[Bug] Inferred types break with certain target types

[C-Higgins]

When the target type is certain combinations of union types, I think when the unions share a property, the type of the ActionFunction breaks and no longer knows the types of the parameters.

** Reproduce **

const test = createSchema<{a:string} | {a:string; b:string},{c:string}>({
  a: it => {} // Parameter 'it' implicitly has 'any' type
  b: it => {} // here 'it' is the correct type
})

As you can see, it should be of type {c:string}, for the first property, but it is not. The other parameters are also any. If you remove a from the second type, it works as expected.

[emyann]

@C-Higgins Thanks for taking the time to report this! Sounds like a weird narrowing/widening issue with Typescript Union Type being processed recursively by:

export type StrictSchema<Target = any, Source = any> = {
  /** `destinationProperty` is the name of the property of the target object you want to produce */
  [destinationProperty in keyof Target]:
    | ActionString<Source>
    | ActionFunction<Target, Source, Target[destinationProperty]>
    | ActionAggregator<Source>
    | ActionSelector<Source, Target>
    | StrictSchema<Target[destinationProperty], Source>;

export interface ActionFunction<D = any, S = any, R = any> {
  (iteratee: S, source: S[], target: D): R;
}

Let me dig this a bit, and come back to you when I have a clue :)

Edit: this is definitely a typescript widening behaviour

replacing

export type StrictSchema<Target = any, Source = any> = {
  /** `destinationProperty` is the name of the property of the target object you want to produce */
  [destinationProperty in keyof Target]:
    | ActionFunction<Target, Source, Target[destinationProperty]>
    ....

by

export type StrictSchema<Target = any, Source = any> = {
  /** `destinationProperty` is the name of the property of the target object you want to produce */
  [destinationProperty in keyof Target]:
    | (iteratee: Source, source: Source[], target:  Target[destinationProperty]) => Target[destinationProperty]

solves the issue 🙃

[emyann]

@C-Higgins This issue should have been fixed in #149 which is version 1.12.2, please let me know if you encounter any issues of this type again :)

https://repl.it/@yrnd1/GlitteringPleasedEquations