[Bug c++/80601] New: spurious -Wconversion warning with explicit class template arguments

["jens.maurer at gmx dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80601

            Bug ID: 80601
           Summary: spurious -Wconversion warning with explicit class
                    template arguments
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jens.maurer at gmx dot net
  Target Milestone: ---

The all-new gcc 7.1.0 has introduced a conversion warning in template
definition context when a value-dependent sizeof is used as a template argument
for a class template. (There is no warning for something like
"f<sizeof(T)>()".) Note that the program below has no instantiation of "g", and
I believe a -Wconversion warning should not trigger when a constant value is
used that fits the target type.

g++ -Wconversion -c x.cc
x.cc: In function ‘int g(T)’:
x.cc:7:11: warning: conversion to ‘unsigned int’ from ‘long unsigned int’ may
alter its value [-Wconversion]
   S<sizeof(T)> s;


template<unsigned int N>
struct S { };

template<class T>
void g(T)
{
  S<sizeof(T)> s;
}