https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118210
Bug ID: 118210
Summary: g++-14.2 fails to parse template lambda expression
operator()<>() call inside the requires expression.
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: piotr.rak at gmail dot com
Target Milestone: ---
Created attachment 59983
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59983&action=edit
Test case
Hello,
g++-14.2 fails to parse template lambda expression template operator()<>() call
inside the requires expression.
Example:
requires{[]<typename T>{}.template operator()<char>();} //error: expected
nested-name-specifier before ‘T’
//error: expected ‘(’ before ‘T’
//error: expected ‘;’ before ‘.’ token
An expression:
[]<typename T>{}.template operator()<char>(); // compiles just fine
Outside the scope requires-expr.
I found this oddity trying to emulate clang __has_constexpr_builtin() invoking
such builtin wrapped in consteval template lambda expression.
As clang __has_constexpr_builtin works sparingly (trait builtin return false)
idea was to check abuse consteval context for both compilers.
As of 14.2 direct lambda calls with template type deduction do work (like
requires {[]<typename T>(T){}(char{});} ).
Please see attached or following example:
// compile -std=c++23 -DSHOW_BUG=1
#if SHOW_BUG
#define requires_expr requires
#else
#define requires_expr
#endif
using any_type_example = char;
void foo1() { requires_expr { []<typename T>(){}.template
operator()<any_type_example>(); }; }
void foo2() { requires_expr { []<typename T>{}.template
operator()<any_type_example>(); }; }
void foo3() { requires_expr { []<int>{}.template operator()<1>(); }; }
Cheers,
/P
