Reduced example from https://stackoverflow.com/q/51053280/2069064:

int global = 0;

constexpr int f(bool arg) {
    if (arg) {
        return 1;
    }

    return global++;
}

Fails on gcc trunk with:

<source>: In function 'constexpr int f(bool)':
<source>:8:1: error: the value of 'global' is not usable in a constant expression
 }
 ^
<source>:1:5: note: 'int global' is not const
 int global = 0;
     ^~~~~~
Compiler returned: 1

But the function is fine. The near-equivalent formulation of

constexpr int g(bool arg) {
    if (arg) {
        return 1;
    } else {
        return global++;
    }
}

also compiles fine.