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Shouldn't this be dmd specific?

With gdc you can tweak real to be 64, 80, 96, or 128 on x86_64 and all are valid (the mantissa bits stay the same, the size is just padding)

Possibly, but phobos statically asserts for the size, so the fact you can change the size in GDC means nothing.

Ah wait, this is for double, not real precision. So you've hit an impossible configuration, fix should be in dmd only then, rather than phobos.

I don't think the library should be masking the compiler in an accepts-invalid situation.

This is not a masking, it is an additional diagnostic.