Specifying return type 'never' on function expression doesn't work, but it works on function declaration.

[yyijoo]

Acknowledgement

• I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

In the code below, mainBar and mainQux are exhibiting errors as if the return type of bar() and qux() were 'undefined'.
Is it by intention or is it kind of a bug?
If it's so by intention, What's inferred differently?

function foo(): never {
 throw new Error();
}

const bar = (): never => {
 throw new Error();
}

const qux = function():never {
 throw new Error();
}

function mainFoo(a: number | undefined): number {
 if (a !== undefined) {
 return a;
 }
 foo();
}

function mainBar(a: number | undefined): number {
 if (a !== undefined) {
 return a;
 }
 bar();
}

function mainQux(a: number | undefined): number {
 if (a !== undefined) {
 return a;
 }
 qux();
}

^(playground)

[fatcerberus]

Duplicate of #46254 and others. It's a known design limitation that all identifiers in a call expression must have explicit type annotations in order for the call to affect control flow, but the type of bar in your example is inferred (the : never return annotation applies to the arrow function expression itself, so doesn't count).

See #32695, which implemented the feature.

A function call is analyzed as an assertion call or never-returning call when

• the call occurs as a top-level expression statement, and
• the call specifies a single identifier or a dotted sequence of identifiers for the function name, and
• each identifier in the function name references an entity with an explicit type, and
• the function name resolves to a function type with an asserts return type or an explicit never return type annotation.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.