Regression in 3.4: Type inference with generic type guard breaks

[timsuchanek]

TypeScript Version: 3.5.0-dev.20190424

Search Terms:
generic type parameters broken, regression 3.4, type inference for mapped conditional types broken

Code

type UserArgs = {
  select?: boolean
}

type Subset<T, U> = { [key in keyof T]: key extends keyof U ? T[key] : never }

function withBoundary<T extends UserArgs>(args?: Subset<T, UserArgs>): T {
  return null as any
}

const boundaryResult = withBoundary({
  select: true,
})

// In TypeScript@3.3:
/**
 * typeof boundaryResult === {
 *   select: true
 * }
 */

// In TypeScript@3.4 @ TypeScript@next:
/**
 * typeof boundaryResult === UserArgs
 */

function withoutBoundary<T extends UserArgs>(args?: T): T {
  return null as any
}

const withoutBoundaryResult = withoutBoundary({
  select: true,
})

// In TypeScript@3.3:
/**
 * typeof withoutBoundaryResult === {
 *   select: true
 * }
 */

// In TypeScript@3.4 & TypeScript@next:
/**
 * typeof withoutBoundaryResult === {
 *   select: true
 * }
 */

Expected behavior:
I expect the return type of the withBoundary function to actually include the fine-grain information of which concrete boolean value is provided for the select property. It should have type {select: true}.

Actual behavior:
Instead, it's typecasted to UserArgs.

We need this behavior to work because we want to make sure that users of our library only pass in a "subset" and not a superset of the UserArgs type into the withBoundary function. As T extends UserArgs is not sufficient to narrow that down, we use (args?: Subset<T, UserArgs>). This worked in 3.3 and is now broken in 3.4.

Playground Link

Related Issues:

• Suggestion: Type guard generic types #4742
• Conditional types are incorrectly narrowed  #30152
• Nested conditional type with generic tuple argument always expands to false branch. #30708
• 3.4 Regression on Type inference with union types #30694