[clang] Allow weak/selectany external definitions in header units. (PR #162713)

[via cfe-commits]

https://github.com/akrieger created https://github.com/llvm/llvm-project/pull/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`.

>From 832e71b4e2501244c1d5b6ea9c4d9d3265ebe6d5 Mon Sep 17 00:00:00 2001
From: Andrew Krieger <akrieger at users.noreply.github.com>
Date: Thu, 9 Oct 2025 10:56:05 -0700
Subject: [PATCH] Allow weak/selectany external definitions in header units.

Summary:
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`.
---
 clang/lib/Sema/SemaDecl.cpp                |  6 ++++--
 clang/test/CXX/module/module.import/p6.cpp | 13 +++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6eaf7b9435491..0063bd51aa612 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -13822,7 +13822,8 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
       !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();
   }
@@ -16162,7 +16163,8 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
       !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();
diff --git a/clang/test/CXX/module/module.import/p6.cpp b/clang/test/CXX/module/module.import/p6.cpp
index cb2d799e5b565..9e378a5fe7759 100644
--- a/clang/test/CXX/module/module.import/p6.cpp
+++ b/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