lower new() to reference the called object instead of re-creating it with apply_type

[JeffBezanson]

addresses #36384

This makes the lowering change suggested there, but I don't see any performance differences yet. However this should go nicely together with #44656.

[Keno]

but I don't see any performance differences yet

That's because you're lowering to:

(T::Type{Val{X}})() where {X} = Expr(:new, :T)

when the fast version is

(T::Type{Val{X}} where X)() = Expr(:new, :T)

Of course ideally the system would see that the typevar is unused and move it for us.

[JeffBezanson]

Found this bug along the way:

julia> struct X{T}
           X()::Int = new{Int}()
       end

julia> X()
X{Int64}()

Processing constructors is hard enough that we were just dropping the type declaration!

[Keno]

Whoops.

[JeffBezanson]

Ok hopefully works now, and I see a speedup.

[Keno]

Can confirm that this addresses the performance issue.

[JeffBezanson]

The subarray test failure on linux64 is weird; can't reproduce locally.

[Keno]

The subarray test failure on linux64 is weird; can't reproduce locally.

#44635

stale