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Exhaustiveness checking doesn't work with variables #12668
Description
TypeScript Version: 2.1.1
If you have exhaustiveness checking where a result is a return from a function it works.
But if result is assigning value to a variable exhaustiveness checking basically does nothing.
Code
interface Square { kind: "square"; size: number; }
interface Rectangle { kind: "rectangle"; width: number; height: number; }
interface Circle { kind: "circle"; radius: number; }
interface Triangle { kind: "triangle"; side: number; }
type Shape = Square | Rectangle | Circle | Triangle;
function area(s: Shape): number {
let area;
switch (s.kind) {
case "square": area = s.size * s.size; break;
case "rectangle": area = s.width * s.height; break;
case "circle": area = Math.PI * s.radius * s.radius; break;
case "triangle": area = Math.sqrt(3) / 4 * s.side * s.side; break;
//default: area = -1; break;
}
//Error because area is number | undefined
return area;
}
function areaWrapped(s: Shape): number {
let area;
area = (() => {
switch (s.kind) {
case "square": return s.size * s.size;
case "rectangle": return s.width * s.height;
case "circle": return Math.PI * s.radius * s.radius;
case "triangle": return Math.sqrt(3) / 4 * s.side * s.side;
}
})();
return area;
}Expected behavior:
Function area: variable aria should be number because the check is exhaustive.
Sidenotes:
- If I uncomment
defaultthentscbelieves that check is exhaustive. areaWrappeduses IIFE to achieve the same effect and it works.
Actual behavior:
Variable area is of type number | undefined while it can't be undefined.
Activity
RyanCavanaugh commentedon Jul 29, 2019
There are architectural / phase-ordering things that make this more complex than it first appears. Anyway adding a sensible
default: throw new Error();case makes the error go away, or addlet! area;