Switch narrowing regression with nested unions

TypeScript Version: 3.2.2, 3.3.0-dev.201xxxxx

Search Terms: 3.2 switch union narrowing regression

Code

type Additive = '+' | '-';
type Multiplicative = '*' | '/';

interface AdditiveObj {
    key: Additive
}

interface MultiplicativeObj {
    key: Multiplicative
}

type Obj = AdditiveObj | MultiplicativeObj

export function foo(obj: Obj) {
    switch (obj.key) {
        case '+': {
            onlyPlus(obj.key);
            return;
        }
    }
}

function onlyPlus(arg: '+') {
  return arg;
}

Expected behavior:
obj.key is narrowed to '+' in the switch and the call to onlyPlus(obj.key) type checks successfully.

Actual behavior:
obj.key is narrowed to Additive in the switch and the call to onlyPlus(obj.key) fails to type check

Playground Link: http://www.typescriptlang.org/play/#src=type%20Additive%20%3D%20'%2B'%20%7C%20'-'%3B%0D%0Atype%20Multiplicative%20%3D%20'*'%20%7C%20'%2F'%3B%0D%0A%0D%0Ainterface%20AdditiveObj%20%7B%0D%0A%20%20%20%20key%3A%20Additive%0D%0A%7D%0D%0A%0D%0Ainterface%20MultiplicativeObj%20%7B%0D%0A%20%20%20%20key%3A%20Multiplicative%0D%0A%7D%0D%0A%0D%0Atype%20Obj%20%3D%20AdditiveObj%20%7C%20MultiplicativeObj%0D%0A%0D%0Aexport%20function%20foo(obj%3A%20Obj)%20%7B%0D%0A%20%20%20%20switch%20(obj.key)%20%7B%0D%0A%20%20%20%20%20%20%20%20case%20'%2B'%3A%20%7B%0D%0A%20%20%20%20%20%20%20%20%20%20%20%20onlyPlus(obj.key)%3B%0D%0A%20%20%20%20%20%20%20%20%20%20%20%20return%3B%0D%0A%20%20%20%20%20%20%20%20%7D%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0Afunction%20onlyPlus(arg%3A%20'%2B')%20%7B%0D%0A%20%20return%20arg%3B%0D%0A%7D%0D%0A

Related Issues:

Note:
This seems to be a regression between TS 3.1 and TS 3.2

Here's another example of a regression in type narrowing behavior from 3.0 that seems related. It seems like the item.user.login is being narrowed correctly in the example but the former is not. If you swap the order of the conditionals in the if statement then the error moves from id to the login property.

export interface FooItem {
  user: {
    id: string;
    login: string;
  };
}

export interface BarItem {
  user: {
    id: string | null;
    login: string | null;
  };
}

function getUser(item: FooItem | BarItem) {
  let user = { id: '', login: '' };
  if (item.user.id && item.user.login) {
    user = {
      id: item.user.id, // Type 'string | null' is not assignable to type 'string'.
      login: item.user.login,
    };
  }
  return user;
}

Playground link (turn on strictNullChecks)

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Switch narrowing regression with nested unions #29012

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Switch narrowing regression with nested unions #29012

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noahm commented on Dec 19, 2018

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