Improve error message when attempting to instantiate unionall

[jakobnissen]

A user might try something like this:

julia> struct Foo{X} end

julia> foo(::Type{<:Foo{X}}, x) where X = Foo{X}(x)
foo (generic function with 2 methods)

julia> foo(Foo, 2)
ERROR: MethodError: no method matching foo(::Type{Foo}, ::Int64)
Closest candidates are:
  foo(::Type{var"#s17"} where var"#s17"<:Foo{X}, ::Any) where X at REPL[2]:1
Stacktrace:
 [1] top-level scope at REPL[4]:1

julia> Foo <: Foo{X} where X
true

And be confused. If Foo <: Foo{X} where X, why does it raise a method error? I ran into it a few days ago (#38111 ), and @juliohm ran into it today. As Jeff Bezanson explained, the reason is that, since foo needs to actually instantiate a Foo, the type parameter X needs to be concrete.

It would be nice if the error message was better. I'm not sure where the error message should be, though.

Edit: Although the mistake may seem obvious in this minimal example, and therefore not need a detailed error message, it's unfortunately not hard to run into this problem with much more complicated type signatures.