lang/funcs: short-circuit with unknown index and tuple collection

Since we need to know the index to know the result type for a tuple, we
need a special case here to deal with that situation and return
cty.DynamicVal; we can't predict the result type exactly until we know the
element type.

lang/funcs/collection.go

@@ -29,6 +29,12 @@ var ElementFunc = function.New(&function.Spec{
 		case listTy.IsListType():
 			return listTy.ElementType(), nil
 		case listTy.IsTupleType():
+			if !args[1].IsKnown() {
+				// If the index isn't known yet then we can't predict the
+				// result type since each tuple element can have its own type.
+				return cty.DynamicPseudoType, nil
+			}
+
 			etys := listTy.TupleElementTypes()
 			var index int
 			err := gocty.FromCtyValue(args[1], &index)

lang/funcs/collection_test.go

@@ -90,6 +90,24 @@ func TestElement(t *testing.T) {
 			cty.NumberIntVal(2),
 			cty.StringVal("hello"),
 		},
+		{
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("hello"),
+				cty.StringVal("bonjour"),
+			}),
+			cty.UnknownVal(cty.Number),
+			cty.DynamicVal,
+		},
+		{
+			cty.UnknownVal(cty.Tuple([]cty.Type{cty.String, cty.Bool})),
+			cty.NumberIntVal(1),
+			cty.UnknownVal(cty.Bool),
+		},
+		{
+			cty.UnknownVal(cty.Tuple([]cty.Type{cty.String, cty.String})),
+			cty.UnknownVal(cty.Number),
+			cty.DynamicVal,
+		},
 	}
 
 	for _, test := range tests {