[Clang] Add the template depth when parsing type constraints (#163960)

The lambdas can introduce new template parameters, and things would go
wrong if the new template parameters still lived in the depth of the
parent, when we evaluate the type constraints.

Fixes llvm/llvm-project#162092

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -478,6 +478,7 @@ Bug Fixes to C++ Support
 - Fix a crash when attempting to deduce a deduction guide from a non deducible template template parameter. (#130604)
 - Fix for clang incorrectly rejecting the default construction of a union with
   nontrivial member when another member has an initializer. (#GH81774)
+- Fixed a template depth issue when parsing lambdas inside a type constraint. (#GH162092)
 - Diagnose unresolved overload sets in non-dependent compound requirements. (#GH51246) (#GH97753)
 
 Bug Fixes to AST Handling

clang/lib/Parse/ParseTemplate.cpp

@@ -533,6 +533,12 @@ bool Parser::isTypeConstraintAnnotation() {
 bool Parser::TryAnnotateTypeConstraint() {
   if (!getLangOpts().CPlusPlus20)
     return false;
+  // The type constraint may declare template parameters, notably
+  // if it contains a generic lambda, so we need to increment
+  // the template depth as these parameters would not be instantiated
+  // at the current depth.
+  TemplateParameterDepthRAII CurTemplateDepthTracker(TemplateParameterDepth);
+  ++CurTemplateDepthTracker;
   CXXScopeSpec SS;
   bool WasScopeAnnotation = Tok.is(tok::annot_cxxscope);
   if (ParseOptionalCXXScopeSpecifier(SS, /*ObjectType=*/nullptr,

clang/test/SemaTemplate/concepts.cpp

@@ -1514,6 +1514,31 @@ static_assert( requires {{ &f } -> C;} ); // expected-error {{reference to overl
 
 }
 
+namespace GH162092 {
+
+template <typename T>
+struct vector;
+
+template <typename T, typename U>
+concept C = __is_same_as(T, U);
+
+template<class T, auto Cpt>
+concept generic_range_value = requires {
+    Cpt.template operator()<int>();
+};
+
+
+template<generic_range_value<[]<
+   C<int>
+   >() {}> T>
+void x() {}
+
+void foo() {
+  x<vector<int>>();
+}
+
+}
+
 namespace GH162770 {
   enum e {};
   template<e> struct s {};