Cannot spread same exact immutable types: null or undefined [1] is incompatible with string [2].

[villesau]

Try Flow

// @flow

type Foo = {| +foo: ?string |};
const test = (bar: Foo): Foo => ({ foo: 'foo', ...bar});

causes:

3: type Foo = {| +foo: ?string |};
                       ^ null or undefined [1] is incompatible with string [2].
References:
3: type Foo = {| +foo: ?string |};
                       ^ [1]
4: const test = (bar: Foo): Foo => ({ foo: 'foo', ...bar});
                                           ^ [2]

This is very strange and frustrating issue, I think there is no any scenario that would make the error valid.

The error location is also very odd: It's in the type, not in the usage. this makes suppressing the error pretty non-optimal.

If I don't spread, the problem disappears.

This happens with all Flow versions.

Happens also without exactness or read-onlyness
possibly undefined version works Unfortunately this is not what our type really is :/

[Gisleburt]

I'm suffering the same problem. This appears to have been a long standing bug in flow. See #5243

[nmote]

This no longer errors on master