make extrema with dims inferrable (JuliaLang#39321)

fixes JuliaLang#39281

Author: JeffBezanson

base/multidimensional.jl

@@ -1698,9 +1698,9 @@ extrema(f, A::AbstractArray; dims=:) = _extrema_dims(f, A, dims)
 _extrema_dims(f, A::AbstractArray, ::Colon) = _extrema_itr(f, A)
 
 function _extrema_dims(f, A::AbstractArray, dims)
-    sz = [size(A)...]
+    sz = size(A)
     for d in dims
-        sz[d] = 1
+        sz = setindex(sz, 1, d)
     end
     T = promote_op(f, eltype(A))
     B = Array{Tuple{T,T}}(undef, sz...)

test/reduce.jl

@@ -643,3 +643,6 @@ end
 
 # make sure we specialize on mapfoldl(::Type, ...)
 @test @inferred(mapfoldl(Int, +, [1, 2, 3]; init=0)) === 6
+
+# issue #39281
+@test @inferred(extrema(rand(2), dims=1)) isa Vector{Tuple{Float64,Float64}}