[Issue 15957] Disabled postblit + template mixin break opAssign with confusing error message

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15957

--- Comment #3 from Walter Bright <bugzilla at digitalmars.com> ---
(In reply to Walter Bright from comment #2)
> https://github.com/dlang/dmd/pull/5713

That's the wrong answer.

What's happening is, when the compiler sees the this(this), is it says "I need
to create an identity assignment operator S.opAssign(S)." It proceeds to do so,
and inserts it into the symbol table of S as a member function.

Then, when:
    s = [1,2,3];
is encountered, it does a lookup of opAssign. It finds S.opAssign(S), and of
course that fails to match.

Mixed in templates are considered imports as far as looking things up goes. The
member functions are not overloaded against them, the member functions override
them. This entirely explains the behavior you are reporting.

Trying to change the lookup rules will have all kinds of ripple effects, and
it's hard to see what they might all be in advance. It would be a high risk
change.

But there's an easy fix. Add opAssign(ref S) to the mixed in template:

    template Impl(S) {
        void opAssign(int[] x) { }
        @disable void opAssign(ref S);
    }

Now, a default opAssign(S) will not be generated, the lookup will find
Impl.opAssign, the two entries will overload against each other, and the first
one is the better match.

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