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Type inference problem with generics and Readonly #26600
Description
TypeScript Version: 3.0.1
Search Terms:
Type inference generics Readonly
Code
class ReactComponent<P>
{
public constructor(public props: Readonly<P>)
{
}
}
interface XampleProps
{
x?: boolean;
}
class Xample<PT extends XampleProps = XampleProps> extends ReactComponent<PT>
{
public render()
{
const localX = this.props.x;
return this.doSthWithX(localX || false);
}
private doSthWithX(data: boolean): boolean
{
return data;
}
}Expected behavior:
Code should compile with no errors.
Actual behavior:
The compiler complains about the return this.doSthWithX(localX || false); line. It says:
Argument of type 'PT["x"]' is not assignable to parameter of type 'boolean'.
Type 'boolean | undefined' is not assignable to type 'boolean'.
Type 'undefined' is not assignable to type 'boolean'.
Adding the explicit type for localX solves the problem.
class Xample<PT extends XampleProps = XampleProps> extends ReactComponent<PT>
{
public render()
{
const localX: boolean | undefined = this.props.x;
return this.doSthWithX(localX || false);
}
private doSthWithX(data: boolean): boolean
{
return data;
}
}Removing the Readonly from the ReactComponent also eliminates the compiler error.
Playground Link:
Related Issues:
Perhaps #26418
Activity
RyanCavanaugh commentedon Oct 10, 2018
Requires strictNullChecks on; not a regression
RyanCavanaugh commentedon Oct 10, 2018
We'd need #22348 or something similar to correctly typecheck this -- it requires the checker understanding that all of the truthy values of the mapped type's
xproperty areboolean, which is not immediately apparent.