Partial application of functions loses type assertion information

[samuela]

Bug Report

🔎 Search Terms

type assertion, assert v is T, asserts condition, partial application, type inference, case splitting

🕗 Version & Regression Information

• This is the behavior in every version I tried, including 4.4.4 and 4.5.0-beta

⏯ Playground Link

Playground link with relevant code

💻 Code

// See https://github.com/Microsoft/TypeScript/issues/7556.
type ErrorT = { new (message: string): Error };
type Either = { tag: "Left"; a: string } | { tag: "Right"; b: number };

// --- Explicit version (assertion type information is propagated):
function assertExplicit(condition: boolean): asserts condition {
  if (!condition) {
    throw new Error("oops");
  }
}
function testWorks(x: Either) {
  assertExplicit(x.tag === "Right");
  console.log(x.b);
}

// --- Partial application version (assertion type information is lost):
function _assert(errorType: ErrorT, condition: boolean): asserts condition {
  if (!condition) {
    throw new errorType("oops");
  }
}
const assert = (condition: boolean) => _assert(Error, condition);
function testFails(x: Either) {
  assert(x.tag === "Left");
  console.log(x.a); // fails with "Property 'a' does not exist on type 'Either'."
}

export {};

🙁 Actual behavior

Type assertion information is lost outside of immediate call-sites. For example, if f(x): asserts <condition> then TS will only respect the assertion of the condition in the direct scope of where f is called and nowhere else, eg if g call f, there is no propagation of the information that asserts <condition> should hold in g.

🙂 Expected behavior

Assertion types to be propagated everywhere they apply.

[MartinJohns]

Working as intended. assert is not an asserting function. See #32695.

[samuela]

But why is assert not an asserting function? Simple control-flow analysis should reveal that it ought to have the same asserting properties as _assert since there's no try/catch in assert.

[devanshj]

As pointed out in #32695 these are the conditions for participation in CFA:

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

assert doesn't fulfil the third condition, it's not explicitly typed. There is a feature request to lift this condition: #45385

[typescript-bot]

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