[Clang] use parameter location for abbreviated function templates

[a-tarasyuk]

Fixes #46386

---

When an abbreviated function template appears in an extern "C" block and all template parameters are invented, TemplateParams->getTemplateLoc() becomes invalid, leading to an incorrect error location. These changes ensure that the error points to the parameter's location when the template location is invalid.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #46386

---

When an abbreviated function template appears in an extern "C" block and all template parameters are invented, TemplateParams->getTemplateLoc() becomes invalid, leading to an incorrect error location. These changes ensure that the error points to the parameter's location when the template location is invalid.

---

Full diff: https://github.com/llvm/llvm-project/pull/129139.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaTemplate.cpp (+6-2)
• (modified) clang/test/CXX/temp/temp.pre/p6.cpp (+15)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2b72143482943..902de9f5f012c 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -249,6 +249,7 @@ Bug Fixes to C++ Support
 - The initialization kind of elements of structured bindings
   direct-list-initialized from an array is corrected to direct-initialization.
 - Clang no longer crashes when a coroutine is declared ``[[noreturn]]``. (#GH127327)
+- Clang now uses the parameter location for abbreviated function templates in ``extern "C"``. (#GH46386)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 38fa3ff3ab5b4..3d26428e0a31e 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -7991,8 +7991,12 @@ Sema::CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams) {
   //   have C linkage.
   DeclContext *Ctx = S->getEntity();
   if (Ctx && Ctx->isExternCContext()) {
-    Diag(TemplateParams->getTemplateLoc(), diag::err_template_linkage)
-        << TemplateParams->getSourceRange();
+    SourceRange Range =
+        TemplateParams->getTemplateLoc().isInvalid() && TemplateParams->size()
+            ? TemplateParams->getParam(TemplateParams->size() - 1)
+                  ->getSourceRange()
+            : TemplateParams->getSourceRange();
+    Diag(Range.getBegin(), diag::err_template_linkage) << Range;
     if (const LinkageSpecDecl *LSD = Ctx->getExternCContext())
       Diag(LSD->getExternLoc(), diag::note_extern_c_begins_here);
     return true;
diff --git a/clang/test/CXX/temp/temp.pre/p6.cpp b/clang/test/CXX/temp/temp.pre/p6.cpp
index cb8c70ca3abed..264972eb44eb3 100644
--- a/clang/test/CXX/temp/temp.pre/p6.cpp
+++ b/clang/test/CXX/temp/temp.pre/p6.cpp
@@ -1,4 +1,19 @@
 // RUN: %clang_cc1 -std=c++20 -verify %s
+// RUN: not %clang_cc1 -std=c++20 -fsyntax-only -fno-diagnostics-show-line-numbers -fcaret-diagnostics-max-lines=1 %s 2>&1 | FileCheck %s -strict-whitespace
+
+namespace GH46386 {
+  extern "C" { // expected-note {{extern "C" language linkage specification begins here}}
+
+  // CHECK:      error: templates must have C++ linkage
+  // CHECK-NEXT: {{^}}  void f(auto) {}
+  // CHECK-NEXT: {{^}}         ^~~~~{{$}}
+  void f(auto) {} // expected-error {{templates must have C++ linkage}}
+
+  void f(void) { // expected-note {{candidate function not viable: requires 0 arguments, but 1 was provided}}
+    f(1);        // expected-error {{no matching function for call to 'f'}}
+  }
+}
+}
 
 // Templates and partial and explicit specializations can't have C linkage.
 namespace extern_c_templates {

[AaronBallman]

LGTM!