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.
