Unexpected type widening when using nested isEnum

[cmfcmf]

The type generated by isEnum behaves unexpectedly when nested inside isObject:

// works as expected:
const simpleEnum = t.isEnum(["a", "b"]); 
// ->
// t.StrictValidator<unknown, "a" | "b">

// unexpected type widening:
const enumInObject = t.isObject({
  enum: t.isEnum(["a", "b"]),
});
// ->
// t.StrictValidator<unknown, ToOptional<{
//   enum: string;
// }>>

Current workarounds:

const workaround1 = t.isObject({
  enum: t.isEnum(["a" as const, "b" as const]),
});
// ->
// t.StrictValidator<unknown, ToOptional<{
//   enum: "a" | "b";
// }>>

const workaround2 = t.isObject({
  enum: t.isOneOf([t.isLiteral("a"), t.isLiteral("b")]),
});
// ->
// t.StrictValidator<unknown, ToOptional<{
//   enum: "a" | "b";
// }>>

Possible workaround, but not yet working due to #7:

const workaround3 = t.isObject({
  enum: t.isEnum(["a", "b"] as const),
});
// -> 
// t.StrictValidator<unknown, ToOptional<{
//   enum: any;
// }>>