Unable to evaluate pointer subtraction between `struct` fields in `comptime`

[rohlem]

Zig Version

0.14.0-dev.1579+f3445f8f6

Steps to Reproduce and Observed Behavior

Reduced from #21478, repro zig test .zig:

pub const S = struct {
    data: [2]u8,
};
const s: S = undefined;
const offset = &s.data[1] - @as(*const u8, @ptrCast(&s)); //triggers compile error
//const offset = @offsetOf(S, "data") + (&s.data[1] - &s.data[0]); //works
test {
    @compileLog(offset);
}

zig test .zig output:

.zig:5:27: error: unable to evaluate comptime expression
const offset = &s.data[1] - @as(*const u8, @ptrCast(&s)); //triggers compile error
               ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.zig:5:27: note: operation is runtime due to this operand

The other definition of offset, which in my understanding should be equivalent, works.
It's generally possible to rewrite code to use @offsetOf and manual calculation, however it's error prone if you already have a way to get a pointer having to duplicate the same logic to calculate the offset.
Furthermore, since the initializer expression is in a comptime context, I think the error stating that an operand is "runtime" is factually incorrect.
The note also doesn't point out one particular operand, which makes it seem incorrect/incomplete.

Expected Behavior

Both equivalent definitions of offset should be resolvable at comptime.