inference: restrict type of type parameter from Vararg

[aviatesk]

Inference has been able to restrict type of Vararg type parameter N to Int for cases like func(..., ::Vararg{T,N}) where {T,N}, but this refinement was not available for signatures like func(::Tuple{Vararg{T,N}}) where {T,N}.

This commit allows the later case to be inferred as well. Now the following kind of case will be inferred, e.g.:

julia> function sub2ind_gen_fallback(dims::NTuple{N,Int}, I) where N # N is knonw to be ::Int
           ind = I[N] - 1
           for i = (N - 1):-1:1
               ind = I[i] - 1 + dims[i]*ind
           end
           return ind + 1
       end;

julia> only(Base.return_types(sub2ind_gen_fallback, (NTuple,Tuple{Vararg{Int}})))
Int64

[aviatesk]

@nanosoldier runbenchmarks("inference", vs=":master")

[nanosoldier]

Your benchmark job has completed - no performance regressions were detected. A full report can be found here.

[simeonschaub]

fixes #37316?

[aviatesk]

Yeah, I didn't know there has
been an issue for this.