[ItaniumDemangle] Set InConstraintExpr to true when demangling a constraint expression.

This prevents demangler failures until the TODO in the [demangler](https://github.com/llvm/llvm-project/blob/3e070906eff720dc44aee86e533e12aafc8bb14b/llvm/include/llvm/Demangle/ItaniumDemangle.h#L5678) is implemented.

This is a temporary fix for [#89914](#89914).

Author: VitaNuo

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -2677,7 +2677,7 @@ template <typename Derived, typename Alloc> struct AbstractManglingParser {
 
   bool TryToParseTemplateArgs = true;
   bool PermitForwardTemplateReferences = false;
-  bool InConstraintExpr = false;
+  bool HasIncompleteTemplateParameterTracking = false;
   size_t ParsingLambdaParamsAtLevel = (size_t)-1;
 
   unsigned NumSyntheticTemplateParameters[3] = {};
@@ -4818,7 +4818,7 @@ template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseConstraintExpr() {
   // Within this expression, all enclosing template parameter lists are in
   // scope.
-  ScopedOverride<bool> SaveInConstraintExpr(InConstraintExpr, true);
+  ScopedOverride<bool> SaveIncompleteTemplateParameterTracking(HasIncompleteTemplateParameterTracking, true);
   return getDerived().parseExpr();
 }
 
@@ -5676,7 +5676,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseTemplateParam() {
   // substitute them all within a <constraint-expression>, so print the
   // parameter numbering instead for now.
   // TODO: Track all enclosing template parameters and substitute them here.
-  if (InConstraintExpr) {
+  if (HasIncompleteTemplateParameterTracking) {
     return make<NameType>(std::string_view(Begin, First - 1 - Begin));
   }
 
@@ -5737,6 +5737,12 @@ Node *AbstractManglingParser<Derived, Alloc>::parseTemplateParamDecl(
   }
 
   if (consumeIf("Tk")) {
+    // We don't track enclosing template parameter levels well enough to reliably
+    // demangle template parameter substitutions, so print an arbitrary
+    // string in place of a parameter for now.
+    // TODO: Track all enclosing template parameters and demangle substitutions.
+    ScopedOverride<bool> SaveIncompleteTemplateParameterTrackingExpr(
+      HasIncompleteTemplateParameterTracking, true);
     Node *Constraint = getDerived().parseName();
     if (!Constraint)
       return nullptr;

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -2677,7 +2677,7 @@ template <typename Derived, typename Alloc> struct AbstractManglingParser {
 
   bool TryToParseTemplateArgs = true;
   bool PermitForwardTemplateReferences = false;
-  bool InConstraintExpr = false;
+  bool HasIncompleteTemplateParameterTracking = false;
   size_t ParsingLambdaParamsAtLevel = (size_t)-1;
 
   unsigned NumSyntheticTemplateParameters[3] = {};
@@ -4818,7 +4818,7 @@ template <typename Derived, typename Alloc>
 Node *AbstractManglingParser<Derived, Alloc>::parseConstraintExpr() {
   // Within this expression, all enclosing template parameter lists are in
   // scope.
-  ScopedOverride<bool> SaveInConstraintExpr(InConstraintExpr, true);
+  ScopedOverride<bool> SaveIncompleteTemplateParameterTracking(HasIncompleteTemplateParameterTracking, true);
   return getDerived().parseExpr();
 }
 
@@ -5676,7 +5676,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseTemplateParam() {
   // substitute them all within a <constraint-expression>, so print the
   // parameter numbering instead for now.
   // TODO: Track all enclosing template parameters and substitute them here.
-  if (InConstraintExpr) {
+  if (HasIncompleteTemplateParameterTracking) {
     return make<NameType>(std::string_view(Begin, First - 1 - Begin));
   }
 
@@ -5737,6 +5737,12 @@ Node *AbstractManglingParser<Derived, Alloc>::parseTemplateParamDecl(
   }
 
   if (consumeIf("Tk")) {
+    // We don't track enclosing template parameter levels well enough to reliably
+    // demangle template parameter substitutions, so print an arbitrary
+    // string in place of a parameter for now.
+    // TODO: Track all enclosing template parameters and demangle substitutions.
+    ScopedOverride<bool> SaveIncompleteTemplateParameterTrackingExpr(
+      HasIncompleteTemplateParameterTracking, true);
     Node *Constraint = getDerived().parseName();
     if (!Constraint)
       return nullptr;

llvm/test/tools/llvm-cxxfilt/demangle-constrained-template.test

@@ -0,0 +1,3 @@
+RUN: llvm-cxxfilt -n _ZN3FooIiE6methodITk4TrueIT_EiEEvS3_ | FileCheck %s
+
+CHECK: void Foo<int>::method<int>(T)