[clang] [C++20][Modules] Allow using stdarg.h with header units (PR #100739)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-modules

Author: Dmitry Polukhin (dmpolukhin)

<details>
<summary>Changes</summary>

Summary:
Macro like `va_start`/`va_end` marked as builtin functions that makes these identifiers special and it results in redefinition of the identifiers as builtins and it hides macro definitions during preloading C++ modules. In case of modules Clang ignores special identifiers but `PP.getCurrentModule()` was not set. This diff fixes IsModule detection logic for this particular case.

Test Plan: check-clang

---
Full diff: https://github.com/llvm/llvm-project/pull/100739.diff


2 Files Affected:

- (modified) clang/lib/Serialization/ASTReader.cpp (+4-5) 
- (added) clang/test/Headers/stdarg-cxx-modules.cpp (+27) 


``````````diff
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 3cb96df12e4da..f3e8027526f6a 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -1051,10 +1051,9 @@ IdentifierID ASTIdentifierLookupTrait::ReadIdentifierID(const unsigned char *d)
   return Reader.getGlobalIdentifierID(F, RawID >> 1);
 }
 
-static void markIdentifierFromAST(ASTReader &Reader, IdentifierInfo &II) {
+static void markIdentifierFromAST(ASTReader &Reader, IdentifierInfo &II, bool IsModule) {
   if (!II.isFromAST()) {
     II.setIsFromAST();
-    bool IsModule = Reader.getPreprocessor().getCurrentModule() != nullptr;
     if (isInterestingIdentifier(Reader, II, IsModule))
       II.setChangedSinceDeserialization();
   }
@@ -1080,7 +1079,7 @@ IdentifierInfo *ASTIdentifierLookupTrait::ReadData(const internal_key_type& k,
     II = &Reader.getIdentifierTable().getOwn(k);
     KnownII = II;
   }
-  markIdentifierFromAST(Reader, *II);
+  markIdentifierFromAST(Reader, *II, Reader.getPreprocessor().getCurrentModule() != nullptr);
   Reader.markIdentifierUpToDate(II);
 
   IdentifierID ID = Reader.getGlobalIdentifierID(F, RawID);
@@ -4543,7 +4542,7 @@ ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName, ModuleKind Type,
 
       // Mark this identifier as being from an AST file so that we can track
       // whether we need to serialize it.
-      markIdentifierFromAST(*this, *II);
+      markIdentifierFromAST(*this, *II, true);
 
       // Associate the ID with the identifier so that the writer can reuse it.
       auto ID = Trait.ReadIdentifierID(Data + KeyDataLen.first);
@@ -8961,7 +8960,7 @@ IdentifierInfo *ASTReader::DecodeIdentifierInfo(IdentifierID ID) {
     auto Key = Trait.ReadKey(Data, KeyDataLen.first);
     auto &II = PP.getIdentifierTable().get(Key);
     IdentifiersLoaded[Index] = &II;
-    markIdentifierFromAST(*this,  II);
+    markIdentifierFromAST(*this,  II, getPreprocessor().getCurrentModule() != nullptr);
     if (DeserializationListener)
       DeserializationListener->IdentifierRead(ID, &II);
   }
diff --git a/clang/test/Headers/stdarg-cxx-modules.cpp b/clang/test/Headers/stdarg-cxx-modules.cpp
new file mode 100644
index 0000000000000..b7c4486c89374
--- /dev/null
+++ b/clang/test/Headers/stdarg-cxx-modules.cpp
@@ -0,0 +1,27 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header h1.h
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header h2.h -fmodule-file=h1.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only main.cpp -fmodule-file=h1.pcm -fmodule-file=h2.pcm
+
+//--- h1.h
+#pragma once
+#include <stdarg.h>
+// expected-no-diagnostics
+
+//--- h2.h
+#pragma once
+import "h1.h";
+// expected-no-diagnostics
+
+//--- main.cpp
+import "h1.h";
+import "h2.h";
+
+void foo(int x, ...) {
+  va_list v;
+  va_start(v, x);
+  va_end(v);
+}
+// expected-no-diagnostics

``````````

</details>


https://github.com/llvm/llvm-project/pull/100739