Bug: return type duplicate infinite amount: | T

[pjonsson]

🐛 Bug Report

• TypeStat version: 0.8.1
• TypeScript version: 5.2
• Node version: 20

Actual Behavior

Grabbing one from the pile here, this isn't the only one, but the rest look similar:

diff --git a/lib/Models/SelectableDimensions/SelectableDimensions.ts b/lib/Models/SelectableDimensions/SelectableDimensions.ts
index b36326626..1747314a8 100644
--- a/lib/Models/SelectableDimensions/SelectableDimensions.ts
+++ b/lib/Models/SelectableDimensions/SelectableDimensions.ts
@@ -19,7 +19,7 @@ export interface EnumDimensionOption<T = string> {
 }

 export interface EnumDimension<T = string> extends Dimension {
-  readonly options?: readonly EnumDimensionOption<T>[];
+  readonly options?: readonly EnumDimensionOption<T>[] | T | T;
   readonly selectedId?: T;
   readonly allowUndefined?: boolean;
   /** If true, then the user can set the value to arbitrary text */

I'm guessing this is related to #1447, but for a different type.

Expected Behavior

I'm not certain that the type on the interface originally is correct because TypeStat has found some similar type issues in other places (relating to promises), so it's quite possible that the interface should have the type EnumDimensionOption<T>[] | T. I expect TypeStat to stop adding | T after the first one is added though.

Is there a way to find the code point that caused TypeStat to add the type? I'm familiar with how Hindley-Milner type inference works, so I realize it might be hard to give a good code point, but a bad code point that at least was involved in constraining the type would still be better than the current "it's somewhere in the source code" black box.

Reproduction

Same as #2014.

[JoshuaKGoldberg]

I'm familiar with how Hindley-Milner type inference works

I'm not 😄. I've tried to keep TypeStat to just use TypeScript compiler APIs and just basic arrays, Maps, and Sets - so that folks don't need to ramp up on deeper type theory to contribute.

If you run with --logfile some-log-file.txt then it'll [log] output the specific fixers that resulted in each mutation. Looking at this file's:

[log] fixReactPropsFromUses found mutations in /Users/josh/repos/terriajs/lib/Models/SelectableDimensions/SelectableDimensions.ts: [
    {
        "mutations": [
            {
                "mutations": [
                    {
                        "insertion": " | T",
                        "range": {
                            "begin": 853
                        },
                        "type": "text-insert"

Digging into the code a bit, fixReactPropsFromUses is calling to the shared createTypeExpansionMutation helper. My hunch is that createTypeExpansionMutation isn't factoring in T generics / deduplicating added type strings as it should. Fun!

[rubiesonthesky]

Is there some minimal reproduction of this? Preferably something that would fit into one file easily. I'd like to add this as a mutation test to see if we already fixed this or if not, then it would be easier to investigate where we are going wrong :)

Just by adding this bit does not seem to trigger the problem

	interface Dimension {
		/** Machine readable ID */
		readonly id?: string;
		/** Human readable name */
		readonly name?: string;
	}

	interface EnumDimensionOption<T = string> {
		readonly id?: T;
		readonly name?: string;
	}

	interface EnumDimension<T = string> extends Dimension {
		readonly options?: readonly EnumDimensionOption<T>[];
		readonly selectedId?: T;
		readonly allowUndefined?: boolean;
		/** If true, then the user can set the value to arbitrary text */
		readonly allowCustomInput?: boolean;
		readonly undefinedLabel?: string;
	}

[JoshuaKGoldberg]

Is there some minimal reproduction of this?

I tried for a bit and couldn't figure it out quickly. So I gave up. But yes +1 that a PR fixing this should ideally have a test case added!