[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).

(cherry picked from commit 19fccc52ff2c1da1f93d9317c34769bd9bab8ac8)

3 files changed, 16 insertions, 6 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index cdc3506d5c6..6353e14bc41 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3252,6 +3252,9 @@ 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);
@@ -3405,6 +3408,9 @@ 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()) {
diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index 3bc4e3596f1..609640576e9 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/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())
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 4f577a3cf74..c38c724ed9b 100755
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/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