[clang][Diagnostic] Clarify error message for auto

[emily-dror]

Show line and file for auto template error in -std=c++20 or -std=c++23
Fixes #147324

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Emily Dror (emily-dror)

Changes

Show line and file for auto template error in -std=c++20 or -std=c++23

---

Full diff: https://github.com/llvm/llvm-project/pull/149781.diff

1 Files Affected:

• (modified) clang/lib/Sema/SemaTemplate.cpp (+18-6)

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index b76619fc50268..4505d5f7f2226 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -8238,12 +8238,24 @@ Sema::CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams) {
     if (CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(Ctx)) {
       // C++ [temp.mem]p2:
       //   A local class shall not have member templates.
-      if (RD->isLocalClass())
-        return Diag(TemplateParams->getTemplateLoc(),
-                    diag::err_template_inside_local_class)
-          << TemplateParams->getSourceRange();
-      else
-        return false;
+      if (RD->isLocalClass()) {
+        SourceLocation DiagLoc = TemplateParams->getTemplateLoc();
+        if (DiagLoc.isInvalid()) {
+          for (const NamedDecl *Param : *TemplateParams) {
+            if (Param && Param->getLocation().isValid()) {
+              DiagLoc = Param->getLocation();
+              break;
+            }
+          }
+        }
+        if (DiagLoc.isInvalid()) {
+          // Still no good location? Fall back to the class declaration itself
+          DiagLoc = RD->getLocation();
+        }
+        return Diag(DiagLoc, diag::err_template_inside_local_class)
+               << TemplateParams->getSourceRange();
+      }
+      return false;
     }
   }
 

[emily-dror]

Fixes #147324

[vbvictor]

Hi, thank you for your contribution!
Please add comprehensive test-cases for your change to make sure everything work as expected and continue to do so in the future.
Please add an entry in clang/docs/ReleaseNotes.rst to let everyone know about your fix.
Please add "Fixes #147324" into PR description (not standalone comment) to automatically link issue to this PR.

[erichkeane]

I think we'd want to extract the determination of a dialog location into a separate function (lambda?) to help calculate it, in both of the diag cases, including hte err_template_outside_namespace_or_class_scope diag.

Also: we need a test for this. A simple reproducer that uses //expected-error lines that point to a line instead of 'in general' i think will work.

[erichkeane]

We need to cover 8261 as well.

[emily-dror]

Good catch, thanks for the review. Would you direct me to the documentation for writing test cases?

Also: we need a test for this. A simple reproducer that uses //expected-error lines that point to a line instead of 'in general' i think will work.