[Bug c++/60715] New: Narrowing conversions not caught in non-type template parameters

["filip.roseen at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60715

            Bug ID: 60715
           Summary: Narrowing conversions not caught in non-type template
                    parameters
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: filip.roseen at gmail dot com

Created attachment 32493
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32493&action=edit
testcase.cpp

template<        char> void   f () {}
template<unsigned int> void   g () {}


template<unsigned int> struct A {};

int main () {

  f<1024> (); // ill-formed, `char { 1024 }` is a narrowing conversion,
              // see [dcl.init.list]p7

  g<-123> (); // ill-formed, `unsigned int { -123 }` is a narrowing
              // conversion, see [dcl.init.list]p7   

  A<-123>  a; // ill-formed, see previous comment
}

/* In the above we assume `char` is 8 bits */

-------------------------------------------------------------------------

[temp.arg.nontype]p1;

  > A template-argument for a non-type, non-template template
  > parameeter shall be one of:
  >
  >   - for a non-type template-aprameter of integral or enumeration
  >     type, a converted constant expression (5.19) of the type of
  >     the template-parameter
  >
  >   <snip />


[expr.const]p3;

  > A converted constant expression of type `T` is an expression,
  > implicitly converted to a prvalue of type `T`, where the converted
  > expression is a core constant expression and the implicit
  > conversion sequence contains only user-defined conversions,
  > lvalue-to-rvalue conversions (4.1), integral promotions (4.5), and
  > integral conversions (4.7) other than narrowing conversions
  > (8.5.4).

-------------------------------------------------------------------------

[ Note: `gcc` accepts testcase.cpp, while `clang` (correctly) issues the
relevant diagnostics. ]

[ Note: `gcc` handles overflow conversions for non-type template
        parameters when used in a class template; ie. it shows a diagnostic
        for `template<char> struct B {};` instantiated as `B<1024> {};` ]