Declaration emit for named inferred types used in further inference inlines inferred types instead of naming them

[chriskrycho]

Steps to reproduce

When emitting types for a type that TS must infer, e.g. the members of a union that derive from some other type, and that inferred type is used in another context where inference is involved (such as const foo = ... where the type of foo is not explicitly specified), tsgo will inline the type even if it is bound to a name.

This is enough to trigger the behavior:

const values = ["a", "b"] as const;
type Value = typeof values[number];

export const getValue = (): Value => "a";

This can cause significantly slower type checking for packages that consume these emitted declarations. For example, we have one project (using tsgo) that regressed from type checking in ~35s to taking over 6m 30s after a package that it depended on started using tsgo for emit.

This repo has several reproductions.

Behavior with typescript@6.0

TS emits named references to the inferred types when they have an intermediate name in cases like this:

export declare const getValue: () => Value;

Behavior with tsgo

tsgo emits an inlined version of the type alias.

export declare const getValue: () => "a" | "b";

[chriskrycho]

Additional info from some exploration with @jakebailey: patching canReuseTypeNode() in Strada to always return false has the same behavior: it inlines the types rather than reusing the reference to the name.

[jakebailey]

I would expect this to be fixed by #1693 / #2459, then.

[chriskrycho]

Local testing confirms that it will indeed be fixed by that. With this change, the consuming package in our monorepo type checks in ~24s; without it, ~469s. So that’ll be a huge win for us—a ~95% reduction/19.5× faster. 🙏🏼