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Don't reduce intersections of mappings of the same object type #64481

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Merged
Anders Hejlsberg (ahejlsberg) merged 4 commits into from
Sep 28, 2026
Merged

Don't reduce intersections of mappings of the same object type #64481

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6dd4013
Don't reduce intersections of mappings of the same object type
ahejlsberg Sep 27, 2026
e065127
Accept new baselines
ahejlsberg Sep 27, 2026
daf10b8
Add regression test
ahejlsberg Sep 27, 2026
d8f563b
Merge branch 'main' into fix-64415
ahejlsberg Sep 28, 2026
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16 changes: 15 additions & 1 deletion tsc/internal/checker/checker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -22192,7 +22192,7 @@ func (c *Checker) getReducedType(t *Type) *Type {
case t.flags&TypeFlagsIntersection != 0:
if t.objectFlags&ObjectFlagsIsNeverIntersectionComputed == 0 {
t.objectFlags |= ObjectFlagsIsNeverIntersectionComputed
if c.somePropertyReducesToNever(t) {
if !c.isMappingOfSameObjectType(t.Types()) && c.somePropertyReducesToNever(t) {
t.objectFlags |= ObjectFlagsIsNeverIntersection
}
}
Expand All @@ -22203,6 +22203,20 @@ func (c *Checker) getReducedType(t *Type) *Type {
return t
}

func (c *Checker) isMappingOfSameObjectType(types []*Type) bool {
if len(types) != 0 && types[0].objectFlags&ObjectFlagsMapped != 0 {
if firstType := c.getModifiersTypeFromMappedType(types[0]); firstType.flags&TypeFlagsObject != 0 {
for _, t := range types[1:] {
if t.objectFlags&ObjectFlagsMapped == 0 || c.getModifiersTypeFromMappedType(t) != firstType {
return false
}
}
return true
}
}
return false
}

func (c *Checker) somePropertyReducesToNever(t *Type) bool {
// Collect declaration counts for each property across all constituent types of the intersection.
counts := make(map[string]int)
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4 changes: 2 additions & 2 deletions tsc/testdata/baselines/reference/compiler/recursiveTypeInference.types
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Expand Up @@ -315,7 +315,7 @@ const Activity = z.object({
},
});
export type ActivityT = output<typeof Activity>;
>ActivityT : { name: string; subactivities: unknown; }
>ActivityT : { name: string; subactivities: any[] | null; }
>Activity : $ZodObject<{ name: $ZodString; readonly subactivities: $ZodNullable<$ZodArray<$ZodObject<any>>>; }>

// (4) reference through a separately-declared const
Expand Down Expand Up @@ -355,6 +355,6 @@ const NodeArray = z.array(Node1);
>Node1 : $ZodObject<{ name: $ZodString; readonly children: $ZodOptional<$ZodArray<$ZodObject<any>>>; }>

export type Node1T = output<typeof Node1>;
>Node1T : { name: string; children?: unknown; }
>Node1T : { name: string; children?: any[] | undefined; }
>Node1 : $ZodObject<{ name: $ZodString; readonly children: $ZodOptional<$ZodArray<$ZodObject<any>>>; }>

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