Fix #26488: don't map over values not provided (#26521)

This is a symptom of the good old how-to-allocate-a-result-array-of-an-arbitrary-transform-of-its-elements problem. Eventually it'd be nice to solve this with `collect` of a lazy implementation, but for now this papers over the egregious problem.

base/reducedim.jl

@@ -140,10 +140,14 @@ let
         [AbstractArray{t} for t in uniontypes(BitIntFloat)]...,
         [AbstractArray{Complex{t}} for t in uniontypes(BitIntFloat)]...}
 
-    global reducedim_init(f, op::Union{typeof(+),typeof(add_sum)}, A::T, region) =
-        reducedim_initarray(A, region, mapreduce_first(f, op, zero(eltype(A))))
-    global reducedim_init(f, op::Union{typeof(*),typeof(mul_prod)}, A::T, region) =
-        reducedim_initarray(A, region, mapreduce_first(f, op, one(eltype(A))))
+    global function reducedim_init(f, op::Union{typeof(+),typeof(add_sum)}, A::T, region)
+        z = zero(f(zero(eltype(A))))
+        reducedim_initarray(A, region, op(z, z))
+    end
+    global function reducedim_init(f, op::Union{typeof(*),typeof(mul_prod)}, A::T, region)
+        u = one(f(one(eltype(A))))
+        reducedim_initarray(A, region, op(u, u))
+    end
 end
 
 ## generic (map)reduction

test/reducedim.jl

@@ -338,6 +338,18 @@ for region in Any[-1, 0, (-1, 2), [0, 1], (1,-2,3), [0 1;
     @test_throws ArgumentError minimum(abs, Areduc, dims=region)
 end
 
+# issue #26488
+@testset "don't map over initial values not provided" begin
+    @test sum(x->x+1, [1], dims=1)[1] === sum(x->x+1, [1]) === 2
+    @test prod(x->x+1, [1], dims=1)[1] === prod(x->x+1, [1]) === 2
+    @test mapreduce(x->x+1, +, [1], dims=1)[1] === mapreduce(x->x+1, +, [1]) === 2
+    @test mapreduce(x->x+1, *, [1], dims=1)[1] === mapreduce(x->x+1, *, [1]) === 2
+    @test mapreduce(!, &, [false], dims=1)[1] === mapreduce(!, &, [false]) === true
+    @test mapreduce(!, |, [true], dims=1)[1] === mapreduce(!, |, [true]) === false
+    @test mapreduce(x->1/x, max, [1], dims=1)[1] === mapreduce(x->1/x, max, [1]) === 1.0
+    @test mapreduce(x->-1/x, min, [1], dims=1)[1] === mapreduce(x->-1/x, min, [1]) === -1.0
+end
+
 # check type of result
 @testset "type of sum(::Array{$T}" for T in [UInt8, Int8, Int32, Int64, BigInt]
     result = sum(T[1 2 3; 4 5 6; 7 8 9], dims=2)