error inside Tangent constructor if incorrect type is used

Author: Miha Zgubic

src/tangent_types/tangent.jl

@@ -25,6 +25,22 @@ struct Tangent{P,T} <: AbstractTangent
     # Note: If T is a Tuple/Dict, then P is also a Tuple/Dict
     # (but potentially a different one, as it doesn't contain differentials)
     backing::T
+
+    function Tangent{P,T}(backing) where {P,T}
+        function backing_error(P, G, E)
+            msg = "Tangent for the primal $P should be backed by a $E type, not by $G."
+            throw(ArgumentError(msg))
+        end
+
+        if P <: Tuple
+            T <: Tuple || backing_error(P, T, Tuple)
+        elseif P <: AbstractDict
+            T <: AbstractDict || backing_error(P, T, AbstractDict)
+        else
+            T <: NamedTuple || backing_error(P, T, NamedTuple)
+        end
+        return new(backing)
+    end
 end
 
 function Tangent{P}(; kwargs...) where {P}

test/tangent_types/tangent.jl

@@ -23,6 +23,26 @@ end
         @test typeof(Tangent{Tuple{}}()) == Tangent{Tuple{},Tuple{}}
     end
 
+    @testset "constructor" begin
+        t = (1.0, 2.0)
+        nt = (x = 1, y=2.0)
+        d = Dict(:x => 1.0, :y => 2.0)
+        vals = [1, 2]
+
+        @test_throws ArgumentError Tangent{typeof(t), typeof(nt)}(nt)
+        @test_throws ArgumentError Tangent{typeof(t), typeof(d)}(d)
+
+        @test_throws ArgumentError Tangent{typeof(d), typeof(nt)}(nt)
+        @test_throws ArgumentError Tangent{typeof(d), typeof(t)}(t)
+
+        @test_throws ArgumentError Tangent{typeof(nt), typeof(vals)}(vals)
+        @test_throws ArgumentError Tangent{typeof(nt), typeof(d)}(d)
+        @test_throws ArgumentError Tangent{typeof(nt), typeof(t)}(t)
+
+        @test_throws ArgumentError Tangent{Foo, typeof(d)}(d)
+        @test_throws ArgumentError Tangent{Foo, typeof(t)}(t)
+    end
+
     @testset "==" begin
         @test Tangent{Foo}(; x=0.1, y=2.5) == Tangent{Foo}(; x=0.1, y=2.5)
         @test Tangent{Foo}(; x=0.1, y=2.5) == Tangent{Foo}(; y=2.5, x=0.1)