Fix minimum/maximum over dimensions with missing values for BigInt

Types such as `BigInt` don't support `typemin` and `typemax` so
the current method to find an initial value different from `missing` fails.
Use the largest/smallest non-missing value to initialize the array instead.
This is an inefficient approach. Faster alternatives would be to avoid using
an initial value at all, and instead keep track of whether a value has been
set in a separate mask; or to use `typemax`/`typemin` for types that support them.

Author: nalimilan

base/reducedim.jl

@@ -146,15 +146,18 @@ for (f1, f2, initval, typeextreme) in ((:min, :max, :Inf, :typemax), (:max, :min
             T = _realtype(f, promote_union(eltype(A)))
             Tr = v0 isa T ? T : typeof(v0)
 
-            # but NaNs and missing need to be avoided as initial values
+            # but NaNs, missing and unordered values need to be avoided as initial values
             if v0 isa Number && isnan(v0)
                 # v0 is NaN
                 v0 = oftype(v0, $initval)
-            elseif isunordered(v0)
-                # v0 is missing or a third-party unordered value
+            elseif ismissing(v0)
+                if !all(ismissing, A)
+                    v0 = mapreduce(f, $f2, skipmissing(A))
+                end
+            elseif isunordered(v0) # v0 is a third-party unordered value
                 Tnm = nonmissingtype(Tr)
                 # TODO: Some types, like BigInt, don't support typemin/typemax.
-                # So a Matrix{Union{BigInt, Missing}} can still error here.
+                # So a Matrix{Union{BigInt, T}} can still error here.
                 v0 = $typeextreme(Tnm)
             end
             # v0 may have changed type.

test/reduce.jl

@@ -377,6 +377,16 @@ end
 @test maximum(Vector(Int16(1):Int16(100))) === Int16(100)
 @test maximum(Int32[1,2]) === Int32(2)
 
+@testset "minimum/maximum over dims with missing (#35308)" begin
+    for T in (Int, Float64, BigInt, BigFloat)
+        x = Union{T, Missing}[1 missing; 2 missing]
+        @test isequal(minimum(x, dims=1), reshape([1, missing], 1, :))
+        @test isequal(maximum(x, dims=1), reshape([2, missing], 1, :))
+        @test isequal(minimum(x, dims=2), reshape([missing, missing], :, 1))
+        @test isequal(maximum(x, dims=2), reshape([missing, missing], :, 1))
+    end
+end
+
 A = circshift(reshape(1:24,2,3,4), (0,1,1))
 @test extrema(A,dims=1) == reshape([(23,24),(19,20),(21,22),(5,6),(1,2),(3,4),(11,12),(7,8),(9,10),(17,18),(13,14),(15,16)],1,3,4)
 @test extrema(A,dims=2) == reshape([(19,23),(20,24),(1,5),(2,6),(7,11),(8,12),(13,17),(14,18)],2,1,4)