[clang] WIP: Improve the -Wundefined-func-template diagnostic note for invisible template functions (PR #129031)

[Haojian Wu via cfe-commits]

https://github.com/hokein created https://github.com/llvm/llvm-project/pull/129031

See discussion in https://github.com/llvm/llvm-project/issues/125071.

>From ccd5da1bdc4c80bd3461458fc1f4360d766fb6e2 Mon Sep 17 00:00:00 2001
From: Haojian Wu <hokein.wu at gmail.com>
Date: Thu, 27 Feb 2025 11:15:09 +0100
Subject: [PATCH] Improve the -Wundefined-func-template diagnostic note for
 invisible function templates

---
 .../clang/Basic/DiagnosticSemaKinds.td        |  2 ++
 clang/include/clang/Sema/Sema.h               |  3 +-
 clang/lib/Sema/SemaTemplate.cpp               |  5 +++-
 .../lib/Sema/SemaTemplateInstantiateDecl.cpp  | 28 +++++++++++++------
 4 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index f10af8f5bd6b2..78ac2de5f1490 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5654,6 +5654,8 @@ def warn_func_template_missing : Warning<"instantiation of function %q0 "
   InGroup<UndefinedFuncTemplate>, DefaultIgnore;
 def note_forward_template_decl : Note<
   "forward declaration of template entity is here">;
+def note_unreachable_template_decl : Note<
+  "declaration of template entity is unreachable here">;
 def note_inst_declaration_hint : Note<"add an explicit instantiation "
   "declaration to suppress this warning if %q0 is explicitly instantiated in "
   "another translation unit">;
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index a501b901862b6..8916124ab73f9 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -11189,7 +11189,8 @@ class Sema final : public SemaBase {
                                       const NamedDecl *Pattern,
                                       const NamedDecl *PatternDef,
                                       TemplateSpecializationKind TSK,
-                                      bool Complain = true);
+                                      bool Complain = true,
+                                      bool* Unreachable = nullptr);
 
   /// DiagnoseTemplateParameterShadow - Produce a diagnostic complaining
   /// that the template parameter 'PrevDecl' is being shadowed by a new
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 38fa3ff3ab5b4..c9cfba3131419 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -765,7 +765,8 @@ bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
                                           const NamedDecl *Pattern,
                                           const NamedDecl *PatternDef,
                                           TemplateSpecializationKind TSK,
-                                          bool Complain /*= true*/) {
+                                          bool Complain /*= true*/,
+                                          bool* Unreachable) {
   assert(isa<TagDecl>(Instantiation) || isa<FunctionDecl>(Instantiation) ||
          isa<VarDecl>(Instantiation));
 
@@ -778,6 +779,8 @@ bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
     if (!hasReachableDefinition(const_cast<NamedDecl *>(PatternDef),
                                 &SuggestedDef,
                                 /*OnlyNeedComplete*/ false)) {
+      if (Unreachable)
+        *Unreachable = true;
       // If we're allowed to diagnose this and recover, do so.
       bool Recover = Complain && !isSFINAEContext();
       if (Complain)
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 1f42f9500959e..399abd43e69ee 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -20,6 +20,7 @@
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/PrettyDeclStackTrace.h"
 #include "clang/AST/TypeLoc.h"
+#include "clang/Basic/DiagnosticParse.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Sema/EnterExpressionEvaluationContext.h"
@@ -34,6 +35,7 @@
 #include "clang/Sema/SemaSwift.h"
 #include "clang/Sema/Template.h"
 #include "clang/Sema/TemplateInstCallback.h"
+#include "llvm/Support/Signals.h"
 #include "llvm/Support/TimeProfiler.h"
 #include <optional>
 
@@ -5052,12 +5054,15 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
       PatternDef = nullptr;
   }
 
+  // True is the template definition is unreachable, otherwise false.
+  bool Unreachable = false;
   // FIXME: We need to track the instantiation stack in order to know which
   // definitions should be visible within this instantiation.
-  if (DiagnoseUninstantiableTemplate(PointOfInstantiation, Function,
-                                Function->getInstantiatedFromMemberFunction(),
-                                     PatternDecl, PatternDef, TSK,
-                                     /*Complain*/DefinitionRequired)) {
+  if (DiagnoseUninstantiableTemplate(
+          PointOfInstantiation, Function,
+          Function->getInstantiatedFromMemberFunction(), PatternDecl,
+          PatternDef, TSK,
+          /*Complain*/ DefinitionRequired, &Unreachable)) {
     if (DefinitionRequired)
       Function->setInvalidDecl();
     else if (TSK == TSK_ExplicitInstantiationDefinition ||
@@ -5082,11 +5087,16 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
       if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
           !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc())) {
         Diag(PointOfInstantiation, diag::warn_func_template_missing)
-          << Function;
-        Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
-        if (getLangOpts().CPlusPlus11)
-          Diag(PointOfInstantiation, diag::note_inst_declaration_hint)
-              << Function;
+            << Function;
+        if (Unreachable) {
+          Diag(PatternDecl->getLocation(),
+               diag::note_unreachable_template_decl);
+        } else {
+          Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
+          if (getLangOpts().CPlusPlus11)
+            Diag(PointOfInstantiation, diag::note_inst_declaration_hint)
+                << Function;
+        }
       }
     }