TS2590: "Expression produces a union type that is too complex to represent" on generics types

[victorgarciaesgi]

Steps to reproduce

Repository: https://github.com/victorgarciaesgi/regle/tree/FRONT/experimental/tsgo
Branch: experimental/tsgo

1. Run npx corepack enable pnpm to install the correct package manager
2. Run nvm use
3. Run pnpm install
4. Run pnpm run build to build all monorepo packages

Then run pnpm run typecheck --filter="@regle/core"
Or pnpm run typecheck --filter="@regle/rules"

Behavior with typescript@5.9

No type error, inference works well as it can be seen here in playground

Behavior with tsgo

tsgo produces the same error on a lot of generics, saying Expression produces a union type that is too complex to represent.

[jakebailey]

It's pretty much guaranteed that this is a union ordering problem. You have a UnionToTuple which is going to be sensitive to this.

You can try the build of tsc on microsoft/TypeScript#63084 (comment) with --stableTypeOrdering and see if this reproduces there too to double check.

[victorgarciaesgi]

Thanks for your response! The UnionToTuple produces a different error (TS2321).
The one that produce the error is a pretty simple infering generic

I'll try the stableTypeOrdering 👍

[victorgarciaesgi]

@jakebailey even with stableTypeOrdering it didn't work.

But I narrowed it down with dichotomy and it wasn't linked to the UnionToTuple type, but to a more complex recursive type.
I fixed type by adding an additionnal isAny<T[K]> and isUnknown<T[K]> on the mapped object type.

I'm having an other problem though but seems more like a crash (only on one package)

I will also use dichotomy to find the cause 👍

Weird thing tho: It doesn't work if I have "include": ["./src/**/*.ts"]" in the tsconfic, but it works when doing independant folders (like "include": ["./src/types/*.ts"]"), and they all build independantly

Otherwise i'll open another issue

[victorgarciaesgi]

Also found the problem for the second, it was a type helper that was augmenting tuple params, and it was crashing if I was augmenting a tuple that had a optional first parameter

The type (simplified):

type ParamsToLooseParams<TParams extends any[], TLoose extends unknown[] = unknown[]> = 
TParams extends [] ? [...TLoose] : TParams extends [infer TFirst, ...infer TRest] ? [TFirst, ...TRest, ...TLoose] : [...TParams, ...TLoose];

Working

ParamsToLooseParams<[foo: number], [condition: boolean]>

Crashing

ParamsToLooseParams<[foo?: number], [condition: boolean]>