[Modules] Fix an identifier hiding a function-like macro definition.

We emit a macro definition only in a module defining it. But it means
that if another module has an identifier with the same name as the macro,
the users of such module won't be able to use the macro anymore.

Fix by storing that an identifier has a macro definition that's not in
a current module (`MacroDirectivesOffset == 0`). This way
`IdentifierLookupVisitor` knows not to stop at the first module with
an identifier but to keep checking included modules for the actual macro
definition.

Fixes issue #32040.

rdar://30258278

Author: vsapsai

clang/lib/Serialization/ASTReader.cpp

@@ -1131,7 +1131,7 @@ IdentifierInfo *ASTIdentifierLookupTrait::ReadData(const internal_key_type& k,
   bool HasRevertedTokenIDToIdentifier = readBit(Bits);
   bool Poisoned = readBit(Bits);
   bool ExtensionToken = readBit(Bits);
-  bool HadMacroDefinition = readBit(Bits);
+  bool HasMacroDefinition = readBit(Bits);
 
   assert(Bits == 0 && "Extra bits in the identifier?");
   DataLen -= sizeof(uint16_t) * 2;
@@ -1151,14 +1151,17 @@ IdentifierInfo *ASTIdentifierLookupTrait::ReadData(const internal_key_type& k,
          "Incorrect C++ operator keyword flag");
   (void)CPlusPlusOperatorKeyword;
 
-  // If this identifier is a macro, deserialize the macro
-  // definition.
-  if (HadMacroDefinition) {
+  // If this identifier has a macro definition, deserialize it or notify the
+  // visitor the actual definition is in a different module.
+  if (HasMacroDefinition) {
     uint32_t MacroDirectivesOffset =
         endian::readNext<uint32_t, llvm::endianness::little>(d);
     DataLen -= 4;
 
-    Reader.addPendingMacro(II, &F, MacroDirectivesOffset);
+    if (MacroDirectivesOffset)
+      Reader.addPendingMacro(II, &F, MacroDirectivesOffset);
+    else
+      hasMacroDefinitionInDependencies = true;
   }
 
   Reader.SetIdentifierInfo(ID, II);
@@ -2419,6 +2422,9 @@ namespace {
       // declarations it needs.
       ++NumIdentifierLookupHits;
       Found = *Pos;
+      if (Trait.hasMoreInformationInDependencies())
+        // Look for the identifier in extra modules as they contain more info.
+        return false;
       return true;
     }
 

clang/lib/Serialization/ASTReaderInternals.h

@@ -286,6 +286,8 @@ class ASTIdentifierLookupTrait : public ASTIdentifierLookupTraitBase {
   // identifier that was constructed before the AST file was read.
   IdentifierInfo *KnownII;
 
+  bool hasMacroDefinitionInDependencies = false;
+
 public:
   using data_type = IdentifierInfo *;
 
@@ -300,6 +302,10 @@ class ASTIdentifierLookupTrait : public ASTIdentifierLookupTraitBase {
   IdentifierID ReadIdentifierID(const unsigned char *d);
 
   ASTReader &getReader() const { return Reader; }
+
+  bool hasMoreInformationInDependencies() const {
+    return hasMacroDefinitionInDependencies;
+  }
 };
 
 /// The on-disk hash table used to contain information about

clang/lib/Serialization/ASTWriter.cpp

@@ -3792,7 +3792,10 @@ bool IsInterestingIdentifier(const IdentifierInfo *II, uint64_t MacroOffset,
       II->getNotableIdentifierID() != tok::NotableIdentifierKind::not_notable ||
       II->getBuiltinID() != Builtin::ID::NotBuiltin ||
       II->getObjCKeywordID() != tok::ObjCKeywordKind::objc_not_keyword;
-  if (MacroOffset || II->isPoisoned() || (!IsModule && IsInteresting) ||
+  if (MacroOffset ||
+      (II->hasMacroDefinition() &&
+       II->hasFETokenInfoChangedSinceDeserialization()) ||
+      II->isPoisoned() || (!IsModule && IsInteresting) ||
       II->hasRevertedTokenIDToIdentifier() ||
       (NeedDecls && II->getFETokenInfo()))
     return true;
@@ -3871,7 +3874,8 @@ class ASTIdentifierTableTrait {
     if (isInterestingIdentifier(II, MacroOffset)) {
       DataLen += 2; // 2 bytes for builtin ID
       DataLen += 2; // 2 bytes for flags
-      if (MacroOffset)
+      if (MacroOffset || (II->hasMacroDefinition() &&
+                          II->hasFETokenInfoChangedSinceDeserialization()))
         DataLen += 4; // MacroDirectives offset.
 
       if (NeedDecls && IdResolver)
@@ -3902,15 +3906,17 @@ class ASTIdentifierTableTrait {
     assert((Bits & 0xffff) == Bits && "ObjCOrBuiltinID too big for ASTReader.");
     LE.write<uint16_t>(Bits);
     Bits = 0;
-    bool HadMacroDefinition = MacroOffset != 0;
-    Bits = (Bits << 1) | unsigned(HadMacroDefinition);
+    bool HasMacroDefinition =
+        (MacroOffset != 0) || (II->hasMacroDefinition() &&
+                               II->hasFETokenInfoChangedSinceDeserialization());
+    Bits = (Bits << 1) | unsigned(HasMacroDefinition);
     Bits = (Bits << 1) | unsigned(II->isExtensionToken());
     Bits = (Bits << 1) | unsigned(II->isPoisoned());
     Bits = (Bits << 1) | unsigned(II->hasRevertedTokenIDToIdentifier());
     Bits = (Bits << 1) | unsigned(II->isCPlusPlusOperatorKeyword());
     LE.write<uint16_t>(Bits);
 
-    if (HadMacroDefinition)
+    if (HasMacroDefinition)
       LE.write<uint32_t>(MacroOffset);
 
     if (NeedDecls && IdResolver) {

clang/test/Modules/shadow-macro.c

@@ -0,0 +1,29 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache \
+// RUN:            -fsyntax-only %t/test.c -verify
+// Test again with the populated module cache.
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache \
+// RUN:            -fsyntax-only %t/test.c -verify
+
+// Test that an identifier with the same name as a macro doesn't hide this
+// macro from the includers.
+
+//--- macro-definition.h
+#define __P(protos) ()
+#define __Q(protos) ()
+
+//--- macro-transitive.h
+#include "macro-definition.h"
+void test(int __P) {} // not "interesting" identifier
+struct __Q {};        // "interesting" identifier
+
+//--- module.modulemap
+module MacroDefinition { header "macro-definition.h" export * }
+module MacroTransitive { header "macro-transitive.h" export * }
+
+//--- test.c
+// expected-no-diagnostics
+#include "macro-transitive.h"
+void foo __P(());
+void bar __Q(());