[Clang] use parameter location for abbreviated function templates (#129139)

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.

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -278,6 +278,7 @@ 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -7991,8 +7991,11 @@ 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(0)->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;

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 {