[Clang] Correctly handle generic lambda used as default template argu…

…ment.

Adjust the template pparameter depth when parsing default
template arguments as they may introduce generic lambda whose parameters
are not substituted at the same depth.

Fixes #62611

Reviewed By: erichkeane, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D151342

clang/docs/ReleaseNotes.rst

@@ -507,6 +507,8 @@ Bug Fixes to C++ Support
   (`#114 <https://github.com/llvm/llvm-project/issues/114>`_)
 - Fix parsing of `auto(x)`, when it is surrounded by parentheses.
   (`#62494 <https://github.com/llvm/llvm-project/issues/62494>`_)
+- Fix handling of generic lambda used as template arguments.
+  (`#62611 <https://github.com/llvm/llvm-project/issues/62611>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseTemplate.cpp

@@ -841,10 +841,17 @@ NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
   // we introduce the type parameter into the local scope.
   SourceLocation EqualLoc;
   ParsedType DefaultArg;
-  if (TryConsumeToken(tok::equal, EqualLoc))
+  if (TryConsumeToken(tok::equal, EqualLoc)) {
+    // The default argument may declare template parameters, notably
+    // if it contains a generic lambda, so we need to increase
+    // the template depth as these parameters would not be instantiated
+    // at the current level.
+    TemplateParameterDepthRAII CurTemplateDepthTracker(TemplateParameterDepth);
+    ++CurTemplateDepthTracker;
     DefaultArg =
         ParseTypeName(/*Range=*/nullptr, DeclaratorContext::TemplateTypeArg)
             .get();
+  }
 
   NamedDecl *NewDecl = Actions.ActOnTypeParameter(getCurScope(),
                                                   TypenameKeyword, EllipsisLoc,
@@ -1030,6 +1037,14 @@ Parser::ParseNonTypeTemplateParameter(unsigned Depth, unsigned Position) {
       //   end of the template-parameter-list rather than a greater-than
       //   operator.
       GreaterThanIsOperatorScope G(GreaterThanIsOperator, false);
+
+      // The default argument may declare template parameters, notably
+      // if it contains a generic lambda, so we need to increase
+      // the template depth as these parameters would not be instantiated
+      // at the current level.
+      TemplateParameterDepthRAII CurTemplateDepthTracker(
+          TemplateParameterDepth);
+      ++CurTemplateDepthTracker;
       EnterExpressionEvaluationContext ConstantEvaluated(
           Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
       DefaultArg =

clang/test/SemaCXX/cxx2a-template-lambdas.cpp

@@ -43,3 +43,30 @@ constexpr T outer() {
 }
 static_assert(outer<int>() == 123);
 template int *outer<int *>(); // expected-note {{in instantiation}}
+
+
+namespace GH62611 {
+template <auto A = [](auto x){}>
+struct C {
+  static constexpr auto B = A;
+};
+
+int test() {
+  C<>::B(42);
+}
+
+namespace AutoParam
+{
+template <auto A = [](auto x) { return x;}>
+auto B = A;
+static_assert(B<>(42) == 42);
+}
+
+namespace TypeParam
+{
+template <typename T = decltype([](auto x) {return x;})>
+auto B = T{};
+static_assert(B<>(42) == 42);
+}
+
+}