c++: lambdas inside constraints [PR92652]

When parsing a constraint-expression, a requires-clause or a
requires-expression, we temporarily increment processing_template_decl
so that we always obtain template trees which we could later reduce via
substitution even when not inside a template.

But incrementing processing_template_decl when we're already inside a
template has the unintended side effect of shifting up the template
parameter levels of a lambda defined inside one of these constructs,
which leads to confusion later during substitution into the lambda.

This patch fixes this issue by incrementing processing_template_decl
during parsing of these constructs only if it is 0.

gcc/cp/ChangeLog:

	PR c++/92652
	PR c++/93698
	PR c++/94128
	* parser.c (cp_parser_requires_clause_expression): Temporarily
	increment processing_template_decl only if it is 0.
	(cp_parser_constraint_expression): Likewise.
	(cp_parser_requires_expression): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/92652
	PR c++/93698
	PR c++/94128
	* g++.dg/cpp2a/concepts-lambda8.C: New test.
	* g++.dg/cpp2a/concepts-lambda9.C: New test.
	* g++.dg/cpp2a/concepts-lambda10.C: New test.

(cherry picked from commit 020d86db8896f088435830595640e6fc21bc64ad)

gcc/cp/parser.c

@@ -27646,11 +27646,16 @@ static tree
 cp_parser_requires_clause_expression (cp_parser *parser, bool lambda_p)
 {
   processing_constraint_expression_sentinel parsing_constraint;
-  ++processing_template_decl;
+  temp_override<int> ovr (processing_template_decl);
+  if (!processing_template_decl)
+    /* Adjust processing_template_decl so that we always obtain template
+       trees here.  We don't do the usual ++processing_template_decl
+       because that would skew the template parameter depth of a lambda
+       within if we're already inside a template.  */
+    processing_template_decl = 1;
   cp_expr expr = cp_parser_constraint_logical_or_expression (parser, lambda_p);
   if (check_for_bare_parameter_packs (expr))
     expr = error_mark_node;
-  --processing_template_decl;
   return expr;
 }
 
@@ -27667,12 +27672,14 @@ static tree
 cp_parser_constraint_expression (cp_parser *parser)
 {
   processing_constraint_expression_sentinel parsing_constraint;
-  ++processing_template_decl;
+  temp_override<int> ovr (processing_template_decl);
+  if (!processing_template_decl)
+    /* As in cp_parser_requires_clause_expression.  */
+    processing_template_decl = 1;
   cp_expr expr = cp_parser_binary_expression (parser, false, true,
 					      PREC_NOT_OPERATOR, NULL);
   if (check_for_bare_parameter_packs (expr))
     expr = error_mark_node;
-  --processing_template_decl;
   expr.maybe_add_location_wrapper ();
   return expr;
 }
@@ -27781,9 +27788,11 @@ cp_parser_requires_expression (cp_parser *parser)
       parms = NULL_TREE;
 
     /* Parse the requirement body. */
-    ++processing_template_decl;
+    temp_override<int> ovr (processing_template_decl);
+    if (!processing_template_decl)
+      /* As in cp_parser_requires_clause_expression.  */
+      processing_template_decl = 1;
     reqs = cp_parser_requirement_body (parser);
-    --processing_template_decl;
     if (reqs == error_mark_node)
       return error_mark_node;
   }

gcc/testsuite/g++.dg/cpp2a/concepts-lambda10.C

+// PR c++/94128
+// { dg-do compile { target c++20 } }
+
+void test(auto param)
+requires requires{ { [](auto p){return p;}(param) }; };
+
+void test2() { test(1); }

gcc/testsuite/g++.dg/cpp2a/concepts-lambda8.C

+// PR c++/92652
+// { dg-do compile { target concepts } }
+
+template < typename T >
+    requires ([]{return true ;}())
+void h() { }
+
+int main()
+{
+    h<int>();
+}

gcc/testsuite/g++.dg/cpp2a/concepts-lambda9.C

+// PR c++/93698
+// { dg-do compile { target concepts } }
+
+#include <utility>
+
+template <int N>
+concept foo = []<std::size_t... Is>(std::index_sequence<Is...>) constexpr {
+  return (Is + ...) > 10;
+}(std::make_index_sequence<N>());
+
+bool a = foo<7>;