[Issue 16481] invalid code accepted leading to linker error

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16481

--- Comment #1 from John Colvin <john.loughran.colvin at gmail.com> ---
Reduced version without phobos imports:

void blah()
{
    int[] a;
    a.map!(x => x.map!to);
}

template to()
{
}

template map(fun...)
{
    auto map(Range)(Range r)
    {
        alias RE = Range;
        alias _fun = unaryFun!fun;
        assert(!is(typeof(_fun(RE.init))));
        MapResult!(_fun, Range)(r);
    }
}

struct MapResult(alias fun, Range)
{
    alias R = Range;
    R _input;

    this(R)
    {
    }

    @property front()
    {
        fun(_input);
    }

}

template unaryFun(alias fun)
{
    alias unaryFun = fun;
}


Undefined symbols for architecture x86_64:
 
"_D3mod26__T9MapResultS73mod2toTAiZ9MapResult6__ctorMFNaNbNcNiNfAiZS3mod26__T9MapResultS73mod2toTAiZ9MapResult",
referenced from:
      _D3mod17__T3mapS73mod2toZ11__T3mapTAiZ3mapFNaNbNiNfAiZv in mod.o


If you comment out the assert then you get this error instead:

mod.d(34): Error: template mod.to cannot deduce function from argument types
!()(int[]), candidates are:
mod.d(8):        mod.to()
mod.d(19): Error: template instance mod.MapResult!(to, int[]) error
instantiating
mod.d(5):        instantiated from here: map!(int[])
mod.d(34):        instantiated from here: __lambda1!(int[])
mod.d(19):        instantiated from here: MapResult!(__lambda1, int[])
mod.d(5):        instantiated from here: map!(int[])

which is what I expect and looks ok.

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