In this testcase, the non-type template parameter has the type of a lambda-expression. This makes no sense because a lambda in template context is specified to be distinct between different specializations of the template, even if the lambda is non-dependent, but here which specialization we are dealing with depends on which lambda we have, and vice versa.
Tested x86_64-pc-linux-gnu, applying to trunk. gcc/cp/ChangeLog: PR c++/99478 * parser.c (cp_parser_lambda_expression): Reject lambda in template parameter type. gcc/testsuite/ChangeLog: PR c++/99478 * g++.dg/cpp2a/lambda-uneval14.C: New test. --- gcc/cp/parser.c | 16 +++++++++++++++- gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C | 6 ++++++ 2 files changed, 21 insertions(+), 1 deletion(-) create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c index aec3aa3587f..3a107206318 100644 --- a/gcc/cp/parser.c +++ b/gcc/cp/parser.c @@ -10796,7 +10796,21 @@ cp_parser_lambda_expression (cp_parser* parser) LAMBDA_EXPR_LOCATION (lambda_expr) = token->location; if (cxx_dialect >= cxx20) - /* C++20 allows lambdas in unevaluated context. */; + { + /* C++20 allows lambdas in unevaluated context, but one in the type of a + non-type parameter is nonsensical. + + Distinguish a lambda in the parameter type from a lambda in the + default argument by looking at local_variables_forbidden_p, which is + only set in default arguments. */ + if (processing_template_parmlist && !parser->local_variables_forbidden_p) + { + error_at (token->location, + "lambda-expression in template parameter type"); + token->error_reported = true; + ok = false; + } + } else if (cp_unevaluated_operand) { if (!token->error_reported) diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C new file mode 100644 index 00000000000..a18035954e1 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval14.C @@ -0,0 +1,6 @@ +// PR c++/99478 +// { dg-do compile { target c++20 } } + +template <decltype ([] {})> auto f() {} // { dg-error "lambda" } + +int main() { f<{}>(); } // { dg-prune-output "no match" } base-commit: 0589be0c59767cf4cbb0ef0e7d918cf6aa3d606c -- 2.27.0