[Bug c++/92214] New: Unhelpful diagnostic for static_assert( some_concept(some_type) )

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92214

            Bug ID: 92214
           Summary: Unhelpful diagnostic for static_assert(
                    some_concept(some_type) )
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 67491
  Target Milestone: ---

If you accidentally use parentheses instead of angle brackets you get a
confusing error:

template<typename> concept foo = true;
static_assert( foo(int) );


Compiled with -std=gnu++2a gives:

x.cc:3:16: error: expected 'auto' or 'decltype(auto)' after 'foo'
    3 | static_assert( foo(int) );
      |                ^~~
x.cc:3:23: error: invalid use of 'auto [requires ::foo<<placeholder>, >]'
    3 | static_assert( foo(int) );
      |                       ^
x.cc:3:19: error: missing template arguments before '(' token
    3 | static_assert( foo(int) );
      |                   ^
x.cc:3:20: error: expected primary-expression before 'int'
    3 | static_assert( foo(int) );
      |                    ^~~


I wasn't trying to declare a variable using a placeholder type (which wouldn't
be allowed in a static_assert anyway), I was trying to say foo<int>.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67491
[Bug 67491] [meta-bug] concepts issues