On Fri, 4 Apr 2025, Jason Merrill wrote:
> Tested x86_64-pc-linux-gnu, applying to trunk.
>
> -- 8< --
>
> Since r10-7441 we set processing_template_decl in a requires-expression so
> that we can use tsubst_expr to evaluate the requirements, but that confuses
> lambdas terribly; begin_lambda_type silently returns error_mark_node and we
> continue into other failures. This patch clears processing_template_decl
> again while we're defining the closure and op() function, so it only remains
> set while parsing the introducer (i.e. any init-captures) and building the
> resulting object. This properly avoids trying to create another lambda in
> tsubst_lambda_expr.
I wonder if we couldn't just set current_template_parms to a dummy
template parameter list if it's empty when parsing a requires-expr?
>
> PR c++/99546
> PR c++/113925
> PR c++/106976
> PR c++/109961
> PR c++/117336
>
> gcc/cp/ChangeLog:
>
> * lambda.cc (build_lambda_object): Handle fake
> requires-expr processing_template_decl.
> * parser.cc (cp_parser_lambda_expression): Likewise.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp2a/lambda-requires2.C: New test.
> * g++.dg/cpp2a/lambda-requires3.C: New test.
> * g++.dg/cpp2a/lambda-requires4.C: New test.
> * g++.dg/cpp2a/lambda-requires5.C: New test.
> ---
> gcc/cp/lambda.cc | 8 +++-
> gcc/cp/parser.cc | 41 ++++++++++++-------
> gcc/testsuite/g++.dg/cpp2a/lambda-requires2.C | 8 ++++
> gcc/testsuite/g++.dg/cpp2a/lambda-requires3.C | 6 +++
> gcc/testsuite/g++.dg/cpp2a/lambda-requires4.C | 6 +++
> gcc/testsuite/g++.dg/cpp2a/lambda-requires5.C | 10 +++++
> 6 files changed, 64 insertions(+), 15 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-requires2.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-requires3.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-requires4.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-requires5.C
>
> diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
> index ed70bb0ba8e..f0a54b60275 100644
> --- a/gcc/cp/lambda.cc
> +++ b/gcc/cp/lambda.cc
> @@ -59,7 +59,13 @@ build_lambda_object (tree lambda_expr)
> vec<constructor_elt, va_gc> *elts = NULL;
> tree node, expr, type;
>
> - if (processing_template_decl || lambda_expr == error_mark_node)
> + if (processing_template_decl && !in_template_context
> + && current_binding_level->requires_expression)
> + /* As in cp_parser_lambda_expression, don't get confused by
> + cp_parser_requires_expression setting processing_template_decl. In
> that
> + case we want to return the result of finish_compound_literal, to avoid
> + tsubst_lambda_expr. */;
> + else if (processing_template_decl || lambda_expr == error_mark_node)
> return lambda_expr;
>
> /* Make sure any error messages refer to the lambda-introducer. */
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index 53e6237f6c4..812a7c5ae7d 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -11736,21 +11736,34 @@ cp_parser_lambda_expression (cp_parser* parser)
> if (cp_parser_error_occurred (parser))
> return error_mark_node;
>
> - type = begin_lambda_type (lambda_expr);
> - if (type == error_mark_node)
> - return error_mark_node;
> -
> - record_lambda_scope (lambda_expr);
> - record_lambda_scope_discriminator (lambda_expr);
> -
> - /* Do this again now that LAMBDA_EXPR_EXTRA_SCOPE is set. */
> - determine_visibility (TYPE_NAME (type));
> -
> - /* Now that we've started the type, add the capture fields for any
> - explicit captures. */
> - register_capture_members (LAMBDA_EXPR_CAPTURE_LIST (lambda_expr));
> -
> {
> + /* OK, this is a bit tricksy. cp_parser_requires_expression sets
> + processing_template_decl to make checking more normal, but that
> confuses
> + lambda parsing terribly. In non-template context, we want to parse
> the
> + lambda once and not tsubst_lambda_expr. So in that case, clear
> + processing_template_decl now, and restore it before the call to
> + build_lambda_object; that way we end up with what looks like a
> templatey
> + functional cast to the closure type, which is suitable for the
> + requires-expression tsubst_expr. This is PR99546 and friends. */
> + processing_template_decl_sentinel ptds (/*reset*/false);
> + if (processing_template_decl && !in_template_context
> + && current_binding_level->requires_expression)
> + processing_template_decl = 0;
> +
> + type = begin_lambda_type (lambda_expr);
> + if (type == error_mark_node)
> + return error_mark_node;
> +
> + record_lambda_scope (lambda_expr);
> + record_lambda_scope_discriminator (lambda_expr);
> +
> + /* Do this again now that LAMBDA_EXPR_EXTRA_SCOPE is set. */
> + determine_visibility (TYPE_NAME (type));
> +
> + /* Now that we've started the type, add the capture fields for any
> + explicit captures. */
> + register_capture_members (LAMBDA_EXPR_CAPTURE_LIST (lambda_expr));
> +
> /* Inside the class, surrounding template-parameter-lists do not apply.
> */
> unsigned int saved_num_template_parameter_lists
> = parser->num_template_parameter_lists;
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-requires2.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-requires2.C
> new file mode 100644
> index 00000000000..be5a71abefe
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-requires2.C
> @@ -0,0 +1,8 @@
> +// PR c++/99546
> +// { dg-do compile { target c++20 } }
> +
> +int main() {
> + constexpr auto b = requires { []{}; };
> + static_assert(b);
> + static_assert(!b); // { dg-error "assertion failed" }
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-requires3.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-requires3.C
> new file mode 100644
> index 00000000000..8c4ef064e52
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-requires3.C
> @@ -0,0 +1,6 @@
> +// PR c++/113925
> +// { dg-do compile { target c++20 } }
> +
> +template<bool B>
> +struct b{};
> +static_assert(requires { b<([]()consteval{ return true; }())>{}; });
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-requires4.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-requires4.C
> new file mode 100644
> index 00000000000..f3bb041310c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-requires4.C
> @@ -0,0 +1,6 @@
> +// PR c++/106976
> +// { dg-do compile { target c++20 } }
> +
> +struct S{
> + constexpr static auto s = requires { []; }; // { dg-error "expected '\{'" }
> +};
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-requires5.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-requires5.C
> new file mode 100644
> index 00000000000..c81831381e3
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-requires5.C
> @@ -0,0 +1,10 @@
> +// PR c++/109961
> +// { dg-do compile { target c++20 } }
> +
> +auto a = requires{
> + []( int b ) consteval {
> + if( b ) {
> + throw b;
> + }
> + }( 0 );
> +};
>
> base-commit: 2a36d22ab52d6ffce9a1fcaf7aca83336679e111
> --
> 2.49.0
>
>
