lang/funcs: fix error when `matchkeys` encountered a variable

`matchkeys` was returning a (false) error if the searchset was a
variable, since then the type of the keylist and searchset parameters
would not match.

This does slightly change the behavior: previously matchkeys would
produce an error if the parameters were not of the same type, for e.g.
if searchset was a list of strings and keylist was a list of integers.
  This no longer produces an error.

lang/funcs/collection.go

@@ -800,7 +800,13 @@ var MatchkeysFunc = function.New(&function.Spec{
 		},
 	},
 	Type: func(args []cty.Value) (cty.Type, error) {
-		if !args[1].Type().Equals(args[2].Type()) {
+		argTypes := make([]cty.Type, 2)
+		for i := 0; i < 2; i++ {
+			argTypes[i] = args[i+1].Type()
+		}
+
+		ty, _ := convert.UnifyUnsafe(argTypes)
+		if ty == cty.NilType {
 			return cty.NilType, errors.New("lists must be of the same type")
 		}
 

lang/funcs/collection_test.go

@@ -1883,8 +1883,7 @@ func TestMatchkeys(t *testing.T) {
 			cty.UnknownVal(cty.List(cty.String)),
 			false,
 		},
-		// errors
-		{ // different types
+		{ // different types that can be unified
 			cty.ListVal([]cty.Value{
 				cty.StringVal("a"),
 			}),
@@ -1894,9 +1893,10 @@ func TestMatchkeys(t *testing.T) {
 			cty.ListVal([]cty.Value{
 				cty.StringVal("a"),
 			}),
-			cty.NilVal,
-			true,
+			cty.ListValEmpty(cty.String),
+			false,
 		},
+		// errors
 		{ // different types
 			cty.ListVal([]cty.Value{
 				cty.StringVal("a"),