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ahejlsberg merged 5 commits into from
Jun 24, 2026
Merged

Prefer covariant inferences with deeper type argument nesting #4389

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4af0548
Track type argument depths of covariant inferences
ahejlsberg Jun 22, 2026
b35f6f1
Accept new baselines
ahejlsberg Jun 22, 2026
b2853de
Add tests
ahejlsberg Jun 22, 2026
a1cb64d
Fix bounds
ahejlsberg Jun 22, 2026
9af8c5f
Fix over-eager lint error
ahejlsberg Jun 22, 2026
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3 changes: 2 additions & 1 deletion internal/checker/checker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -287,7 +287,8 @@ type InferenceContext struct {

type InferenceInfo struct {
typeParameter *Type // Type parameter for which inferences are being made
candidates []*Type // Candidates in covariant positions
candidates []*Type // Candidates in covariant positions in decreasing depth order
candidateDepths []int // Type argument depths of covariant inferences
contraCandidates []*Type // Candidates in contravariant positions
inferredType *Type // Cache for resolved inferred type
priority InferencePriority // Priority of current inference set
Expand Down
36 changes: 30 additions & 6 deletions internal/checker/inference.go
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Expand Up @@ -27,6 +27,7 @@ type InferenceState struct {
sourceStack []*Type
targetStack []*Type
next *InferenceState
depth int
}

func (c *Checker) getInferenceState() *InferenceState {
Expand Down Expand Up @@ -187,6 +188,7 @@ func (c *Checker) inferFromTypes(n *InferenceState, source *Type, target *Type)
}
if n.priority < inference.priority {
inference.candidates = nil
inference.candidateDepths = nil
inference.contraCandidates = nil
inference.topLevel = true
inference.priority = n.priority
Expand All @@ -199,9 +201,28 @@ func (c *Checker) inferFromTypes(n *InferenceState, source *Type, target *Type)
inference.contraCandidates = append(inference.contraCandidates, candidate)
clearCachedInferences(n.inferences)
}
} else if !slices.Contains(inference.candidates, candidate) {
inference.candidates = append(inference.candidates, candidate)
clearCachedInferences(n.inferences)
} else {
index := slices.Index(inference.candidates, candidate)
if index < 0 || inference.candidateDepths[index] < n.depth {
// Candidate isn't present or is present with lower depth
if index >= 0 {
// Remove candidate with lower depth
inference.candidates = slices.Delete(inference.candidates, index, index+1)
inference.candidateDepths = slices.Delete(inference.candidateDepths, index, index+1)
}
index = 0
for index < len(inference.candidateDepths) {
if inference.candidateDepths[index] < n.depth {
break
}
index++
}
// Insert candidate at end or immediately before first candidate with lower depth.
// This ensures candidates with the highest depth are stored first.
inference.candidates = slices.Insert(inference.candidates, index, candidate)
inference.candidateDepths = slices.Insert(inference.candidateDepths, index, n.depth)
clearCachedInferences(n.inferences)
}
}
}
if n.priority&InferencePriorityReturnType == 0 && target.flags&TypeFlagsTypeParameter != 0 && inference.topLevel && !c.isTypeParameterAtTopLevel(n.originalTarget, target, 0) {
Expand Down Expand Up @@ -282,13 +303,15 @@ func (c *Checker) inferFromTypes(n *InferenceState, source *Type, target *Type)
}

func (c *Checker) inferFromTypeArguments(n *InferenceState, sourceTypes []*Type, targetTypes []*Type, variances []VarianceFlags) {
n.depth++
for i := range min(len(sourceTypes), len(targetTypes)) {
if i < len(variances) && variances[i]&VarianceFlagsVarianceMask == VarianceFlagsContravariant {
c.inferFromContravariantTypes(n, sourceTypes[i], targetTypes[i])
} else {
c.inferFromTypes(n, sourceTypes[i], targetTypes[i])
}
}
n.depth--
}

func (c *Checker) inferWithPriority(n *InferenceState, source *Type, target *Type, newPriority InferencePriority) {
Expand Down Expand Up @@ -1451,9 +1474,9 @@ func (c *Checker) isTypeParameterAtTopLevelInReturnType(signature *Signature, ty

func (c *Checker) getTypeFromInference(inference *InferenceInfo) *Type {
switch {
case inference.candidates != nil:
case len(inference.candidates) != 0:
return c.getUnionTypeEx(inference.candidates, UnionReductionSubtype, nil, nil)
case inference.contraCandidates != nil:
case len(inference.contraCandidates) != 0:
return c.getIntersectionType(inference.contraCandidates)
}
return nil
Expand Down Expand Up @@ -1573,6 +1596,7 @@ func cloneInferenceInfo(info *InferenceInfo) *InferenceInfo {
return &InferenceInfo{
typeParameter: info.typeParameter,
candidates: slices.Clone(info.candidates),
candidateDepths: slices.Clone(info.candidateDepths),
contraCandidates: slices.Clone(info.contraCandidates),
inferredType: info.inferredType,
priority: info.priority,
Expand All @@ -1595,7 +1619,7 @@ func hasInferenceCandidates(info *InferenceInfo) bool {
}

func hasInferenceCandidatesOrDefault(info *InferenceInfo) bool {
return info.candidates != nil || info.contraCandidates != nil || hasTypeParameterDefault(info.typeParameter)
return hasInferenceCandidates(info) || hasTypeParameterDefault(info.typeParameter)
}

func hasTypeParameterDefault(tp *Type) bool {
Expand Down
90 changes: 90 additions & 0 deletions testdata/baselines/reference/compiler/nestedGenericTypeInference.symbols
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@@ -0,0 +1,90 @@
//// [tests/cases/compiler/nestedGenericTypeInference.ts] ////

=== nestedGenericTypeInference.ts ===
// https://github.com/microsoft/typescript-go/issues/1789

declare function flat<T>(args: T[] | T[][]): void;
>flat : Symbol(flat, Decl(nestedGenericTypeInference.ts, 0, 0))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 2, 22))
>args : Symbol(args, Decl(nestedGenericTypeInference.ts, 2, 25))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 2, 22))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 2, 22))

type Value = 1 | 2;
>Value : Symbol(Value, Decl(nestedGenericTypeInference.ts, 2, 50))

declare const n: Value[] | Value[][];
>n : Symbol(n, Decl(nestedGenericTypeInference.ts, 4, 13))
>Value : Symbol(Value, Decl(nestedGenericTypeInference.ts, 2, 50))
>Value : Symbol(Value, Decl(nestedGenericTypeInference.ts, 2, 50))

flat(n);
>flat : Symbol(flat, Decl(nestedGenericTypeInference.ts, 0, 0))
>n : Symbol(n, Decl(nestedGenericTypeInference.ts, 4, 13))

type Box<T> = { value: T };
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 7, 9))
>value : Symbol(value, Decl(nestedGenericTypeInference.ts, 7, 15))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 7, 9))

declare function flat0<T>(args: Box<T> | Box<Box<T>>): void;
>flat0 : Symbol(flat0, Decl(nestedGenericTypeInference.ts, 7, 27))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 9, 23))
>args : Symbol(args, Decl(nestedGenericTypeInference.ts, 9, 26))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 9, 23))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 9, 23))

declare const arg0: Box<string> | Box<Box<string>>;
>arg0 : Symbol(arg0, Decl(nestedGenericTypeInference.ts, 10, 13))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))

flat0(arg0);
>flat0 : Symbol(flat0, Decl(nestedGenericTypeInference.ts, 7, 27))
>arg0 : Symbol(arg0, Decl(nestedGenericTypeInference.ts, 10, 13))

declare function flat1<T>(args: Array<T> | Array<Box<T>>): void;
>flat1 : Symbol(flat1, Decl(nestedGenericTypeInference.ts, 11, 12))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 13, 23))
>args : Symbol(args, Decl(nestedGenericTypeInference.ts, 13, 26))
>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --) ... and 4 more)
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 13, 23))
>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --) ... and 4 more)
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 13, 23))

declare const arg1: Array<string> | Array<Box<string>>;
>arg1 : Symbol(arg1, Decl(nestedGenericTypeInference.ts, 14, 13))
>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --) ... and 4 more)
>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --) ... and 4 more)
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))

flat1(arg1);
>flat1 : Symbol(flat1, Decl(nestedGenericTypeInference.ts, 11, 12))
>arg1 : Symbol(arg1, Decl(nestedGenericTypeInference.ts, 14, 13))

declare function flat2<T>(args: Box<T> | Box<Array<T>>): void;
>flat2 : Symbol(flat2, Decl(nestedGenericTypeInference.ts, 15, 12))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 17, 23))
>args : Symbol(args, Decl(nestedGenericTypeInference.ts, 17, 26))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 17, 23))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --) ... and 4 more)
>T : Symbol(T, Decl(nestedGenericTypeInference.ts, 17, 23))

declare const arg2: Box<string> | Box<Array<string>>;
>arg2 : Symbol(arg2, Decl(nestedGenericTypeInference.ts, 18, 13))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>Box : Symbol(Box, Decl(nestedGenericTypeInference.ts, 5, 8))
>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --) ... and 4 more)

flat2(arg2);
>flat2 : Symbol(flat2, Decl(nestedGenericTypeInference.ts, 15, 12))
>arg2 : Symbol(arg2, Decl(nestedGenericTypeInference.ts, 18, 13))

60 changes: 60 additions & 0 deletions testdata/baselines/reference/compiler/nestedGenericTypeInference.types
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Original file line number Diff line number Diff line change
@@ -0,0 +1,60 @@
//// [tests/cases/compiler/nestedGenericTypeInference.ts] ////

=== nestedGenericTypeInference.ts ===
// https://github.com/microsoft/typescript-go/issues/1789

declare function flat<T>(args: T[] | T[][]): void;
>flat : <T>(args: T[] | T[][]) => void
>args : T[] | T[][]

type Value = 1 | 2;
>Value : Value

declare const n: Value[] | Value[][];
>n : Value[][] | Value[]

flat(n);
>flat(n) : void
>flat : <T>(args: T[] | T[][]) => void
>n : Value[][] | Value[]

type Box<T> = { value: T };
>Box : Box<T>
>value : T

declare function flat0<T>(args: Box<T> | Box<Box<T>>): void;
>flat0 : <T>(args: Box<T> | Box<Box<T>>) => void
>args : Box<T> | Box<Box<T>>

declare const arg0: Box<string> | Box<Box<string>>;
>arg0 : Box<string> | Box<Box<string>>

flat0(arg0);
>flat0(arg0) : void
>flat0 : <T>(args: Box<T> | Box<Box<T>>) => void
>arg0 : Box<string> | Box<Box<string>>

declare function flat1<T>(args: Array<T> | Array<Box<T>>): void;
>flat1 : <T>(args: Array<T> | Array<Box<T>>) => void
>args : T[] | Box<T>[]

declare const arg1: Array<string> | Array<Box<string>>;
>arg1 : string[] | Box<string>[]

flat1(arg1);
>flat1(arg1) : void
>flat1 : <T>(args: Array<T> | Array<Box<T>>) => void
>arg1 : string[] | Box<string>[]

declare function flat2<T>(args: Box<T> | Box<Array<T>>): void;
>flat2 : <T>(args: Box<T> | Box<Array<T>>) => void
>args : Box<T> | Box<T[]>

declare const arg2: Box<string> | Box<Array<string>>;
>arg2 : Box<string> | Box<string[]>

flat2(arg2);
>flat2(arg2) : void
>flat2 : <T>(args: Box<T> | Box<Array<T>>) => void
>arg2 : Box<string> | Box<string[]>

11 changes: 5 additions & 6 deletions ...nes/reference/submodule/conformance/assignmentCompatWithGenericCallSignatures2.errors.txt
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@@ -1,7 +1,6 @@
assignmentCompatWithGenericCallSignatures2.ts(15,1): error TS2322: Type 'B' is not assignable to type 'A'.
Types of parameters 'y' and 'y' are incompatible.
Type 'T[]' is not assignable to type 'T'.
'T' could be instantiated with an arbitrary type which could be unrelated to 'T[]'.
Types of parameters 'x' and 'x' are incompatible.
Type 'T' is not assignable to type 'T[]'.
assignmentCompatWithGenericCallSignatures2.ts(16,1): error TS2322: Type 'A' is not assignable to type 'B'.
Types of parameters 'y' and 'y' are incompatible.
Type 'S' is not assignable to type 'S[]'.
Expand All @@ -25,9 +24,9 @@ assignmentCompatWithGenericCallSignatures2.ts(16,1): error TS2322: Type 'A' is n
a = b;
~
!!! error TS2322: Type 'B' is not assignable to type 'A'.
!!! error TS2322: Types of parameters 'y' and 'y' are incompatible.
!!! error TS2322: Type 'T[]' is not assignable to type 'T'.
!!! error TS2322: 'T' could be instantiated with an arbitrary type which could be unrelated to 'T[]'.
!!! error TS2322: Types of parameters 'x' and 'x' are incompatible.
!!! error TS2322: Type 'T' is not assignable to type 'T[]'.
!!! related TS2208 assignmentCompatWithGenericCallSignatures2.ts:4:6: This type parameter might need an `extends T[]` constraint.
b = a;
~
!!! error TS2322: Type 'A' is not assignable to type 'B'.
Expand Down
25 changes: 25 additions & 0 deletions ...eference/submodule/conformance/assignmentCompatWithGenericCallSignatures2.errors.txt.diff
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@@ -0,0 +1,25 @@
--- old.assignmentCompatWithGenericCallSignatures2.errors.txt
+++ new.assignmentCompatWithGenericCallSignatures2.errors.txt
@@= skipped -0, +0 lines =@@
assignmentCompatWithGenericCallSignatures2.ts(15,1): error TS2322: Type 'B' is not assignable to type 'A'.
- Types of parameters 'y' and 'y' are incompatible.
- Type 'T[]' is not assignable to type 'T'.
- 'T' could be instantiated with an arbitrary type which could be unrelated to 'T[]'.
+ Types of parameters 'x' and 'x' are incompatible.
Comment on lines +4 to +8

@ahejlsberg ahejlsberg Jun 22, 2026

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This is intended.

+ Type 'T' is not assignable to type 'T[]'.
assignmentCompatWithGenericCallSignatures2.ts(16,1): error TS2322: Type 'A' is not assignable to type 'B'.
Types of parameters 'y' and 'y' are incompatible.
Type 'S' is not assignable to type 'S[]'.
@@= skipped -24, +23 lines =@@
a = b;
~
!!! error TS2322: Type 'B' is not assignable to type 'A'.
-!!! error TS2322: Types of parameters 'y' and 'y' are incompatible.
-!!! error TS2322: Type 'T[]' is not assignable to type 'T'.
-!!! error TS2322: 'T' could be instantiated with an arbitrary type which could be unrelated to 'T[]'.
+!!! error TS2322: Types of parameters 'x' and 'x' are incompatible.
+!!! error TS2322: Type 'T' is not assignable to type 'T[]'.
+!!! related TS2208 assignmentCompatWithGenericCallSignatures2.ts:4:6: This type parameter might need an `extends T[]` constraint.
b = a;
~
!!! error TS2322: Type 'A' is not assignable to type 'B'.
22 changes: 22 additions & 0 deletions testdata/tests/cases/compiler/nestedGenericTypeInference.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,22 @@
// @noEmit: true

// https://github.com/microsoft/typescript-go/issues/1789

declare function flat<T>(args: T[] | T[][]): void;
type Value = 1 | 2;
declare const n: Value[] | Value[][];
flat(n);

type Box<T> = { value: T };

declare function flat0<T>(args: Box<T> | Box<Box<T>>): void;
declare const arg0: Box<string> | Box<Box<string>>;
flat0(arg0);

declare function flat1<T>(args: Array<T> | Array<Box<T>>): void;
declare const arg1: Array<string> | Array<Box<string>>;
flat1(arg1);

declare function flat2<T>(args: Box<T> | Box<Array<T>>): void;
declare const arg2: Box<string> | Box<Array<string>>;
flat2(arg2);