Type guard on conditional parameter type

[jer-sen]

TypeScript Version: 3.9.2

Search Terms:

Expected behavior: No error, Typescript should guess that a variable of type T extends '1' ? '1' : never will never be false, so that NonFalse<TypeWithCondition<T>> is exactly the same than true | (T extends '1' ? '1' : never)

Actual behavior: Error on logNonFalse(a);

Related Issues: none

Code

type TypeWithCondition<T> = boolean | (T extends '1' ? '1' : never);

type NonFalse<T> = Exclude<T, false>;

function logNonFalse<T>(b: NonFalse<TypeWithCondition<T>>) {
    console.log(b);
};

function test1<T>(a: TypeWithCondition<T>) {
    if (a === false) throw new Error("Can't be false");
    logNonFalse(a);
    // Error : Argument of type 'true | ([T] extends ["1"] ? "1" : never)' is not assignable to parameter of type 'true'.
    //  Type '[T] extends ["1"] ? "1" : never' is not assignable to type 'true'.
    //    Type '"1"' is not assignable to type 'true'.(2345)
};

Output

"use strict";
function logNonFalse(b) {
    console.log(b);
}
;
function test1(a) {
    if (a === false)
        throw new Error("Can't be false");
    logNonFalse(a);
    // Error : Argument of type 'true | ([T] extends ["1"] ? "1" : never)' is not assignable to parameter of type 'true'.
    //  Type '[T] extends ["1"] ? "1" : never' is not assignable to type 'true'.
    //    Type '"1"' is not assignable to type 'true'.(2345)
}
;

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "ES2017",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

[RyanCavanaugh]

We can't analyze narrowing like this without negated types (#29317). Even then this might require higher order reasoning than that