[Bug c/91093] Error on implicit int by default

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91093

--- Comment #2 from Florian Weimer <fw at gcc dot gnu.org> ---
It seems that

auto x = 1.0;

will change meaning in C2X (as implemented by GCC) and use type double instead
of int for x. We are probably way too late to fix this (by rejecting the
construct in earlier language modes) before C2X support arrives in GCC.

This may require the same careful treatment as bug 91092 and bug 106416.