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

[via cfe-commits]

https://github.com/akrieger updated https://github.com/llvm/llvm-project/pull/162713

>From 21cf21227e3f5f456ce50a51b115b9a84a5d3704 Mon Sep 17 00:00:00 2001
From: Andrew Krieger <akrieger at meta.com>
Date: Thu, 9 Oct 2025 13:33:22 -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                | 29 +++++++++++++++++-----
 clang/test/CXX/module/module.import/p6.cpp | 13 ++++++++++
 2 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6eaf7b9435491..43492f40b8041 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -13816,13 +13816,20 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
       VDecl->setInvalidDecl();
   }
 
-  // C++ [module.import/6] external definitions are not permitted in header
-  // units.
+  // C++ [module.import/6]
+  //   ...
+  //   A header unit shall not contain a definition of a non-inline function or
+  //   variable whose name has external linkage.
+  //
+  // We choose to allow weak & selectany definitions, as they are common in
+  // headers, and have semantics similar to inline definitions which are allowed
+  // in header units.
   if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
       !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();
   }
@@ -16153,16 +16160,26 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
     }
   }
 
-  // C++ [module.import/6] external definitions are not permitted in header
-  // units.  Deleted and Defaulted functions are implicitly inline (but the
+  // C++ [module.import/6]
+  //   ...
+  //   A header unit shall not contain a definition of a non-inline function or
+  //   variable whose name has external linkage.
+  //
+  // Deleted and Defaulted functions are implicitly inline (but the
   // inline state is not set at this point, so check the BodyKind explicitly).
+  // We choose to allow weak & selectany definitions, as they are common in
+  // headers, and have semantics similar to inline definitions which are allowed
+  // in header units.
   // FIXME: Consider an alternate location for the test where the inlined()
   // state is complete.
   if (getLangOpts().CPlusPlusModules && currentModuleIsHeaderUnit() &&
       !FD->isInvalidDecl() && !FD->isInlined() &&
       BodyKind != FnBodyKind::Delete && BodyKind != FnBodyKind::Default &&
       FD->getFormalLinkage() == Linkage::External && !FD->isTemplated() &&
-      !FD->isTemplateInstantiation()) {
+      !FD->isTemplateInstantiation() &&
+      // The intent of the above section seems to be to prevent ODR issues.
+      // Weak symbols do the same but are not part of the c++ spec.
+      !(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