strictNullChecks: false positive with switch in for loop #27239
Description
TypeScript Version: 3.1.0-dev.20180920
Search Terms: strictNullChecks
Code
This might seem weird, but is based on actual real-world code.
declare function baz(): Promise<true | "a" | "b" | "c" | Error>;
async function foo() {
let bar: "x" | Error;
for (let i = 0; i < 3; i++) {
const test = await baz();
switch (test) {
case true: return;
case "a": throw new Error("a");
case "b": {
bar = "x";
continue;
}
default: {
if (test instanceof Error) {
bar = new Error("foo");
continue;
} else {
throw new Error("c");
}
}
}
}
// false positive: bar is used before being assigned
if (bar === "x") {
} else {
bar.message = "something";
}
}Expected behavior:
No error after the for loop. The only way to get there is if bar has the value "x" or is an instance of Error. If baz resolves to true, the function returns, "b" sets bar to "x", and "a" and everything that is not an Error (including "c" throw, and an Error sets it to an Error.
Actual behavior:
Variable "bar" is used before being assigned.
Related Issues: #24091