You can play with the above example in the TypeScript Playground.
This would be a duplicate of this existing issue.
Update: this is another example where it's clear that explicitly creating a valid action works while asking TS to check all the possibible cases fails, probably because of an internal, hardcoded, limit.
| /* | |
| Narrowing down type issue: the 26th "success action" breaks TS at | |
| `const action: Action<AcceptedType> = { type, success: true }` | |
| with the following error: | |
| ``` | |
| Type | |
| '{ type: "Action_1" | "Action_2" | "Action_3" | "Action_4" | "Action_5" | "Action_6" | "Action_7" | "Action_8" | "Action_9" | "Action_10" | "Action_11" | "Action_12" | "Action_13" | ... 12 more ... | "Action_26"; success: true; }' | |
| is not assignable to type 'ActionSuccess'. | |
| Type | |
| '{ type: "Action_1" | "Action_2" | "Action_3" | "Action_4" | "Action_5" | "Action_6" | "Action_7" | "Action_8" | "Action_9" | "Action_10" | "Action_11" | "Action_12" | "Action_13" | ... 12 more ... | "Action_26"; success: true; }' | |
| is not assignable to type 'Action_26_Success'. | |
| Types of property 'type' are incompatible. | |
| Type | |
| '"Action_1" | "Action_2" | "Action_3" | "Action_4" | "Action_5" | "Action_6" | "Action_7" | "Action_8" | "Action_9" | "Action_10" | "Action_11" | "Action_12" | "Action_13" | "Action_14" | ... 11 more ... | "Action_26"' | |
| is not assignable to type '"Action_26"'. | |
| Type '"Action_1"' is not assignable to type '"Action_26"'. | |
| ``` | |
| by removing the `success: true` from the 26th action, or from another random action, the error disappears. | |
| */ | |
| interface Action_0_Failure { type: 'Action_0', failure: true } | |
| interface Action_1_Success { type: 'Action_1', success: true } | |
| interface Action_2_Success { type: 'Action_2', success: true } | |
| interface Action_3_Success { type: 'Action_3', success: true } | |
| interface Action_4_Success { type: 'Action_4', success: true } | |
| interface Action_5_Success { type: 'Action_5', success: true } | |
| interface Action_6_Success { type: 'Action_6', success: true } | |
| interface Action_7_Success { type: 'Action_7', success: true } | |
| interface Action_8_Success { type: 'Action_8', success: true } | |
| interface Action_9_Success { type: 'Action_9', success: true } | |
| interface Action_10_Success { type: 'Action_10', success: true } | |
| interface Action_11_Success { type: 'Action_11', success: true } | |
| interface Action_12_Success { type: 'Action_12', success: true } | |
| interface Action_13_Success { type: 'Action_13', success: true } | |
| interface Action_14_Success { type: 'Action_14', success: true } | |
| interface Action_15_Success { type: 'Action_15', success: true } | |
| interface Action_16_Success { type: 'Action_16', success: true } | |
| interface Action_17_Success { type: 'Action_17', success: true } | |
| interface Action_18_Success { type: 'Action_18', success: true } | |
| interface Action_19_Success { type: 'Action_19', success: true } | |
| interface Action_20_Success { type: 'Action_20', success: true } | |
| interface Action_21_Success { type: 'Action_21', success: true } | |
| interface Action_22_Success { type: 'Action_22', success: true } | |
| interface Action_23_Success { type: 'Action_23', success: true } | |
| interface Action_24_Success { type: 'Action_24', success: true } | |
| interface Action_25_Success { type: 'Action_25', success: true } | |
| interface Action_26_Success { type: 'Action_26', success: true } // the 26th action with `success: true` causes the problem. By removing `success: true` the error disappears | |
| type ActionSuccess = Action_1_Success | Action_2_Success | Action_3_Success | Action_4_Success | Action_5_Success | Action_6_Success | Action_7_Success | Action_8_Success | Action_9_Success | Action_10_Success | Action_11_Success | Action_12_Success | Action_13_Success | Action_14_Success | Action_15_Success | Action_16_Success | Action_17_Success | Action_18_Success | Action_19_Success | Action_20_Success | Action_21_Success | Action_22_Success | Action_23_Success | Action_24_Success | Action_25_Success | Action_26_Success | |
| // see https://stackoverflow.com/a/50499316/3508387 | |
| type NarrowAction<T, K> = T extends { type: K } ? T : never | |
| type Action<K extends ActionSuccess['type']> = NarrowAction<ActionSuccess, K> | |
| type GenericSuccess<T> = T extends { success: true } ? T : never | |
| type AcceptedType = GenericSuccess<ActionSuccess>['type'] | |
| const callback = (type: AcceptedType) => { | |
| const action: Action<AcceptedType> = { type, success: true } // here's the TS error | |
| return action | |
| } | |
| // correct usage | |
| callback('Action_1') | |
| callback('Action_25') | |
| // wrong usage, protected by TS | |
| // @ts-expect-error | |
| callback('Action_0') | |
| // @ts-expect-error | |
| callback('unexisting action') |