Allow weak/selectany external definitions in header units. (#162713)

weak and selectany are mechanisms for allowing the linker to resolve ODR
violations. [module.import/6] states

> A header unit shall not contain a definition of a non-inline function
or variable whose name has external linkage.

But this prevents compiling any headers with such weak symbols defined.
These occur in eg. some Windows SDK headers like `DirectXMath.h`.

```
#ifndef XMGLOBALCONST
#if defined(__GNUC__) && !defined(__MINGW32__)
#define XMGLOBALCONST extern const __attribute__((weak))
#else
#define XMGLOBALCONST extern const __declspec(selectany)
#endif
#endif

    XMGLOBALCONST XMVECTORF32 g_XMSinCoefficients0 = { { { -0.16666667f, +0.0083333310f, -0.00019840874f, +2.7525562e-06f } } };
```

Proposed solution: Do not emit `diag::err_extern_def_in_header_unit` if
the `FD` or `VDecl` have either `SelectAnyAttr` or `WeakAttr`.

Author: akrieger

clang/lib/Sema/SemaDecl.cpp

@@ -13816,13 +13816,20 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
       VDecl->setInvalidDecl();
   }
 
-  // C++ [module.import/6] external definitions are not permitted in header
-  // units.
+  // C++ [module.import/6]
+  //   ...
+  //   A header unit shall not contain a definition of a non-inline function or
+  //   variable whose name has external linkage.
+  //
+  // We choose to allow weak & selectany definitions, as they are common in
+  // headers, and have semantics similar to inline definitions which are allowed
+  // in header units.
   if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
       !VDecl->isInvalidDecl() && VDecl->isThisDeclarationADefinition() &&
       VDecl->getFormalLinkage() == Linkage::External && !VDecl->isInline() &&
       !VDecl->isTemplated() && !isa<VarTemplateSpecializationDecl>(VDecl) &&
-      !VDecl->getInstantiatedFromStaticDataMember()) {
+      !VDecl->getInstantiatedFromStaticDataMember() &&
+      !(VDecl->hasAttr<SelectAnyAttr>() || VDecl->hasAttr<WeakAttr>())) {
     Diag(VDecl->getLocation(), diag::err_extern_def_in_header_unit);
     VDecl->setInvalidDecl();
   }
@@ -16153,16 +16160,24 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
     }
   }
 
-  // C++ [module.import/6] external definitions are not permitted in header
-  // units.  Deleted and Defaulted functions are implicitly inline (but the
+  // C++ [module.import/6]
+  //   ...
+  //   A header unit shall not contain a definition of a non-inline function or
+  //   variable whose name has external linkage.
+  //
+  // Deleted and Defaulted functions are implicitly inline (but the
   // inline state is not set at this point, so check the BodyKind explicitly).
+  // We choose to allow weak & selectany definitions, as they are common in
+  // headers, and have semantics similar to inline definitions which are allowed
+  // in header units.
   // FIXME: Consider an alternate location for the test where the inlined()
   // state is complete.
   if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
       !FD->isInvalidDecl() && !FD->isInlined() &&
       BodyKind != FnBodyKind::Delete && BodyKind != FnBodyKind::Default &&
       FD->getFormalLinkage() == Linkage::External && !FD->isTemplated() &&
-      !FD->isTemplateInstantiation()) {
+      !FD->isTemplateInstantiation() &&
+      !(FD->hasAttr<SelectAnyAttr>() || FD->hasAttr<WeakAttr>())) {
     assert(FD->isThisDeclarationADefinition());
     Diag(FD->getLocation(), diag::err_extern_def_in_header_unit);
     FD->setInvalidDecl();

clang/test/CXX/module/module.import/p6.cpp

@@ -3,6 +3,9 @@
 
 // RUN: %clang_cc1 -std=c++20 -x c++-header %t/bad-header-unit.h \
 // RUN:  -emit-header-unit -o %t/bad-header-unit.pcm -verify
+// RUN: %clang_cc1 -std=c++20 -x c++-header %t/bad-header-unit-declspec.h \
+// RUN:  -emit-header-unit -o %t/bad-header-unit.pcm -verify \
+// RUN:  -fdeclspec
 
 //--- bad-header-unit.h
 
@@ -77,3 +80,13 @@ template <typename T> bool b() {
 }
 
 inline bool B = b<int>();
+
+__attribute__((weak)) int weak_fun_definition() { return 42; }
+
+__attribute__((weak)) int weak_var_definition = 42;
+
+//--- bad-header-unit-declspec.h
+
+/* The cases below should compile without diagnostics.  */
+
+__declspec(selectany) int selectany_var_definition = 42; // expected-no-diagnostics