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Fix translation error that disabled union discrimination optimization #1757

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Sep 29, 2025
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Fix translation error that disabled union discrimination optimization #1757

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5c43e6d
Fix translation error that disabled union discrimination optimization
ahejlsberg Sep 27, 2025
b568352
Fix union discrimination optimization according to #1085.
ahejlsberg Sep 27, 2025
d88bda4
Add test for discriminated union with >10 cases
ahejlsberg Sep 27, 2025
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2 changes: 1 addition & 1 deletion internal/checker/checker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -29417,7 +29417,7 @@ func (c *Checker) discriminateContextualTypeByObjectMembers(node *ast.Node, cont

func (c *Checker) getMatchingUnionConstituentForObjectLiteral(unionType *Type, node *ast.Node) *Type {
keyPropertyName := c.getKeyPropertyName(unionType)
if keyPropertyName == "" {
if keyPropertyName != "" {
propNode := core.Find(node.AsObjectLiteralExpression().Properties.Nodes, func(p *ast.Node) bool {
return p.Symbol() != nil && ast.IsPropertyAssignment(p) && p.Symbol().Name == keyPropertyName && c.isPossiblyDiscriminantValue(p.Initializer())
})
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30 changes: 14 additions & 16 deletions internal/checker/relater.go
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Expand Up @@ -1167,24 +1167,22 @@ func (c *Checker) mapTypesByKeyProperty(types []*Type, keyPropertyName string) m
for _, t := range types {
if t.flags&(TypeFlagsObject|TypeFlagsIntersection|TypeFlagsInstantiableNonPrimitive) != 0 {
discriminant := c.getTypeOfPropertyOfType(t, keyPropertyName)
if discriminant != nil {
if !isLiteralType(discriminant) {
return nil
}
duplicate := false
for _, d := range discriminant.Distributed() {
key := c.getRegularTypeOfLiteralType(d)
if existing := typesByKey[key]; existing == nil {
typesByKey[key] = t
} else if existing != c.unknownType {
typesByKey[key] = c.unknownType
duplicate = true
}
}
if !duplicate {
count++
if discriminant == nil || !isLiteralType(discriminant) {
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Doesn't this behave differently from the original code? The original code would still continue the loop of the discriminate was undefined.

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Yes, but the exit is intended. We only want to build a map when all constituents have a discriminant. When some constituents lack a discriminant, we want to skip the map optimization and go through the regular discriminateTypeByDiscriminableItems code path.

return nil
}
duplicate := false
for _, d := range discriminant.Distributed() {
key := c.getRegularTypeOfLiteralType(d)
if existing := typesByKey[key]; existing == nil {
typesByKey[key] = t
} else if existing != c.unknownType {
typesByKey[key] = c.unknownType
duplicate = true
}
}
if !duplicate {
count++
}
}
}
if count >= 10 && count*2 >= len(types) {
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43 changes: 43 additions & 0 deletions testdata/baselines/reference/compiler/missingDiscriminants2.errors.txt
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@@ -0,0 +1,43 @@
missingDiscriminants2.ts(32,23): error TS2353: Object literal may only specify known properties, and 'subkind' does not exist in type '{ kind: "b"; }'.
missingDiscriminants2.ts(33,34): 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

// This tests ensures the change also works for discriminated unions with more than 10 cases

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"; }'.

105 changes: 105 additions & 0 deletions testdata/baselines/reference/compiler/missingDiscriminants2.symbols
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@@ -0,0 +1,105 @@
//// [tests/cases/compiler/missingDiscriminants2.ts] ////

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

// This tests ensures the change also works for discriminated unions with more than 10 cases

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

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

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

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

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

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

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

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

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

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

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

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

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

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

| { str: "n" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 18, 5))

| { str: "o" }
>str : Symbol(str, Decl(missingDiscriminants2.ts, 19, 5))

| { num: 1 }
>num : Symbol(num, Decl(missingDiscriminants2.ts, 20, 5))

const thing1: Thing = { str: "a", num: 0 }
>thing1 : Symbol(thing1, Decl(missingDiscriminants2.ts, 22, 5))
>Thing : Symbol(Thing, Decl(missingDiscriminants2.ts, 0, 0))
>str : Symbol(str, Decl(missingDiscriminants2.ts, 22, 23))
>num : Symbol(num, Decl(missingDiscriminants2.ts, 22, 33))

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

const thing3: Thing = { num: 1, str: "b" } // Shouldn't be error
>thing3 : Symbol(thing3, Decl(missingDiscriminants2.ts, 24, 5))
>Thing : Symbol(Thing, Decl(missingDiscriminants2.ts, 0, 0))
>num : Symbol(num, Decl(missingDiscriminants2.ts, 24, 23))
>str : Symbol(str, Decl(missingDiscriminants2.ts, 24, 31))

type Item =
>Item : Symbol(Item, Decl(missingDiscriminants2.ts, 24, 42))

| { kind: "a", subkind: 0, value: string }
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 27, 5))
>subkind : Symbol(subkind, Decl(missingDiscriminants2.ts, 27, 16))
>value : Symbol(value, Decl(missingDiscriminants2.ts, 27, 28))

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

| { kind: "b" }
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 29, 5))

const item1: Item = { subkind: 1, kind: "b" } // Error, type "b" not assignable to type "a"
>item1 : Symbol(item1, Decl(missingDiscriminants2.ts, 31, 5))
>Item : Symbol(Item, Decl(missingDiscriminants2.ts, 24, 42))
>subkind : Symbol(subkind, Decl(missingDiscriminants2.ts, 31, 21))
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 31, 33))

const item2: Item = { kind: "b", subkind: 1 } // Error, 'subkind' isn't a known property
>item2 : Symbol(item2, Decl(missingDiscriminants2.ts, 32, 5))
>Item : Symbol(Item, Decl(missingDiscriminants2.ts, 24, 42))
>kind : Symbol(kind, Decl(missingDiscriminants2.ts, 32, 21))
>subkind : Symbol(subkind, Decl(missingDiscriminants2.ts, 32, 32))

115 changes: 115 additions & 0 deletions testdata/baselines/reference/compiler/missingDiscriminants2.types
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@@ -0,0 +1,115 @@
//// [tests/cases/compiler/missingDiscriminants2.ts] ////

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

// This tests ensures the change also works for discriminated unions with more than 10 cases

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

36 changes: 36 additions & 0 deletions testdata/tests/cases/compiler/missingDiscriminants2.ts
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@@ -0,0 +1,36 @@
// @strict: true
// @noEmit: true

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

// This tests ensures the change also works for discriminated unions with more than 10 cases

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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