styled-components broken by #41841

[sandersn]

Found by overnight DT run for #41841

Here's a standalone repro; it's not minimal but it doesn't depend on anything else.

interface FunctionComponent<P = {}> {
    defaultProps?: Partial<P>;
}
type ComponentType<P = {}> = FunctionComponent<P>;
export type KProps<C extends string | ComponentType<any>> =
    object extends object ? Omit<C, any> & Partial<C> : never;
export interface StyledK<C extends string | ComponentType<any>> {
    (props: KProps<C>): never
}
type Constraint<T extends ComponentType> = T ;
export type StyledComponentInnerComponent<C> = C extends StyledK<infer I> ? I : C;
export type SProblem<C extends ComponentType<any>> = C extends StyledK<any> ? Constraint<StyledComponentInnerComponent<C>> : number;

Expected behavior:
No error, as in 4.1.

Actual behavior:
Error in SProblem on Constraint<StyledComponentInnerComponent<C>>:

error TS2344: Type 'StyledComponentInnerComponent<C>' does not satisfy the constraint 'ComponentType<{}>'.
  Type 'string | ComponentType<any> | C' is not assignable to type 'FunctionComponent<{}>'.
    Type 'string' has no properties in common with type 'FunctionComponent<{}>'.
      Type 'string | ComponentType<any>' is not assignable to type 'FunctionComponent<{}>'.
        Type 'string' has no properties in common with type 'FunctionComponent<{}>'.

[sandersn]

Here's a smaller repo. I think it better illustrates the problem:

interface StyledK<C extends string> {
    (props: {}): never
}
type Constraint<T extends { p }> = T ;
type Inner<C> = C extends StyledK<infer I> ? I : C;
export type SProblem<C extends { p }> = C extends StyledK<any> ? Constraint<Inner<C>> : number;

I guess before the string constraint on C from the outer C extends StyledK<any> ? ... got dropped? It kind of looks like this new error is correct.

[ahejlsberg]

I guess before the string constraint on C from the outer C extends StyledK ? ... got dropped? It kind of looks like this new error is correct.

Yes, that was the problem with not preserving substitution types in conditional types--constraints implied by outer conditional types were dropped. They're now preserved, which in this case reveals an error. If C is a StyledK<any> then Inner<C> is constrained to string | C, and string is not assignable to { p: any }.

BTW, I have no earthly idea what these types are supposed to be doing!

[sandersn]

Presumably they made sense in situ. The standalone repro chops them up pretty badly. I'll go back to the original and try to figure out a workaround.

(There are nearby comments in the original along the lines of "I'm quitting now before my head starts hurting again." So.)

[sandersn]

Later: It seems like the string constraint was a low-fi keyof JSX.IntrinsicElements substitute, because switching to that now compiles successfully. I'm afraid that it's going to be a lot slower though.

DefinitelyTyped/DefinitelyTyped#50046