[clang][modules] giving the __stddef_ headers their own modules can cause redeclaration errors with -fbuiltin-headers-in-system-modules (#84127)

On Apple platforms, some of the stddef.h types are also declared in
system headers. In particular NULL has a conflicting declaration in
<sys/_types/_null.h>. When that's in a different module from
<__stddef_null.h>, redeclaration errors can occur.

Make the \_\_stddef_ headers be non-modular in
-fbuiltin-headers-in-system-modules and restore them back to not
respecting their header guards. Still define the header guards though.
__stddef_max_align_t.h was in _Builtin_stddef_max_align_t prior to the
addition of _Builtin_stddef, and it needs to stay in a module because
struct's can't be type merged. __stddef_wint_t.h didn't used to have a
module, but leave it in it current module since it doesn't really belong
to stddef.h.

1 files changed, 6 insertions, 3 deletions

diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
index afb2948..10c475f 100644
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -2498,9 +2498,12 @@ void ModuleMapParser::parseHeaderDecl(MMToken::TokenKind LeadingToken,
   }
 
   bool NeedsFramework = false;
-  // Don't add the top level headers to the builtin modules if the builtin headers
-  // belong to the system modules.
-  if (!Map.LangOpts.BuiltinHeadersInSystemModules || ActiveModule->isSubModule() || !isBuiltInModuleName(ActiveModule->Name))
+  // Don't add headers to the builtin modules if the builtin headers belong to
+  // the system modules, with the exception of __stddef_max_align_t.h which
+  // always had its own module.
+  if (!Map.LangOpts.BuiltinHeadersInSystemModules ||
+      !isBuiltInModuleName(ActiveModule->getTopLevelModuleName()) ||
+      ActiveModule->fullModuleNameIs({"_Builtin_stddef", "max_align_t"}))
     Map.addUnresolvedHeader(ActiveModule, std::move(Header), NeedsFramework);
 
   if (NeedsFramework)