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Merged
merged 2 commits into from
Sep 29, 2025
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Merged

Port microsoft/typescript-go#1757 #62501

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4e59e52
Port https://github.com/microsoft/typescript-go/pull/1757
ahejlsberg Sep 28, 2025
f939285
Add regression test
ahejlsberg Sep 28, 2025
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32 changes: 15 additions & 17 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -27888,24 +27888,22 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
for (const type of types) {
if (type.flags & (TypeFlags.Object | TypeFlags.Intersection | TypeFlags.InstantiableNonPrimitive)) {
const discriminant = getTypeOfPropertyOfType(type, name);
if (discriminant) {
if (!isLiteralType(discriminant)) {
return undefined;
}
let duplicate = false;
forEachType(discriminant, t => {
const id = getTypeId(getRegularTypeOfLiteralType(t));
const existing = map.get(id);
if (!existing) {
map.set(id, type);
}
else if (existing !== unknownType) {
map.set(id, unknownType);
duplicate = true;
}
});
if (!duplicate) count++;
if (!discriminant || !isLiteralType(discriminant)) {
return undefined;
}
let duplicate = false;
forEachType(discriminant, t => {
const id = getTypeId(getRegularTypeOfLiteralType(t));
const existing = map.get(id);
if (!existing) {
map.set(id, type);
}
else if (existing !== unknownType) {
map.set(id, unknownType);
duplicate = true;
}
});
if (!duplicate) count++;
}
}
return count >= 10 && count * 2 >= types.length ? map : undefined;
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41 changes: 41 additions & 0 deletions tests/baselines/reference/missingDiscriminants2.errors.txt
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@@ -0,0 +1,41 @@
missingDiscriminants2.ts(30,23): error TS2353: Object literal may only specify known properties, and 'subkind' does not exist in type '{ kind: "b"; }'.
missingDiscriminants2.ts(31,34): error TS2353: Object literal may only specify known properties, and 'subkind' does not exist in type '{ kind: "b"; }'.


==== missingDiscriminants2.ts (2 errors) ====
// https://github.com/microsoft/typescript-go/issues/1020

type Thing =
| { str: "a", num: 0 }
| { str: "b" }
| { str: "c" }
| { str: "d" }
| { str: "e" }
| { str: "f" }
| { str: "g" }
| { str: "h" }
| { str: "i" }
| { str: "j" }
| { str: "k" }
| { str: "l" }
| { str: "m" }
| { str: "n" }
| { str: "o" }
| { num: 1 }

const thing1: Thing = { str: "a", num: 0 }
const thing2: Thing = { str: "b", num: 1 } // Shouldn't be error
const thing3: Thing = { num: 1, str: "b" } // Shouldn't be error

type Item =
| { kind: "a", subkind: 0, value: string }
| { kind: "a", subkind: 1, value: number }
| { kind: "b" }

const item1: Item = { subkind: 1, kind: "b" } // Error, type "b" not assignable to type "a"
~~~~~~~
!!! error TS2353: Object literal may only specify known properties, and 'subkind' does not exist in type '{ kind: "b"; }'.
const item2: Item = { kind: "b", subkind: 1 } // Error, 'subkind' isn't a known property
~~~~~~~
!!! error TS2353: Object literal may only specify known properties, and 'subkind' does not exist in type '{ kind: "b"; }'.

103 changes: 103 additions & 0 deletions tests/baselines/reference/missingDiscriminants2.symbols
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//// [tests/cases/compiler/missingDiscriminants2.ts] ////

=== missingDiscriminants2.ts ===
// https://github.com/microsoft/typescript-go/issues/1020

type Thing =
>Thing : Symbol(Thing, Decl(missingDiscriminants2.ts, 0, 0))

| { str: "a", num: 0 }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 3, 5))
>num : Symbol(num, Decl(missingDiscriminants2.ts, 3, 15))

| { str: "b" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 4, 5))

| { str: "c" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 5, 5))

| { str: "d" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 6, 5))

| { str: "e" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 7, 5))

| { str: "f" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 8, 5))

| { str: "g" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 9, 5))

| { str: "h" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 10, 5))

| { str: "i" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 11, 5))

| { str: "j" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 12, 5))

| { str: "k" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 13, 5))

| { str: "l" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 14, 5))

| { str: "m" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 15, 5))

| { str: "n" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 16, 5))

| { str: "o" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 17, 5))

| { num: 1 }
>num : Symbol(num, Decl(missingDiscriminants2.ts, 18, 5))

const thing1: Thing = { str: "a", num: 0 }
>thing1 : Symbol(thing1, Decl(missingDiscriminants2.ts, 20, 5))
>Thing : Symbol(Thing, Decl(missingDiscriminants2.ts, 0, 0))
>str : Symbol(str, Decl(missingDiscriminants2.ts, 20, 23))
>num : Symbol(num, Decl(missingDiscriminants2.ts, 20, 33))

const thing2: Thing = { str: "b", num: 1 } // Shouldn't be error
>thing2 : Symbol(thing2, Decl(missingDiscriminants2.ts, 21, 5))
>Thing : Symbol(Thing, Decl(missingDiscriminants2.ts, 0, 0))
>str : Symbol(str, Decl(missingDiscriminants2.ts, 21, 23))
>num : Symbol(num, Decl(missingDiscriminants2.ts, 21, 33))

const thing3: Thing = { num: 1, str: "b" } // Shouldn't be error
>thing3 : Symbol(thing3, Decl(missingDiscriminants2.ts, 22, 5))
>Thing : Symbol(Thing, Decl(missingDiscriminants2.ts, 0, 0))
>num : Symbol(num, Decl(missingDiscriminants2.ts, 22, 23))
>str : Symbol(str, Decl(missingDiscriminants2.ts, 22, 31))

type Item =
>Item : Symbol(Item, Decl(missingDiscriminants2.ts, 22, 42))

| { kind: "a", subkind: 0, value: string }
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 25, 5))
>subkind : Symbol(subkind, Decl(missingDiscriminants2.ts, 25, 16))
>value : Symbol(value, Decl(missingDiscriminants2.ts, 25, 28))

| { kind: "a", subkind: 1, value: number }
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 26, 5))
>subkind : Symbol(subkind, Decl(missingDiscriminants2.ts, 26, 16))
>value : Symbol(value, Decl(missingDiscriminants2.ts, 26, 28))

| { kind: "b" }
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 27, 5))

const item1: Item = { subkind: 1, kind: "b" } // Error, type "b" not assignable to type "a"
>item1 : Symbol(item1, Decl(missingDiscriminants2.ts, 29, 5))
>Item : Symbol(Item, Decl(missingDiscriminants2.ts, 22, 42))
>subkind : Symbol(subkind, Decl(missingDiscriminants2.ts, 29, 21))
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 29, 33))

const item2: Item = { kind: "b", subkind: 1 } // Error, 'subkind' isn't a known property
>item2 : Symbol(item2, Decl(missingDiscriminants2.ts, 30, 5))
>Item : Symbol(Item, Decl(missingDiscriminants2.ts, 22, 42))
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 30, 21))
>subkind : Symbol(subkind, Decl(missingDiscriminants2.ts, 30, 32))

169 changes: 169 additions & 0 deletions tests/baselines/reference/missingDiscriminants2.types
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//// [tests/cases/compiler/missingDiscriminants2.ts] ////

=== missingDiscriminants2.ts ===
// https://github.com/microsoft/typescript-go/issues/1020

type Thing =
>Thing : Thing
> : ^^^^^

| { str: "a", num: 0 }
>str : "a"
> : ^^^
>num : 0
> : ^

| { str: "b" }
>str : "b"
> : ^^^

| { str: "c" }
>str : "c"
> : ^^^

| { str: "d" }
>str : "d"
> : ^^^

| { str: "e" }
>str : "e"
> : ^^^

| { str: "f" }
>str : "f"
> : ^^^

| { str: "g" }
>str : "g"
> : ^^^

| { str: "h" }
>str : "h"
> : ^^^

| { str: "i" }
>str : "i"
> : ^^^

| { str: "j" }
>str : "j"
> : ^^^

| { str: "k" }
>str : "k"
> : ^^^

| { str: "l" }
>str : "l"
> : ^^^

| { str: "m" }
>str : "m"
> : ^^^

| { str: "n" }
>str : "n"
> : ^^^

| { str: "o" }
>str : "o"
> : ^^^

| { num: 1 }
>num : 1
> : ^

const thing1: Thing = { str: "a", num: 0 }
>thing1 : Thing
> : ^^^^^
>{ str: "a", num: 0 } : { str: "a"; num: 0; }
> : ^^^^^^^^^^^^^^^^^^^^^
>str : "a"
> : ^^^
>"a" : "a"
> : ^^^
>num : 0
> : ^
>0 : 0
> : ^

const thing2: Thing = { str: "b", num: 1 } // Shouldn't be error
>thing2 : Thing
> : ^^^^^
>{ str: "b", num: 1 } : { str: "b"; num: 1; }
> : ^^^^^^^^^^^^^^^^^^^^^
>str : "b"
> : ^^^
>"b" : "b"
> : ^^^
>num : 1
> : ^
>1 : 1
> : ^

const thing3: Thing = { num: 1, str: "b" } // Shouldn't be error
>thing3 : Thing
> : ^^^^^
>{ num: 1, str: "b" } : { num: 1; str: "b"; }
> : ^^^^^^^^^^^^^^^^^^^^^
>num : 1
> : ^
>1 : 1
> : ^
>str : "b"
> : ^^^
>"b" : "b"
> : ^^^

type Item =
>Item : Item
> : ^^^^

| { kind: "a", subkind: 0, value: string }
>kind : "a"
> : ^^^
>subkind : 0
> : ^
>value : string
> : ^^^^^^

| { kind: "a", subkind: 1, value: number }
>kind : "a"
> : ^^^
>subkind : 1
> : ^
>value : number
> : ^^^^^^

| { kind: "b" }
>kind : "b"
> : ^^^

const item1: Item = { subkind: 1, kind: "b" } // Error, type "b" not assignable to type "a"
>item1 : Item
> : ^^^^
>{ subkind: 1, kind: "b" } : { subkind: number; kind: "b"; }
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>subkind : number
> : ^^^^^^
>1 : 1
> : ^
>kind : "b"
> : ^^^
>"b" : "b"
> : ^^^

const item2: Item = { kind: "b", subkind: 1 } // Error, 'subkind' isn't a known property
>item2 : Item
> : ^^^^
>{ kind: "b", subkind: 1 } : { kind: "b"; subkind: number; }
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>kind : "b"
> : ^^^
>"b" : "b"
> : ^^^
>subkind : number
> : ^^^^^^
>1 : 1
> : ^

34 changes: 34 additions & 0 deletions tests/cases/compiler/missingDiscriminants2.ts
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// @strict: true
// @noEmit: true

// https://github.com/microsoft/typescript-go/issues/1020

type Thing =
| { str: "a", num: 0 }
| { str: "b" }
| { str: "c" }
| { str: "d" }
| { str: "e" }
| { str: "f" }
| { str: "g" }
| { str: "h" }
| { str: "i" }
| { str: "j" }
| { str: "k" }
| { str: "l" }
| { str: "m" }
| { str: "n" }
| { str: "o" }
| { num: 1 }

const thing1: Thing = { str: "a", num: 0 }
const thing2: Thing = { str: "b", num: 1 } // Shouldn't be error
const thing3: Thing = { num: 1, str: "b" } // Shouldn't be error

type Item =
| { kind: "a", subkind: 0, value: string }
| { kind: "a", subkind: 1, value: number }
| { kind: "b" }

const item1: Item = { subkind: 1, kind: "b" } // Error, type "b" not assignable to type "a"
const item2: Item = { kind: "b", subkind: 1 } // Error, 'subkind' isn't a known property
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