Description
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".