[Issue 18287] several std.math functions no longer work with alias this

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18287

--- Comment #1 from hsteoh at quickfur.ath.cx ---
Has signbit() ever worked with alias this types?  I tried commenting out the
new sig constraint of signbit(), but this code still fails to compile on git
master:

--------
struct Proxy {
    double f;
    alias f this;
}
auto p = Proxy(1.61803);
auto sign = signbit(p);
--------

The compiler says:
---------
std/math.d(267): Error: cannot implicitly convert expression 4.50360e+15 of
type double to Proxy
std/math.d(5762): Error: template instance std.math.floatTraits!(Proxy) error
instantiating
std/math.d(5806):        instantiated from here: signbit!(Proxy)
--------

This is caused by signbit() attempting to instantiate floatTraits with Proxy,
but floatTraits does not take alias this into account.

(Not that it *shouldn't* work with alias this, but just that it doesn't seem to
have worked before. Or perhaps that only broke after floatTraits was
introduced?)

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