By the way, fixes #399 (in issue's example a problem in concating mixed and date, not in when).

Additional commit (can be separated into separate PR, but then i will have to make rebase after this PR will be merged). Small one, but fixes #381.

Smallest repo of a problem:

yup.object({
  conditional: yup.object().when("$check", {
    is: true,
    then: yup.object({
      lazy: yup.lazy(val => yup.number()),
    })
  }),
}).validateSync({}, {context: {check: true}});

It fixes it, but underlying cause of the issue still there:

• validate in non-strict mode calls cast.

• In object._cast field get resolved then casted.

• cast calls resolve inside itself. So you end up with schema.resolve().resolve().

• after resolve with conditions is executed resulted schema still contains original schema conditions, they are not stripped. So when you call resolve on resulted schema, those conditions will be called again and their result will be concated again.

• and if in a result of conditions was a lazy schema, it ended up concating with itself, but Lazy do not have concat method and error was thrown.

Current solution solves the last point: skip merging same schema's or values.

Breaking addition will be a removal of original _conditions from resolve result. It will also allow to easily implement nested whens.

thanks!