[clang] [Sema] Don't drop weak_import from a declaration that follows a declaration directly contained in a linkage-specification (PR #85886)

[Akira Hatanaka via cfe-commits]

https://github.com/ahatanak created https://github.com/llvm/llvm-project/pull/85886

Only drop it if the declaration follows a definition. I believe this is what 33e022650adee965c65f9aea086ee74f3fd1bad5 was trying to do.

rdar://61865848

>From 1ccbc2e2a3bdf127f5baeb22123c67428da4e656 Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <ahatanaka at apple.com>
Date: Tue, 19 Mar 2024 15:50:00 -0700
Subject: [PATCH] [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 33e022650adee965c65f9aea086ee74f3fd1bad5 was trying to do.

rdar://61865848
---
 clang/lib/Sema/SemaDecl.cpp      | 3 +--
 clang/test/SemaCXX/attr-weak.cpp | 7 +++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 5850cd0ab6b9aa..2e45f1191273a4 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/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);
diff --git a/clang/test/SemaCXX/attr-weak.cpp b/clang/test/SemaCXX/attr-weak.cpp
index f065bfd9483f8a..a2c5fd4abd35f6 100644
--- a/clang/test/SemaCXX/attr-weak.cpp
+++ b/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 at -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}}