reduce: unalias arrays for hcat/vcat (map)reduce (#58490)

Implements mapreduce_first for hcat and vcat

Author: Keno

base/reduce.jl

@@ -407,6 +407,8 @@ reduce_first(::typeof(add_sum), x::BitUnsignedSmall) = UInt(x)
 reduce_first(::typeof(mul_prod), x) = reduce_first(*, x)
 reduce_first(::typeof(mul_prod), x::BitSignedSmall)   = Int(x)
 reduce_first(::typeof(mul_prod), x::BitUnsignedSmall) = UInt(x)
+reduce_first(::typeof(vcat), x) = vcat(x)
+reduce_first(::typeof(hcat), x) = hcat(x)
 
 """
     Base.mapreduce_first(f, op, x)

test/reduce.jl

@@ -771,3 +771,9 @@ end
         Val(any(in((:one,:two,:three)),(:four,:three)))
     end |> only == Val{true}
 end
+
+# `reduce(vcat, A)` should not alias the input for length-1 collections
+let A=[1;;]
+    @test reduce(vcat, Any[A]) !== A
+    @test reduce(hcat, Any[A]) !== A
+end