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Keep inferring from type arguments when the same generic type is nested inside them #65077

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Keep inferring from type arguments when the same generic type is nested inside them #65077

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d1c1f14
Keep inferring from type arguments when the same generic type is nest…
kennysarnoski-jarvis Oct 31, 2026
132d27c
Cover the Prisma shape of #64757 with the reported repro
kennysarnoski-jarvis Oct 31, 2026
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55 changes: 36 additions & 19 deletions tsc/internal/checker/inference.go
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Original file line number Diff line number Diff line change
Expand Up @@ -81,7 +81,7 @@ func (c *Checker) inferFromTypes(n *InferenceState, source *Type, target *Type)
}
if source.alias != nil && target.alias != nil && source.alias.symbol == target.alias.symbol {
if len(source.alias.typeArguments) != 0 || len(target.alias.typeArguments) != 0 {
c.invokeOnce(n, source, target, (*Checker).inferFromAliasTypeArguments)
c.invokeOnceWithoutRecursionCheck(n, source, target, (*Checker).inferFromAliasTypeArguments)
}
// And if there weren't any type arguments, there's no reason to run inference as the types must be the same.
return
Expand Down Expand Up @@ -227,7 +227,7 @@ func (c *Checker) inferFromTypes(n *InferenceState, source *Type, target *Type)
switch {
case source.objectFlags&ObjectFlagsReference != 0 && target.objectFlags&ObjectFlagsReference != 0 && (source.AsTypeReference().target == target.AsTypeReference().target || c.isArrayType(source) && c.isArrayType(target)) && !(source.AsTypeReference().node != nil && target.AsTypeReference().node != nil):
// If source and target are references to the same generic type, infer from type arguments
c.invokeOnce(n, source, target, (*Checker).inferFromReferenceTypeArguments)
c.invokeOnceWithoutRecursionCheck(n, source, target, (*Checker).inferFromReferenceTypeArguments)
case source.flags&TypeFlagsIndex != 0 && target.flags&TypeFlagsIndex != 0:
c.inferFromContravariantTypes(n, source.AsIndexType().target, target.AsIndexType().target)
case (isLiteralType(source) || source.flags&TypeFlagsString != 0) && target.flags&TypeFlagsIndex != 0:
Expand Down Expand Up @@ -344,6 +344,19 @@ func (c *Checker) inferFromContravariantTypesIfStrictFunctionTypes(n *InferenceS
// such that we would go on inferring forever, even though we would never infer
// between the same pair of types.
func (c *Checker) invokeOnce(n *InferenceState, source *Type, target *Type, action func(c *Checker, n *InferenceState, source *Type, target *Type)) {
c.invokeOnceWorker(n, source, target, action, true /*checkRecursion*/)
}

// invokeOnceWithoutRecursionCheck caches like invokeOnce but leaves the recursion stacks alone and
// never reports a circularity. It serves inference between the type arguments of two references to
// the same generic type (or alias), which is bounded by the arguments themselves and needs no cycle
// guard. Pushing those references onto the stacks would make an outer `Box<() => Box<T>>` look like
// a cycle at depth 2 and drop the inferences for the inner `T` (#64757).
func (c *Checker) invokeOnceWithoutRecursionCheck(n *InferenceState, source *Type, target *Type, action func(c *Checker, n *InferenceState, source *Type, target *Type)) {
c.invokeOnceWorker(n, source, target, action, false /*checkRecursion*/)
}

func (c *Checker) invokeOnceWorker(n *InferenceState, source *Type, target *Type, action func(c *Checker, n *InferenceState, source *Type, target *Type), checkRecursion bool) {
key := CacheHashKey{
Lo: uint64(source.id)<<32 | uint64(target.id),
Hi: uint64(uint32(n.priority))<<2 | uint64(core.IfElse(n.contravariant, 2, 0)) | uint64(core.IfElse(n.bivariant, 1, 0)),
Expand All @@ -355,25 +368,29 @@ func (c *Checker) invokeOnce(n *InferenceState, source *Type, target *Type, acti
n.visited.set(key, InferencePriorityCircularity)
saveInferencePriority := n.inferencePriority
n.inferencePriority = InferencePriorityMaxValue
// We stop inferring and report a circularity if we encounter duplicate recursion identities on both
// the source side and the target side.
saveExpandingFlags := n.expandingFlags
n.sourceStack = append(n.sourceStack, source)
n.targetStack = append(n.targetStack, target)
if c.isDeeplyNestedType(source, n.sourceStack, 2) {
n.expandingFlags |= ExpandingFlagsSource
}
if c.isDeeplyNestedType(target, n.targetStack, 2) {
n.expandingFlags |= ExpandingFlagsTarget
}
if n.expandingFlags != ExpandingFlagsBoth {
action(c, n, source, target)
if checkRecursion {
// We stop inferring and report a circularity if we encounter duplicate recursion identities on both
// the source side and the target side.
saveExpandingFlags := n.expandingFlags
n.sourceStack = append(n.sourceStack, source)
n.targetStack = append(n.targetStack, target)
if c.isDeeplyNestedType(source, n.sourceStack, 2) {
n.expandingFlags |= ExpandingFlagsSource
}
if c.isDeeplyNestedType(target, n.targetStack, 2) {
n.expandingFlags |= ExpandingFlagsTarget
}
if n.expandingFlags != ExpandingFlagsBoth {
action(c, n, source, target)
} else {
n.inferencePriority = InferencePriorityCircularity
}
n.targetStack = n.targetStack[:len(n.targetStack)-1]
n.sourceStack = n.sourceStack[:len(n.sourceStack)-1]
n.expandingFlags = saveExpandingFlags
} else {
n.inferencePriority = InferencePriorityCircularity
action(c, n, source, target)
}
n.targetStack = n.targetStack[:len(n.targetStack)-1]
n.sourceStack = n.sourceStack[:len(n.sourceStack)-1]
n.expandingFlags = saveExpandingFlags
n.visited.set(key, n.inferencePriority)
n.inferencePriority = min(n.inferencePriority, saveInferencePriority)
}
Expand Down
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