[Clang][Concepts] Correct the CurContext for friend declarations (llvm#106890)

`FindInstantiatedDecl()` relies on the `CurContext` to find the
corresponding class template instantiation for a class template
declaration.

Previously, we pushed the semantic declaration context for constraint
comparison, which is incorrect for constraints on friend declarations.
In issue llvm#78101, the semantic context of the friend is the TU, so we
missed the implicit template specialization `Template<void, 4>` when
looking for the instantiation of the primary template `Template` at the
time of checking the member instantiation; instead, we mistakenly picked
up the explicit specialization `Template<float, 5>`, hence the error.

As a bonus, this also fixes a crash when diagnosing constraints. The
DeclarationName is not necessarily an identifier, so it's incorrect to
call `getName()` on e.g. overloaded operators. Since the
DiagnosticBuilder has correctly handled Decl printing, we don't need to
find the printable name ourselves.

Fixes llvm#78101

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -338,6 +338,7 @@ Bug Fixes to C++ Support
 - Fixed an assertion failure when converting vectors to int/float with invalid expressions. (#GH105486)
 - Template parameter names are considered in the name lookup of out-of-line class template
   specialization right before its declaration context. (#GH64082)
+- Fixed a constraint comparison bug for friend declarations. (#GH78101)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5603,7 +5603,7 @@ def note_checking_constraints_for_function_here : Note<
 def note_constraint_substitution_here : Note<
   "while substituting template arguments into constraint expression here">;
 def note_constraint_normalization_here : Note<
-  "while calculating associated constraint of template '%0' here">;
+  "while calculating associated constraint of template %0 here">;
 def note_parameter_mapping_substitution_here : Note<
   "while substituting into concept arguments here; substitution failures not "
   "allowed in concept arguments">;

clang/lib/Sema/SemaConcept.cpp

@@ -1012,7 +1012,14 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
   // possible that e.g. constraints involving C<Class<T>> and C<Class> are
   // perceived identical.
   std::optional<Sema::ContextRAII> ContextScope;
-  if (auto *RD = dyn_cast<CXXRecordDecl>(DeclInfo.getDeclContext())) {
+  const DeclContext *DC = [&] {
+    if (!DeclInfo.getDecl())
+      return DeclInfo.getDeclContext();
+    return DeclInfo.getDecl()->getFriendObjectKind()
+               ? DeclInfo.getLexicalDeclContext()
+               : DeclInfo.getDeclContext();
+  }();
+  if (auto *RD = dyn_cast<CXXRecordDecl>(DC)) {
     ThisScope.emplace(S, const_cast<CXXRecordDecl *>(RD), Qualifiers());
     ContextScope.emplace(S, const_cast<DeclContext *>(cast<DeclContext>(RD)),
                          /*NewThisContext=*/false);

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1226,8 +1226,7 @@ void Sema::PrintInstantiationStack() {
     case CodeSynthesisContext::ConstraintNormalization:
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_constraint_normalization_here)
-          << cast<NamedDecl>(Active->Entity)->getName()
-          << Active->InstantiationRange;
+          << cast<NamedDecl>(Active->Entity) << Active->InstantiationRange;
       break;
     case CodeSynthesisContext::ParameterMappingSubstitution:
       Diags.Report(Active->PointOfInstantiation,

clang/test/CXX/temp/temp.constr/temp.constr.normal/p1.cpp

@@ -15,7 +15,7 @@ template<typename T> requires Bar2<T> struct S2 { };
 // expected-note@-1{{template is declared here}}
 template<typename T> requires Bar2<T> && true struct S2<T> { };
 // expected-error@-1{{class template partial specialization is not more specialized than the primary template}}
-// expected-note@-2{{while calculating associated constraint of template 'S2' here}}
+// expected-note@-2{{while calculating associated constraint of template 'S2<T>' here}}
 
 namespace type_pack {
   template<typename... Args>

clang/test/SemaTemplate/concepts-friends.cpp

@@ -525,3 +525,26 @@ struct S {
 };
 
 }
+
+namespace GH78101 {
+
+template <typename T, int i>
+concept True = true;
+
+template <typename T, int I> struct Template {
+  static constexpr int i = I;
+
+  friend constexpr auto operator+(True<i> auto f) { return i; }
+};
+
+template <int I> struct Template<float, I> {
+  static constexpr int i = I;
+
+  friend constexpr auto operator+(True<i> auto f) { return i; }
+};
+
+Template<void, 4> f{};
+
+static_assert(+Template<float, 5>{} == 5);
+
+} // namespace GH78101