[Bug c++/83181] [C++17] Invalid deduction guide accepted

[eddiejnolan at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83181

Eddie Nolan  changed:

   What|Removed |Added

 CC||eddiejnolan at gmail dot com

--- Comment #3 from Eddie Nolan  ---
Here's another example:

template 
struct foo {};

template
foo(T&&) -> foo;

Clang says:

:5:1: error: deduction guide template contains a template parameter
that cannot be deduced
5 | foo(T&&) -> foo;
  | ^
:4:31: note: non-deducible template parameter 'U'
4 | template
  |   ^

But GCC accepts it.

[Bug c++/83181] [C++17] Invalid deduction guide accepted

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83181

Marek Polacek  changed:

   What|Removed |Added

   Last reconfirmed||2020-09-16
 Status|UNCONFIRMED |NEW
 Ever confirmed|0   |1
 CC||mpolacek at gcc dot gnu.org

--- Comment #2 from Marek Polacek  ---
Still accepted.

[Bug c++/83181] [C++17] Invalid deduction guide accepted

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83181

TC  changed:

   What|Removed |Added

 CC||rs2740 at gmail dot com

--- Comment #1 from TC  ---
What's happening here is that a deduction failure occurs with the deduction
guide (substitution yields a template non-type parameter of type void whose
argument also can't be deduced) and so it is eliminated from the overload set,
but there's still an implicit guide generated from the constructor and the
deduction is done from that.

[temp.param]/15 proscribes deduction guide templates with a template parameter
that neither is deducible nor has a default argument, so this should be
diagnosed.