[Issue 19703] std.typecons.ReplaceType wrongly evaluates alias this

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19703

Simen Kjaeraas <simen.kjaras at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |simen.kjaras at gmail.com
            Summary|Algebraic doesn't work with |std.typecons.ReplaceType
                   |alias this of templated     |wrongly evaluates alias
                   |type                        |this

--- Comment #1 from Simen Kjaeraas <simen.kjaras at gmail.com> ---
Reduced example:

import std.typecons : ReplaceType;

struct S1(int n) { }
struct S2 {
    S1!4 s1;
    alias s1 this;
}

static assert(is(ReplaceType!(int, int, S2)[0] == S2));


The problem is this line in std.typecons:

https://github.com/dlang/phobos/blob/13705f53bd3322168de5701d64a5f7c8cf791867/std/typecons.d#L8682

It aggressively matches alias this where it shouldn't.

It should check if T[0]'s alias this is what's being matched instead of T[0]
itself. Here's one possible fix:

private template isAliasThis(T, alias U, V...) {
    static foreach (at; __traits(getAliasThis, T)) {
        static if (is(typeof(__traits(getMember, T, at)) : U!V)) {
            enum isAliasThis = true;
        }
    }
    static if (!__traits(compiles, assert(isAliasThis))) {
        enum isAliasThis = false;
    }
}

And replacing the offending line with this:

   else static if (is(T[0] : U!V, alias U, V...) && !isAliasThis!(T, U, V))

--