[Sema] Don't drop weak_import from a declaration that follows a

declaration directly contained in a linkage-specification

Only drop it if the declaration follows a definition. I believe this is
what 33e0226 was trying to do.

rdar://61865848

Author: ahatanaka

clang/lib/Sema/SemaDecl.cpp

@@ -4613,8 +4613,7 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
   mergeDeclAttributes(New, Old);
   // Warn if an already-declared variable is made a weak_import in a subsequent
   // declaration
-  if (New->hasAttr<WeakImportAttr>() &&
-      Old->getStorageClass() == SC_None &&
+  if (New->hasAttr<WeakImportAttr>() && Old->isThisDeclarationADefinition() &&
       !Old->hasAttr<WeakImportAttr>()) {
     Diag(New->getLocation(), diag::warn_weak_import) << New->getDeclName();
     Diag(Old->getLocation(), diag::note_previous_declaration);

clang/test/SemaCXX/attr-weak.cpp

@@ -55,3 +55,10 @@ constexpr bool weak_method_is_non_null = &WithWeakMember::weak_method != nullptr
 // virtual member function is present.
 constexpr bool virtual_weak_method_is_non_null = &WithWeakMember::virtual_weak_method != nullptr; // expected-error {{must be initialized by a constant expression}}
 // expected-note@-1 {{comparison against pointer to weak member 'WithWeakMember::virtual_weak_method' can only be performed at runtime}}
+
+// Check that no warnings are emitted.
+extern "C" int g0;
+extern int g0 __attribute__((weak_import));
+
+extern "C" int g1 = 0; // expected-note {{previous definition is here}}
+extern int g1 __attribute__((weak_import)); // expected-warning {{attribute declaration must precede definition}}