSeq()) {
           ModuleImportLoc = Result.getLocation();
           ModuleImportPath.clear();
           ModuleImportExpectsIdentifier = true;
           CurLexerKind = CLK_LexAfterModuleImport;
         }
         break;
+      } else if (Result.getIdentifierInfo() == getIdentifierInfo("module")) {
+        TrackGMFState.handleModule(ImportSeqState.afterTopLevelSeq());
+        break;
       }
       LLVM_FALLTHROUGH;
     default:
+      TrackGMFState.handleMisc();
       ImportSeqState.handleMisc();
       break;
     }
   }
 
   LastTokenWasAt = Result.is(tok::at);
   --LexLevel;
 
case ImportAction::ModuleBegin:
       Suffix.back().startToken();
       Suffix.back().setKind(tok::annot_module_begin);
       Suffix.back().setLocation(SemiLoc);
       Suffix.back().setAnnotationEndLoc(SemiLoc);
       Suffix.back().setAnnotationValue(Action.ModuleForHeader);
       LLVM_FALLTHROUGH;
 
     case ImportAction::ModuleImport:
+    case ImportAction::HeaderUnitImport:
     case ImportAction::SkippedModuleImport:
       // We chose to import (or textually enter) the file. Convert the
       // header-name token into a header unit annotation token.
       Suffix[0].setKind(tok::annot_header_unit);
       Suffix[0].setAnnotationEndLoc(Suffix[0].getLocation());
       Suffix[0].setAnnotationValue(Action.ModuleForHeader);
       // FIXME: Call the moduleImport callback?
       break;

clang/lib/Parse/Parser.cpp

bool Parser::ParseTopLevelDecl(DeclGroupPtrTy &Result,
 
   case tok::kw_import:
   import_decl: {
     Decl *ImportDecl = ParseModuleImport(SourceLocation(), ImportState);
     Result = Actions.ConvertDeclToDeclGroup(ImportDecl);
     return false;
   }
 
-  case tok::annot_module_include:
-    Actions.ActOnModuleInclude(Tok.getLocation(),
-                               reinterpret_cast<Module *>(
-                                   Tok.getAnnotationValue()));
+  case tok::annot_module_include: {
+    auto Loc = Tok.getLocation();
+    Module *Mod = reinterpret_cast<Module *>(Tok.getAnnotationValue());
+    // FIXME: We need a better way to disambiguate C++ clang modules and
+    // standard C++ modules.
+    if (!getLangOpts().CPlusPlusModules || !Mod->isHeaderUnit())
+      Actions.ActOnModuleInclude(Loc, Mod);
+    else {
+      DeclResult Import =
+          Actions.ActOnModuleImport(Loc, SourceLocation(), Loc, Mod);
+      Decl *ImportDecl = Import.isInvalid() ? nullptr : Import.get();
+      Result = Actions.ConvertDeclToDeclGroup(ImportDecl);
+    }
     ConsumeAnnotationToken();
     return false;
+  }
 
   case tok::annot_module_begin:
     Actions.ActOnModuleBegin(Tok.getLocation(), reinterpret_cast<Module *>(
                                                     Tok.getAnnotationValue()));
     ConsumeAnnotationToken();
     ImportState = Sema::ModuleImportState::NotACXX20Module;
     return false;
 

clang/test/Modules/cxx20-include-translation.cpp

+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 -xc++-user-header h1.h -emit-header-unit -o h1.pcm
+// RUN: %clang_cc1 -std=c++20 -xc++-user-header h2.h -emit-header-unit -o h2.pcm
+// RUN: %clang_cc1 -std=c++20 -xc++-user-header h3.h -emit-header-unit -o h3.pcm
+// RUN: %clang_cc1 -std=c++20 -xc++-user-header h4.h -emit-header-unit -o h4.pcm
+
+// RUN: %clang_cc1 -std=c++20 Xlate.cpp -emit-module-interface -o Xlate.pcm \
+// RUN: -fmodule-file=h1.pcm -fmodule-file=h2.pcm -fmodule-file=h3.pcm \
+// RUN: -fmodule-file=h4.pcm -fsyntax-only -Wauto-import -verify
+
+// Check that we do the intended translation and not more.
+// RUN: %clang_cc1 -std=c++20 Xlate.cpp \
+// RUN: -fmodule-file=h1.pcm -fmodule-file=h2.pcm -fmodule-file=h3.pcm \
+// RUN: -fmodule-file=h4.pcm  -E -undef | FileCheck %s
+
+// We expect no diagnostics here, the used functions should all be available.
+// RUN: %clang_cc1 -std=c++20 Xlate.cpp -emit-module-interface \
+// RUN: -fmodule-file=h1.pcm -fmodule-file=h2.pcm -fmodule-file=h3.pcm \
+// RUN: -fmodule-file=h4.pcm -fsyntax-only
+
+// The content of the headers is not terribly important, we just want to check
+// whether they are textually included or include-translated.
+//--- h1.h
+#ifndef H1_GUARD
+#define H1_GUARD
+
+#define ONE 1
+
+void foo();
+
+#endif // H1_GUARD
+
+//--- h2.h
+#ifndef H2_GUARD
+#define H2_GUARD
+
+#define TWO 2
+
+void bar();
+
+#endif // H2_GUARD
+
+//--- h3.h
+#ifndef H3_GUARD
+#define H3_GUARD
+
+#define THREE 3
+
+void baz();
+
+#endif // H3_GUARD
+
+//--- h4.h
+#ifndef H4_GUARD
+#define H4_GUARD
+
+#define FOUR 4
+
+void boo();
+
+#endif // H4_GUARD
+
+//--- h5.h
+#ifndef H5_GUARD
+#define H5_GUARD
+
+#define FIVE 5
+
+void five();
+
+#endif // H4_GUARD
+
+//--- Xlate.cpp
+/* some comment ...
+  ... */
+module /*nothing here*/;
+
+// This should be include-translated, when the header unit for h1 is available.
+#include "h1.h" // expected-warning {{treating #include as an import of module './h1.h'}}
+// Import of a header unit is allowed, named modules are not.
+import "h2.h";
+// A regular, untranslated, header
+#include "h5.h"
+
+export module Xlate;
+
+// This is OK, the import immediately follows the module decl.
+import "h3.h";
+
+// This should *not* be include-translated, even if header unit for h4 is
+// available.
+#include "h4.h"
+
+export void charlie() {
+  foo();
+  bar();
+  baz();
+  boo();
+  five();
+}
+
+// CHECK: #pragma clang module import "./h1.h"
+// CHECK: import ./h2.h
+// CHECK: import ./h3.h
+// CHECK-NOT: #pragma clang module import "./h4.h"