Fix argmin/argmax for non-vectors

Make sure `argmin` and `argmax` yield the correct return type when
falling back to `indmin` and `indmax`, at least for `AbstractArray` and
`Associative` arguments.

src/Compat.jl

@@ -1311,8 +1311,17 @@ if !isdefined(Base, :findall)
 end
 
 @static if !isdefined(Base, :argmin)
-    const argmin = indmin
-    const argmax = indmax
+    if VERSION >= v"0.7.0-DEV.1660" # indmin/indmax return key
+        const argmin = indmin
+        const argmax = indmax
+    else
+        argmin(x::AbstractArray) = CartesianIndex(ind2sub(x, indmin(x)))
+        argmin(x::AbstractVector) = indmin(x)
+        argmin(x::Associative) = first(Iterators.drop(keys(x), indmin(values(x))-1))
+        argmax(x::AbstractArray) = CartesianIndex(ind2sub(x, indmax(x)))
+        argmax(x::AbstractVector) = indmax(x)
+        argmax(x::Associative) = first(Iterators.drop(keys(x), indmax(values(x))-1))
+    end
     export argmin, argmax
 end
 

test/runtests.jl

@@ -1170,7 +1170,11 @@ end
 
 # 0.7.0-DEV.3516
 @test argmax([10,12,9,11]) == 2
+@test argmax([10 12; 9 11]) == CartesianIndex(1, 2)
+@test argmax(Dict(:z=>10, :y=>12, :x=>9, :w=>11)) == :y
 @test argmin([10,12,9,11]) == 3
+@test argmin([10 12; 9 11]) == CartesianIndex(2, 1)
+@test argmin(Dict(:z=>10, :y=>12, :x=>9, :w=>11)) == :x
 
 # 0.7.0-DEV.3415
 @test findall(x -> x==1, [1, 2, 3, 2, 1]) == [1, 5]