Re: [PATCH] c++: Allow template lambdas without lambda-declarator [PR97839]

[Jason Merrill via Gcc-patches]

On 11/18/20 1:16 PM, Marek Polacek wrote:

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.

Agreed.

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