Resolve instability from reducedim_init (#43467)

base/reducedim.jl

@@ -139,7 +139,7 @@ for (f1, f2, initval, typeextreme) in ((:min, :max, :Inf, :typemax), (:max, :min
 
         if isempty(A1)
             # If the slice is empty just return non-view version as the initial array
-            return copy(A1)
+            return map(f, A1)
         else
             # otherwise use the min/max of the first slice as initial value
             v0 = mapreduce(f, $f2, A1)
@@ -148,9 +148,9 @@ for (f1, f2, initval, typeextreme) in ((:min, :max, :Inf, :typemax), (:max, :min
             Tr = v0 isa T ? T : typeof(v0)
 
             # but NaNs and missing need to be avoided as initial values
-            if (v0 == v0) === false
+            if v0 isa Number && isnan(v0)
                 # v0 is NaN
-                v0 = $initval
+                v0 = oftype(v0, $initval)
             elseif isunordered(v0)
                 # v0 is missing or a third-party unordered value
                 Tnm = nonmissingtype(Tr)

test/reducedim.jl

@@ -445,8 +445,8 @@ end
 
 @testset "argmin/argmax" begin
     B = reshape(3^3:-1:1, (3, 3, 3))
-    @test B[argmax(B, dims=[2, 3])] == maximum(B, dims=[2, 3])
-    @test B[argmin(B, dims=[2, 3])] == minimum(B, dims=[2, 3])
+    @test B[argmax(B, dims=[2, 3])] == @inferred(maximum(B, dims=[2, 3]))
+    @test B[argmin(B, dims=[2, 3])] == @inferred(minimum(B, dims=[2, 3]))
 end
 
 @testset "in-place reductions with mismatched dimensionalities" begin
@@ -506,3 +506,7 @@ end
         @test r_red == [3]
     end
 end
+
+@testset "type stability (issue #43461)" begin
+    @test (@inferred maximum(Float64, reshape(1:4,2,:); dims = 2)) == reshape([3,4],2,1)
+end