add tests

test/basics/test_defaultarrayinterface.jl

@@ -0,0 +1,32 @@
+using Test: @testset, @test, @inferred
+using DerivableInterfaces: @interface, DefaultArrayInterface
+
+# function wrappers to test type-stability
+_getindex(A, i...) = @interface DefaultArrayInterface() A[i...]
+_setindex!(A, v, i...) = @interface DefaultArrayInterface() A[i...] = v
+_map!(args...) = @interface DefaultArrayInterface() map!(args...)
+function _mapreduce(args...; kwargs...)
+  @interface DefaultArrayInterface() mapreduce(args...; kwargs...)
+end
+
+@testset "indexing" begin
+  for (A, i) in ((zeros(2), 2), (zeros(2, 2), (2, 1)), (zeros(1, 2, 3), (1, 2, 3)))
+    a = @inferred _getindex(A, i...)
+    @test a == A[i...]
+    v = 1.1
+    A′ = @inferred _setindex!(A, v, i...)
+    @test_broken A′ == (A[i...] = v) # FIXME: `setindex` and `A[i] = v` have a slight semantic difference
+  end
+end
+
+@testset "map!" begin
+  A = zeros(3)
+  a = @inferred _map!(Returns(2), copy(A), A)
+  @test a == map!(Returns(2), copy(A), A)
+end
+
+@testset "mapreduce" begin
+  A = zeros(3)
+  a = @inferred _mapreduce(Returns(2), +, A)
+  @test a == mapreduce(Returns(2), +, A)
+end