[Sema] Don't drop weak_import from a declaration if its definition isn't seen (llvm#85886)

I believe this is what the original commit (33e0226) was trying to do.

This fixes a bug where clang removes the attribute from a declaration that follows a declaration directly contained in a linkage-specification.

rdar://61865848
(cherry picked from commit 884772f)

Conflicts:
	clang/test/Sema/attr-weak.c

Author: ahatanak

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5925,7 +5925,7 @@ def note_extern_c_global_conflict : Note<
 def note_extern_c_begins_here : Note<
   "extern \"C\" language linkage specification begins here">;
 def warn_weak_import : Warning <
-  "an already-declared variable is made a weak_import declaration %0">;
+  "%0 cannot be declared 'weak_import' because its definition has been provided">;
 def ext_static_non_static : Extension<
   "redeclaring non-static %0 as static is a Microsoft extension">,
   InGroup<MicrosoftRedeclareStatic>;

clang/lib/Sema/SemaDecl.cpp

@@ -4587,16 +4587,18 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
   }
 
   mergeDeclAttributes(New, Old);
-  // Warn if an already-declared variable is made a weak_import in a subsequent
+  // Warn if an already-defined variable is made a weak_import in a subsequent
   // declaration
-  if (New->hasAttr<WeakImportAttr>() &&
-      Old->getStorageClass() == SC_None &&
-      !Old->hasAttr<WeakImportAttr>()) {
-    Diag(New->getLocation(), diag::warn_weak_import) << New->getDeclName();
-    Diag(Old->getLocation(), diag::note_previous_declaration);
-    // Remove weak_import attribute on new declaration.
-    New->dropAttr<WeakImportAttr>();
-  }
+  if (New->hasAttr<WeakImportAttr>())
+    for (auto *D = Old; D; D = D->getPreviousDecl()) {
+      if (D->isThisDeclarationADefinition() != VarDecl::DeclarationOnly) {
+        Diag(New->getLocation(), diag::warn_weak_import) << New->getDeclName();
+        Diag(D->getLocation(), diag::note_previous_definition);
+        // Remove weak_import attribute on new declaration.
+        New->dropAttr<WeakImportAttr>();
+        break;
+      }
+    }
 
   if (const auto *ILA = New->getAttr<InternalLinkageAttr>())
     if (!Old->hasAttr<InternalLinkageAttr>()) {

clang/test/Sema/attr-weak.c

@@ -17,8 +17,12 @@ static int f(void) __attribute__((weak)); // expected-error {{weak declaration c
 static int x __attribute__((weak)); // expected-error {{weak declaration cannot have internal linkage}}
 
 // rdar://9538608
-int C; // expected-note {{previous declaration is here}}
-extern int C __attribute__((weak_import)); // expected-warning {{an already-declared variable is made a weak_import declaration}}
+int C; // expected-note {{previous definition is here}}
+extern int C __attribute__((weak_import)); // expected-warning {{'C' cannot be declared 'weak_import'}}
+
+int C2; // expected-note {{previous definition is here}}
+extern int C2;
+extern int C2 __attribute__((weak_import)); // expected-warning {{'C2' cannot be declared 'weak_import'}}
 
 static int pr14946_x;
 extern int pr14946_x  __attribute__((weak)); // expected-error {{weak declaration cannot have internal linkage}}

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}}