[Bug c++/68666] New: [concepts] "is not a function template" using class-scoped variable template in constraint expression

[Casey at Carter dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68666

            Bug ID: 68666
           Summary: [concepts] "is not a function template" using
                    class-scoped variable template in constraint
                    expression
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Casey at Carter dot net
  Target Milestone: ---

Created attachment 36892
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36892&action=edit
Test case

Compiling this program with r231219:

struct A {
  template <class>
  static constexpr bool val = true;
};

template <class T>
concept bool C = A::val<T>;

C{T} struct B {};

results in:

~/gcc6-r231219/bin/g++ -std=c++1z foo.cpp -c
foo.cpp:9:15: error: ‘template<class> constexpr const bool A::val<
<template-parameter-1-1> >’ is not a function template
 C{T} struct B {};
               ^

I don't disagree with the diagnostic per se - val certainly is not a function
template - but I disagree that the program is ill-formed due to that fact.
Notably the program compiles correctly if A is a namespace instead of a class:

namespace A {
  template <class>
  static constexpr bool val = true;
}

template <class T>
concept bool C = A::val<T>;

C{T} struct B {};