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From: "ryan.burn at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/67658] New: [concepts] invalid code with constrained concepts compiles
Date: Sun, 20 Sep 2015 20:40:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67658

            Bug ID: 67658
           Summary: [concepts] invalid code with constrained concepts
                    compiles
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ryan.burn at gmail dot com
  Target Milestone: ---

This code compiles with gcc

g++ (GCC) 6.0.0 20150915 (experimental)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

//////////////////////////////////////////////////////
template<class T> concept bool C1() { return false; }                           
template<C1 T> concept bool C2() { return true; }                               

void f(C2 x) {                                                                  
}                                                                               

struct A {} a;                                                                  

int main() {                                                                    
  f(a);                                                                         
}
//////////////////////////////////////////////////////

>>From page 17 example 6.3 of N4377
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4377.pdf), it looks
like constrained concepts are not allowed. And even if they were, the
constraints on concept C2 should not be satisfied.