[Bug c++/105774] New: Bogus overflow in constant expression

["jeff at jgarrett dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105774

            Bug ID: 105774
           Summary: Bogus overflow in constant expression
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jeff at jgarrett dot org
  Target Milestone: ---

The following is diagnosed as ill-formed by GCC but not by Clang:

    int main() {
      constexpr auto _ = [] {
        char x = 127;
        return ++x;
      }();
    }

<source>:5:5: error: overflow in constant expression [-fpermissive]

On godbolt https://godbolt.org/z/91oeGsEbh
Originally from
https://stackoverflow.com/questions/72425404/still-unsure-about-signed-integer-overflow-in-c

I believe that this is well-formed. [expr.pre.incr]/1 says x++ is equivalent to
x+=1. [expr.ass]/6 says that x+=1 is equivalent to x=x+1 except that x is only
evaluated once. That expression x=x+1 avoids overflow through integer
promotion.

The same code with x+=1 instead of ++x is allowed by GCC.