make Tuple{Union{}} unconstructable (#49111)

Type intersection assumed it was equal to Union{}, so this makes it
unconstructable so that holds true. This is similar to what the
NamedTuple constructor does.

Secondarily, this fixes an inference bug where it would create
Vararg{Union{}} and then incorrectly handle that fieldtype.

- Fixes #32392
- Addresses part of the concerns discussed in
  JuliaLang/julia#24614 (comment)
- Addresses part of the issues presented in
  JuliaLang/julia#26175
- May allow improving jl_type_equality_is_identity
  (https://github.com/JuliaLang/julia/pull/49017/files#diff-882927c6e612596e22406ae0d06adcee88a9ec05e8b61ad81b48942e2cb266e9R986)
- May allow improving intersection (finish_unionall can be more
  aggressive at computing varval for any typevars that appears in
  covariant position and has lb=Union{} and ub=leaf type)

src/uniformscaling.jl

@@ -179,7 +179,7 @@ for (t1, t2) in ((:UnitUpperTriangular, :UpperTriangular),
                  (:UnitLowerTriangular, :LowerTriangular))
     @eval begin
         function (+)(UL::$t1, J::UniformScaling)
-            ULnew = copymutable_oftype(UL.data, Base._return_type(+, Tuple{eltype(UL), typeof(J)}))
+            ULnew = copymutable_oftype(UL.data, Base.promote_op(+, eltype(UL), typeof(J)))
             for i in axes(ULnew, 1)
                 ULnew[i,i] = one(ULnew[i,i]) + J
             end
@@ -193,7 +193,7 @@ end
 # However, to preserve type stability, we do not special-case a
 # UniformScaling{<:Complex} that happens to be real.
 function (+)(A::Hermitian, J::UniformScaling{<:Complex})
-    TS = Base._return_type(+, Tuple{eltype(A), typeof(J)})
+    TS = Base.promote_op(+, eltype(A), typeof(J))
     B = copytri!(copymutable_oftype(parent(A), TS), A.uplo, true)
     for i in diagind(B)
         B[i] = A[i] + J
@@ -202,7 +202,7 @@ function (+)(A::Hermitian, J::UniformScaling{<:Complex})
 end
 
 function (-)(J::UniformScaling{<:Complex}, A::Hermitian)
-    TS = Base._return_type(+, Tuple{eltype(A), typeof(J)})
+    TS = Base.promote_op(+, eltype(A), typeof(J))
     B = copytri!(copymutable_oftype(parent(A), TS), A.uplo, true)
     B .= .-B
     for i in diagind(B)
@@ -213,7 +213,7 @@ end
 
 function (+)(A::AbstractMatrix, J::UniformScaling)
     checksquare(A)
-    B = copymutable_oftype(A, Base._return_type(+, Tuple{eltype(A), typeof(J)}))
+    B = copymutable_oftype(A, Base.promote_op(+, eltype(A), typeof(J)))
     for i in intersect(axes(A,1), axes(A,2))
         @inbounds B[i,i] += J
     end
@@ -222,7 +222,7 @@ end
 
 function (-)(J::UniformScaling, A::AbstractMatrix)
     checksquare(A)
-    B = convert(AbstractMatrix{Base._return_type(+, Tuple{eltype(A), typeof(J)})}, -A)
+    B = convert(AbstractMatrix{Base.promote_op(+, eltype(A), typeof(J))}, -A)
     for i in intersect(axes(A,1), axes(A,2))
         @inbounds B[i,i] += J
     end