Unreachable returns badly break return type inference

リンクされている TypeScript Playground の再現例から始め、allowUnreachableCode 下での f、g、h の推論された宣言を比較します。コンパイラによる戻り値の型の推論と到達不能コードの処理を追跡します。g と h が f と同様に number を推論し、関連する noImplicitAny 診断が生成されれば完了です。

🔎 Search Terms

unreachable any return

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about let

⏯ Playground Link

https://www.typescriptlang.org/play?target=7&jsx=0&ts=5.2.0-beta&allowUnreachableCode=true#code/GYVwdgxgLglg9mABMAFASgN6IPTcQBwCc58BTQgGwE9FDThy6ATRAQwGdEwQBbAI3IAoAJAVSURAA8A3FMQBeRAEZZdKCEJIZI3IgBypSRIowwpREzilOYOBNbAG0WuI1IoVMolMM6kUgBcIpIKiABEwHBwYYIAvoKgkLAIiADm6Fi6PoykLBxsYFQiYhIycooqLuqaUtI6eAZGiCZmiADWMBQUnGpuiB5e2X4QgSK9NdrxidDwSAAWGTh4Q8xsnKyFY641KvX6hsam5h1dPdvunuZDpP5BouKIVLI0Farnj3XxQA

💻 Code

function f(){ // properly referred as number
	let x; x = 1; return x;
	// Next line does not affect return type inference:
	x = "foo"
}
function g(){ // inferred as any
	let x; x = 1; return x;
	// Next line kills return type inference:
	return x;
}
function h(){ // inferred as any
	return 1;
	// Next line kills return type inference:
	let y; y = 1; return y;
}

🙁 Actual behavior

The types are inferred as:

declare function f(): number;
declare function g(): any;
declare function h(): any;

The case of f demonstrates that return types based on assignment to mutable variables can work. The unreachable assignment to x after return has no effect on the return type.

In the case of g, the return type should be unaffected - x is already inferred to be number and an extra return does not change the type of x.

In the case of h, the return type should be unaffected as well. The second return can be inferred to be number.

🙂 Expected behavior

The types should be inferred as:

declare function f(): number;
declare function g(): number;
declare function h(): number;

Either unreachable return statements should not affect return type or return should not stop type inference.

Also, the code should be flagged by the noImplicitAny rule.

Additional information about the issue

possibly related to #40393
likely related to #26914

Disabling allowUnreachableCode does not alleviate these issues.