[Concepts] Fix incorrect control flow when TryAnnotateTypeConstraint annotates an invalid template-id

TryAnnotateTypeConstraint could annotate a template-id which doesn't end up being a type-constraint,
in which case control flow would incorrectly flow into ParseImplicitInt.

Reenter the loop in this case.
Enable relevant tests for C++20. This required disabling typo-correction during TryAnnotateTypeConstraint
and changing a test case which is broken due to a separate bug (will be reported and handled separately).

clang/include/clang/Sema/Sema.h

@@ -6986,7 +6986,8 @@ public:
                           QualType ObjectType, bool EnteringContext,
                           bool &MemberOfUnknownSpecialization,
                           SourceLocation TemplateKWLoc = SourceLocation(),
-                          AssumedTemplateKind *ATK = nullptr);
+                          AssumedTemplateKind *ATK = nullptr,
+                          bool Disambiguation = false);
 
   TemplateNameKind isTemplateName(Scope *S,
                                   CXXScopeSpec &SS,
@@ -6995,7 +6996,8 @@ public:
                                   ParsedType ObjectType,
                                   bool EnteringContext,
                                   TemplateTy &Template,
-                                  bool &MemberOfUnknownSpecialization);
+                                  bool &MemberOfUnknownSpecialization,
+                                  bool Disambiguation = false);
 
   /// Try to resolve an undeclared template name as a type template.
   ///

clang/lib/Parse/ParseDecl.cpp

@@ -3253,6 +3253,9 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
           goto DoneWithDeclSpec;
         if (isTypeConstraintAnnotation())
           continue;
+        if (NextToken().is(tok::annot_template_id))
+          // Might have been annotated by TryAnnotateTypeConstraint.
+          continue;
         // Eat the scope spec so the identifier is current.
         ConsumeAnnotationToken();
         ParsedAttributesWithRange Attrs(AttrFactory);
@@ -3406,6 +3409,9 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
           goto DoneWithDeclSpec;
         if (isTypeConstraintAnnotation())
           continue;
+        if (Tok.is(tok::annot_template_id))
+          // Might have been annotated by TryAnnotateTypeConstraint.
+          continue;
         ParsedAttributesWithRange Attrs(AttrFactory);
         if (ParseImplicitInt(DS, nullptr, TemplateInfo, AS, DSContext, Attrs)) {
           if (!Attrs.empty()) {

clang/lib/Parse/ParseTemplate.cpp

@@ -710,7 +710,8 @@ bool Parser::TryAnnotateTypeConstraint() {
                                       /*ObjectType=*/ParsedType(),
                                       /*EnteringContext=*/false,
                                       PossibleConcept,
-                                      MemberOfUnknownSpecialization);
+                                      MemberOfUnknownSpecialization,
+                                      /*Disambiguation=*/true);
     if (MemberOfUnknownSpecialization || !PossibleConcept ||
         TNK != TNK_Concept_template) {
       if (SS.isNotEmpty())

clang/lib/Sema/SemaTemplate.cpp

@@ -174,7 +174,8 @@ TemplateNameKind Sema::isTemplateName(Scope *S,
                                       ParsedType ObjectTypePtr,
                                       bool EnteringContext,
                                       TemplateTy &TemplateResult,
-                                      bool &MemberOfUnknownSpecialization) {
+                                      bool &MemberOfUnknownSpecialization,
+                                      bool Disambiguation) {
   assert(getLangOpts().CPlusPlus && "No template names in C!");
 
   DeclarationName TName;
@@ -204,7 +205,7 @@ TemplateNameKind Sema::isTemplateName(Scope *S,
   LookupResult R(*this, TName, Name.getBeginLoc(), LookupOrdinaryName);
   if (LookupTemplateName(R, S, SS, ObjectType, EnteringContext,
                          MemberOfUnknownSpecialization, SourceLocation(),
-                         &AssumedTemplate))
+                         &AssumedTemplate, Disambiguation))
     return TNK_Non_template;
 
   if (AssumedTemplate != AssumedTemplateKind::None) {
@@ -371,7 +372,8 @@ bool Sema::LookupTemplateName(LookupResult &Found,
                               bool EnteringContext,
                               bool &MemberOfUnknownSpecialization,
                               SourceLocation TemplateKWLoc,
-                              AssumedTemplateKind *ATK) {
+                              AssumedTemplateKind *ATK,
+                              bool Disambiguation) {
   if (ATK)
     *ATK = AssumedTemplateKind::None;
 
@@ -494,8 +496,9 @@ bool Sema::LookupTemplateName(LookupResult &Found,
     }
   }
 
-  if (Found.empty() && !IsDependent) {
-    // If we did not find any names, attempt to correct any typos.
+  if (Found.empty() && !IsDependent && !Disambiguation) {
+    // If we did not find any names, and this is not a disambiguation, attempt
+    // to correct any typos.
     DeclarationName Name = Found.getLookupName();
     Found.clear();
     // Simple filter callback that, for keywords, only accepts the C++ *_cast

clang/test/SemaCXX/invalid-member-expr.cpp

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++98 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
 class X {};
 

clang/test/SemaCXX/typo-correction.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fspell-checking-limit 0 -verify -Wno-c++11-extensions %s
+// RUN: %clang_cc1 -fspell-checking-limit 0 -verify -Wno-c++11-extensions -std=c++20 %s
 
 namespace PR21817{
 int a(-rsing[2]); // expected-error {{undeclared identifier 'rsing'; did you mean 'using'?}}
@@ -523,8 +524,8 @@ PR18685::BitVector Map;  // expected-error-re {{no type named 'BitVector' in nam
 namespace shadowed_template {
 template <typename T> class Fizbin {};  // expected-note {{'::shadowed_template::Fizbin' declared here}}
 class Baz {
-   int Fizbin();
-   Fizbin<int> qux;  // expected-error {{no template named 'Fizbin'; did you mean '::shadowed_template::Fizbin'?}}
+   int Fizbin;
+   Fizbin<int> qux; // expected-error {{no template named 'Fizbin'; did you mean '::shadowed_template::Fizbin'?}}
 };
 }
 

clang/test/SemaTemplate/ms-lookup-template-base-classes.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fcxx-exceptions -fexceptions -std=c++1y -fms-compatibility -fno-spell-checking -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -std=c++20 -fms-compatibility -fno-spell-checking -fsyntax-only -verify %s
 
 
 template <class T>