From: Egas Ribeiro <egas.g.ribeiro@gmail.com>
Date: Thu, 25 Dec 2025 12:33:47 +0000 (+0000)
Subject: c++: Fix ICE with requires-expression in lambda requires-clause [PR123080]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=549bfeaa0bd2641ed99f1525caf3d54e2126396a;p=thirdparty%2Fgcc.git

c++: Fix ICE with requires-expression in lambda requires-clause [PR123080]

When parsing a lambda with a trailing requires-clause, calling
cp_parser_requires_clause_opt confused generic lambda parsing when
implicit template parameters were involved.
After failing to parse a type-requirement during tentative parsing and
hitting error recovery code in cp_parser_skip_to_end_of_statement that
aborted the current implicit template, attemping to finish the lambda
declaration caused ICEs.

Fix this by not aborting the current implicit template during tentative
parsing and adding cleanup for fully_implicit_function_template_p.

	PR c++/123080

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_skip_to_end_of_statement): Don't abort
	implicit templates during tentative parsing.
	(cp_parser_lambda_declarator_opt): Add cleanup for
	fully_implicit_function_template_p before parsing
	trailing_requires_clause.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-requires6.C: New test.
	* g++.dg/cpp2a/lambda-requires6a.C: New test.

Signed-off-by: Egas Ribeiro <egas.g.ribeiro@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
---

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 26da789f821..9ec3b21ab7f 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -4288,7 +4288,8 @@ cp_parser_skip_to_end_of_statement (cp_parser* parser)
   unsigned nesting_depth = 0;
 
   /* Unwind generic function template scope if necessary.  */
-  if (parser->fully_implicit_function_template_p)
+  if (parser->fully_implicit_function_template_p
+      && !cp_parser_parsing_tentatively (parser))
     abort_fully_implicit_template (parser);
 
   while (true)
@@ -12748,6 +12749,9 @@ cp_parser_lambda_declarator_opt (cp_parser* parser, tree lambda_expr,
 
   if (has_param_list)
     {
+      /* Avoid ICE on lambda-requires6a.C.  */
+      auto cleanup
+	= make_temp_override (parser->fully_implicit_function_template_p);
       /* Parse optional trailing requires clause.  */
       trailing_requires_clause = cp_parser_requires_clause_opt (parser, false);
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-requires6.C b/gcc/testsuite/g++.dg/cpp2a/lambda-requires6.C
new file mode 100644
index 00000000000..eada6779ac3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-requires6.C
@@ -0,0 +1,5 @@
+// PR c++/123080
+// { dg-do compile { target c++20 } }
+template<typename T> struct type_identity { using type = T; };
+auto f = [](auto x) requires requires { typename type_identity<decltype(x)>::type(0); } {};
+int main() { f(0); }
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-requires6a.C b/gcc/testsuite/g++.dg/cpp2a/lambda-requires6a.C
new file mode 100644
index 00000000000..54e72345c9a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-requires6a.C
@@ -0,0 +1,3 @@
+// PR c++/123080
+// { dg-do compile { target c++20 } }
+auto f = [](auto x) requires requires { // { dg-error "expected | invalid member template | constraints" }