Type guard function does not work for string literal types

[kimamula]

TypeScript Version:

1.8.2

Code

type Click = 'click';

function isClick(event: string): event is Click {
 return event === 'click';
}

function validateClickEvent(event: string): Click {
 if (isClick(event)) {
 return event; // this line raises a compilation error: Type 'string' is not assignable to type '"click"'
 }
 throw new Error('not click');
}

type Creature = {};
type Animal = {} & Creature;

function isAnimal(creature: Creature): creature is Animal {
 return true;
}

function validateAnimalCreature(creature: Creature): Animal {
 if (isAnimal(creature)) {
 return creature; // this line does not raise any compilation error
 }
 throw new Error('not animal');
}

Expected behavior:

Can apply type guard to string literal types.

Actual behavior:

Cannot apply type guard to string literal types.

[DanielRosenwasser]

Related to #7224, and a fix exists at #7235.

[sandersn]

Fixed by #7235