[Bug c++/86439] New: CTAD with deleted copy constructor fails due to deduction-guide taking by value

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86439

            Bug ID: 86439
           Summary: CTAD with deleted copy constructor fails due to
                    deduction-guide taking by value
           Product: gcc
           Version: 8.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Reduced from: https://stackoverflow.com/q/51244047/2069064

struct NC {
    NC() = default;
    NC(NC const&) = delete;
    NC& operator=(NC const&) = delete;
};

template <int>
struct C {
    C(NC const&);
};

C(NC) -> C<0>;

int main() {
    NC nc;
    C c(nc);
}

clang accepts. 
gcc rejects this program with:

<source>: In function 'int main()':
<source>:16:11: error: class template argument deduction failed:
     C c(nc);
           ^
<source>:16:11: error: use of deleted function 'NC::NC(const NC&)'
<source>:3:5: note: declared here
     NC(NC const&) = delete;
     ^~
<source>:12:1: note:   initializing argument 1 of 'C(NC)-> C<0>'
 C(NC) -> C<0>;
 ^

But nowhere do we need to copy NC here. CTAD doesn't require invoking the
function, just picking the best viable candidate. And once we pick C<0>,
actually constructing a C<0> is fine since it doesn't require a copy.