[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 updated https://github.com/llvm/llvm-project/pull/85886

>From d39667c7e65c10babb478d8f8d54fecb66d90568 Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <ahatanak at gmail.com>
Date: Tue, 19 Mar 2024 15:50:00 -0700
Subject: [PATCH 1/2] [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 5850cd0ab6b9a..2e45f1191273a 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 f065bfd9483f8..a2c5fd4abd35f 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}}

>From 33e3219c721d0c03f445c9bb977cb9f3809e74ac Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <ahatanak at gmail.com>
Date: Wed, 8 May 2024 20:40:58 -0700
Subject: [PATCH 2/2] Check whether there's a definition at all

---
 clang/lib/Sema/SemaDecl.cpp | 23 +++++++++++++++++------
 clang/test/Sema/attr-weak.c |  4 ++++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 2e45f1191273a..0be6906026a85 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -4613,12 +4613,23 @@ 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->isThisDeclarationADefinition() &&
-      !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>()) {
+    // We know there's no full definition as the attribute on New would have
+    // been removed otherwise. Just look for the most recent tentative
+    // definition.
+    VarDecl *TentativeDef = nullptr;
+    for (auto *D = Old; D; D = D->getPreviousDecl())
+      if (D->isThisDeclarationADefinition() == VarDecl::TentativeDefinition) {
+        TentativeDef = D;
+        break;
+      }
+
+    if (TentativeDef) {
+      Diag(New->getLocation(), diag::warn_weak_import) << New->getDeclName();
+      Diag(TentativeDef->getLocation(), diag::note_previous_declaration);
+      // Remove weak_import attribute on new declaration.
+      New->dropAttr<WeakImportAttr>();
+    }
   }
 
   if (const auto *ILA = New->getAttr<InternalLinkageAttr>())
diff --git a/clang/test/Sema/attr-weak.c b/clang/test/Sema/attr-weak.c
index b827d1539b997..94e1723e125b6 100644
--- a/clang/test/Sema/attr-weak.c
+++ b/clang/test/Sema/attr-weak.c
@@ -19,6 +19,10 @@ static int x __attribute__((weak)); // expected-error {{weak declaration cannot
 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 C2; // expected-note {{previous declaration is here}}
+extern int C2;
+extern int C2 __attribute__((weak_import)); // expected-warning {{an already-declared variable is made a weak_import declaration}}
+
 static int pr14946_x;
 extern int pr14946_x  __attribute__((weak)); // expected-error {{weak declaration cannot have internal linkage}}