Type guards fail in switch (true) constructs.

[sccolbert]

Sometimes it's nice to use switch (true) along with boolean expression case clauses to make code cleaner than a bunch of if/else statements. Unfortunately, type guards do not work here:

TypeScript Version: 2.3.1

Code

interface Square {
  type: 'square';
  size: number;
}

interface Rectangle {
  type: 'rectangle';
  width: number;
  height: number;
}

type Shape = Square | Rectangle;

function isSquare(shape: Shape): shape is Square {
  return shape.type === 'square';
}

function isRectangle(shape: Shape): shape is Rectangle {
  return shape.type === 'rectangle';
}


let shape: Shape;

switch (true) {
  case shape.type === 'square':
    shape.size;  // fail
    break;
  case shape.type === 'rectangle':
    shape.width; // fail
    break;
}


switch (true) {
  case isSquare(shape):
    shape.size;  // fail
    break;
  case isRectangle(shape):
    shape.width; // fail
    break;
}


switch (shape.type) {
  case 'square':
    shape.size;  // okay
    break;
  case 'rectangle':
    shape.width; // okay
    break;
}


if (isSquare(shape)) {
  shape.size;  // okay
}

if (isRectangle(shape)) {
  shape.width;  // okay
  shape.height; // okay
}

Expected behavior:

All forms should compile.

Actual behavior:

The switch (true) forms do not compile.

[yortus]

Duplicate of #8934