Inconsistent shift-overflow warnings in C++20

[manxorist]

Consider the following code:

int foo4s() {
    return (0x7000'0000) << 4;
}

unsigned int foo4u() {
    return (0x7000'0000u) << 4;
}

int foo1s() {
    return (0x7000'0000) << 1;
}

unsigned int foo1u() {
    return (0x7000'0000u) << 1;
}

which when compiled with -std=c++20 -Wall -Wextra -Wpedantic results in:

<source>:2:26: warning: signed shift result (0x700000000) requires 36 bits to represent, but 'int' only has 32 bits [-Wshift-overflow]
    return (0x7000'0000) << 4;
           ~~~~~~~~~~~~~ ^  ~
1 warning generated.

In C++20, the semantics are just as well defined for the signed case as they are for the unsigned case, thus generating a warning when shifting bits outside of a signed target type and not when shifting outside of a unsigned target type seems inconsistent to me.

GCC does not warn in this case in C++20 mode. See https://godbolt.org/z/9rbafz3h3.

This came up when discussing a similar (but not identical) issue in GCC. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103826