Null check does not cause control flow analysis to narrow nullable interface property

[btoo]

Bug Report

🔎 Search Terms

interface optional properties props nullable nonnullable non-nullable control flow analysis type narrow

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed all the FAQ entries

⏯ Playground Link

Playground link with relevant code

💻 Code

interface NullableProps {
  nullableProp?: string
}

type RequiredProps = Required<NullableProps>

declare function withRequiredProps(requiredProps: RequiredProps): any;

function withNullableProps(props: NullableProps) {
  if (!props.nullableProp) return;

  /** this is correct */
  type ProperlyTypeNarrowedToBeNonNullable = typeof props.nullableProp

  /** but this is incorrect */
  type IncorrectlyStillNullableProps = typeof props;

  /**
   * Argument of type 'NullableProps' is not assignable to parameter of type 'Required<NullableProps>'.
   *   Types of property 'someNullableProp' are incompatible.
   *     Type 'string | undefined' is not assignable to type 'string'.
   *       Type 'undefined' is not assignable to type 'string'.(2345)
   */
  withRequiredProps(props)
}

🙁 Actual behavior

the line

if (!props.nullableProp) return;

does not narrow props to be { nullableProp: NonNullable }

🙂 Expected behavior

the line

if (!props.nullableProp) return;

should narrow props to be { nullableProp: NonNullable }

[jcalz]

duplicate of #31755 ?

[MartinJohns]

Duplicate of #42384 (which refers to #31755).

[btoo]

glad i'm not the only one. let's hope #42384 (comment) leads somewhere!