Wrong type inference for block scoped variables

[DavideCanton]

TypeScript Version: 4.1.3

Search Terms: inference, variable, block, wrong

Expected behavior:
It should compile just fine.

Actual behavior:
Compilation error on row 16 due to wrong inference of variable.

Related Issues: not found

Code:

function setToTrue(func: (v: boolean) => void) {
    func(true);
}

function expect<T>(v: T): { toEqual: (t2: T) => void } {
    return {
        toEqual: v2 => {
            if (v !== v2) throw new Error('different values');
        }
    };
}

function foo() {
    let b: boolean | null = null;    
    setToTrue(x => b = x);
    expect(b).toEqual(true);
}

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

Type inference for variable "b" at row 14 is wrong, even if I specify explicitly "boolean | null". The compiler is unable to understand that b is effectively assigned immediately at line 15, and I find it to be correct, but it also infers that the actual type of the variable is just "null" and not "boolean | null" as I provided.
As a result, the code at line 16 (which I loosely borrowed from jasmine), does not compile, as the expect function is unable to get the property generic type instantiation, because T gets replaced with "null".

[MartinJohns]

Duplicate of #9998.

[DavideCanton]

Sorry, I read the whole issue but I did not find which of the examples is covering my case.

Nobody in #9998 is complaining about block-scoped variables with explicit type being wrongly narrowed.

[MartinJohns]

Type annotations for local variables are not considered if the variable doesn't get any other value assigned anywhere.

Some reason why foo is typed undefined and not number | undefined in this example:

const foo: number | undefined = undefined;
foo; // Hover over me.

This behavior is intentional, but causes problems with the issue from #9998.

Same issue as #40860. See also #8513.

[typescript-bot]

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