__traits(isDisabled) with invalid template function

[dlangBugzillaToGithub]

John Hall reported this on 2019-11-13T17:24:41Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20389

Description

If you @disable a templated function that is not valid and then try to call __traits(isDisabled) on it with the template instantiated with something other than a type, then you will get a message that the template instance does not match the declaration. However, it does not tell you why. For instance, in the code below, T should also be part of the template, but you can only find that out if you actually call foo. 

@disable T[] foo(U)(T[] x, T rhs)
{
    return x ~ rhs;
}

unittest {
    static assert(__traits(isDisabled, foo!"~"));
}

I would reiterate that this seems to be related to using something other than a type in the instantiation. If I try the code below, then it tells me that T is an undefined identifier.

unittest {
    static assert(__traits(isDisabled, foo!(int)));
}

[dlangBugzillaToGithub]

john.michael.hall commented on 2019-11-13T18:10:52Z

It seems that this is related to not having a module defined...

When I add something like
module A;
to the top and then 
static assert(__traits(isDisabled, A.foo));
compiles without error when U is there and if U is removed then it gives the error that T is not defined.