On Tue, Nov 17, 2020 at 01:05:20PM -0500, Marek Polacek via Gcc-patches wrote:
> Our implementation of template lambdas incorrectly requires the optional
> lambda-declarator. This was probably required by an early draft of
> generic lambdas, but now the production is [expr.prim.lambda.general]:
>
> lambda-expression:
> lambda-introducer lambda-declarator [opt] compound-statement
> lambda-introducer < template-parameter-list > requires-clause [opt]
> lambda-declarator [opt] compound-statement
>
> Therefore, we should accept the following test.
>
> Incidentally, I noticed we give a terrible diagnostic when the user uses
> 'mutable', but forgets to type '()' before it, which sounds like a common
> mistake. So it seems to me we should handle that specifically, rather
> than to emit this:
This might be necessary to handle <wg21.link/p1102> anyway.
> lambda-generic8.C: In lambda function:
> lambda-generic8.C:8:18: error: expected '{' before 'mutable'
> 8 | []<typename T> mutable {}.operator()<int>();
> | ^~~~~~~
> lambda-generic8.C: In function 'int main()':
> lambda-generic8.C:8:17: error: expected ';' before 'mutable'
> 8 | []<typename T> mutable {}.operator()<int>();
> | ^~~~~~~~
> | ;
> lambda-generic8.C:8:28: error: expected primary-expression before '.' token
> 8 | []<typename T> mutable {}.operator()<int>();
> | ^
> lambda-generic8.C:8:40: error: expected primary-expression before 'int'
> 8 | []<typename T> mutable {}.operator()<int>();
> | ^~~
>
> Is it okay to fix this in stage3?
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> gcc/cp/ChangeLog:
>
> PR c++/97839
> * parser.c (cp_parser_lambda_declarator_opt): Don't require ().
>
> gcc/testsuite/ChangeLog:
>
> PR c++/97839
> * g++.dg/cpp2a/lambda-generic8.C: New test.
> ---
> gcc/cp/parser.c | 14 ++++++--------
> gcc/testsuite/g++.dg/cpp2a/lambda-generic8.C | 9 +++++++++
> 2 files changed, 15 insertions(+), 8 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-generic8.C
>
> diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> index 42f705266bb..9f09c778c29 100644
> --- a/gcc/cp/parser.c
> +++ b/gcc/cp/parser.c
> @@ -10604,6 +10604,8 @@ cp_parser_trait_expr (cp_parser* parser, enum rid
> keyword)
>
> lambda-expression:
> lambda-introducer lambda-declarator [opt] compound-statement
> + lambda-introducer < template-parameter-list > requires-clause [opt]
> + lambda-declarator [opt] compound-statement
>
> Returns a representation of the expression. */
>
> @@ -11061,13 +11063,11 @@ cp_parser_lambda_introducer (cp_parser* parser,
> tree lambda_expr)
> /* Parse the (optional) middle of a lambda expression.
>
> lambda-declarator:
> - < template-parameter-list [opt] >
> - requires-clause [opt]
> - ( parameter-declaration-clause [opt] )
> - attribute-specifier [opt]
> + ( parameter-declaration-clause )
> decl-specifier-seq [opt]
> - exception-specification [opt]
> - lambda-return-type-clause [opt]
> + noexcept-specifier [opt]
> + attribute-specifier-seq [opt]
> + trailing-return-type [opt]
> requires-clause [opt]
>
> LAMBDA_EXPR is the current representation of the lambda expression. */
> @@ -11217,8 +11217,6 @@ cp_parser_lambda_declarator_opt (cp_parser* parser,
> tree lambda_expr)
> trailing-return-type in case of decltype. */
> pop_bindings_and_leave_scope ();
> }
> - else if (template_param_list != NULL_TREE) // generate diagnostic
> - cp_parser_require (parser, CPP_OPEN_PAREN, RT_OPEN_PAREN);
>
> /* Create the function call operator.
>
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-generic8.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-generic8.C
> new file mode 100644
> index 00000000000..f3c3809b36d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-generic8.C
> @@ -0,0 +1,9 @@
> +// PR c++/97839
> +// { dg-do compile { target c++20 } }
> +// Test that a lambda with <template-param-list> doesn't require
> +// a lambda-declarator.
> +
> +int main()
> +{
> + []<typename T>{}.operator()<int>();
> +}
>
> base-commit: 8661f4faa875f361cd22a197774c1fa04cd0580b
> --
> 2.28.0
>
Marek
