From 16c1d9dc9e6b468055a8d1052ef270f1946828b0 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 22 Nov 2023 14:01:35 -0800
Subject: [PATCH 1/3] Reduce intersections of constrained type variables and
 primitive types

---
 src/compiler/checker.ts | 84 +++++++++++++++++++++++++++++++++++++++--
 src/compiler/types.ts   |  4 ++
 2 files changed, 85 insertions(+), 3 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 8ae0ea8980e21..03bdbf2a73423 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -16794,6 +16794,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         if (!(flags & TypeFlags.Never)) {
             includes |= flags & TypeFlags.IncludesMask;
             if (flags & TypeFlags.Instantiable) includes |= TypeFlags.IncludesInstantiable;
+            if (flags & TypeFlags.Intersection && getObjectFlags(type) & ObjectFlags.IsConstrainedTypeVariable) includes |= TypeFlags.IncludesConstrainedTypeVariable;
             if (type === wildcardType) includes |= TypeFlags.IncludesWildcard;
             if (!strictNullChecks && flags & TypeFlags.Nullable) {
                 if (!(getObjectFlags(type) & ObjectFlags.ContainsWideningType)) includes |= TypeFlags.IncludesNonWideningType;
@@ -16938,6 +16939,49 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         }
     }
 
+    function removeConstrainedTypeVariables(types: Type[]) {
+        const typeVariables: TypeVariable[] = [];
+        // First collect a list of the type variables occurring in constraining intersections.
+        for (const type of types) {
+            if (getObjectFlags(type) & ObjectFlags.IsConstrainedTypeVariable) {
+                const index = (type as IntersectionType).types[0].flags & TypeFlags.TypeVariable ? 0 : 1;
+                pushIfUnique(typeVariables, (type as IntersectionType).types[index]);
+            }
+        }
+        // For each type variable, check if the constraining intersections for that type variable fully
+        // cover the constraint of the type variable; if so, remove the constraining intersections and
+        // substitute the type variable.
+        for (const typeVariable of typeVariables) {
+            const primitives: Type[] = [];
+            // First collect the primitive types from the constraining intersections.
+            for (const type of types) {
+                if (getObjectFlags(type) & ObjectFlags.IsConstrainedTypeVariable) {
+                    const index = (type as IntersectionType).types[0].flags & TypeFlags.TypeVariable ? 0 : 1;
+                    if ((type as IntersectionType).types[index] === typeVariable) {
+                        insertType(primitives, (type as IntersectionType).types[1 - index]);
+                    }
+                }
+            }
+            // If every constituent in the type variable's constraint is covered by an intersection of the type
+            // variable and that constituent, remove those intersections and substitute the type variable.
+            const constraint = getBaseConstraintOfType(typeVariable)!;
+            if (everyType(constraint, t => containsType(primitives, t))) {
+                let i = types.length;
+                while (i > 0) {
+                    i--;
+                    const type = types[i];
+                    if (getObjectFlags(type) & ObjectFlags.IsConstrainedTypeVariable) {
+                        const index = (type as IntersectionType).types[0].flags & TypeFlags.TypeVariable ? 0 : 1;
+                        if ((type as IntersectionType).types[index] === typeVariable && containsType(primitives, (type as IntersectionType).types[1 - index])) {
+                            orderedRemoveItemAt(types, i);
+                        }
+                    }
+                }
+                insertType(types, typeVariable);
+            }
+        }
+    }
+
     function isNamedUnionType(type: Type) {
         return !!(type.flags & TypeFlags.Union && (type.aliasSymbol || (type as UnionType).origin));
     }
@@ -17012,6 +17056,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (includes & TypeFlags.StringLiteral && includes & TypeFlags.TemplateLiteral) {
                 removeStringLiteralsMatchedByTemplateLiterals(typeSet);
             }
+            if (includes & TypeFlags.IncludesConstrainedTypeVariable) {
+                removeConstrainedTypeVariables(typeSet);
+            }
             if (unionReduction === UnionReduction.Subtype) {
                 typeSet = removeSubtypes(typeSet, !!(includes & TypeFlags.Object));
                 if (!typeSet) {
@@ -17276,9 +17323,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return true;
     }
 
-    function createIntersectionType(types: Type[], aliasSymbol?: Symbol, aliasTypeArguments?: readonly Type[]) {
+    function createIntersectionType(types: Type[], objectFlags: ObjectFlags, aliasSymbol?: Symbol, aliasTypeArguments?: readonly Type[]) {
         const result = createType(TypeFlags.Intersection) as IntersectionType;
-        result.objectFlags = getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
+        result.objectFlags = objectFlags | getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
         result.types = types;
         result.aliasSymbol = aliasSymbol;
         result.aliasTypeArguments = aliasTypeArguments;
@@ -17299,6 +17346,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const typeMembershipMap = new Map<string, Type>();
         const includes = addTypesToIntersection(typeMembershipMap, 0 as TypeFlags, types);
         const typeSet: Type[] = arrayFrom(typeMembershipMap.values());
+        let objectFlags = ObjectFlags.None;
         // An intersection type is considered empty if it contains
         // the type never, or
         // more than one unit type or,
@@ -17350,6 +17398,36 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         if (typeSet.length === 1) {
             return typeSet[0];
         }
+        if (typeSet.length === 2) {
+            const typeVarIndex = typeSet[0].flags & TypeFlags.TypeVariable ? 0 : 1;
+            const typeVariable = typeSet[typeVarIndex];
+            const primitiveType = typeSet[1 - typeVarIndex];
+            if (typeVariable.flags & TypeFlags.TypeVariable && (primitiveType.flags & (TypeFlags.Primitive | TypeFlags.NonPrimitive) || includes & TypeFlags.IncludesEmptyObject)) {
+                // We have an intersection T & P or P & T, where T is a type variable and P is a primitive type, the object type, or {}.
+                const constraint = getBaseConstraintOfType(typeVariable);
+                // Check that T's constraint is similarly composed of primitive types, the object type, or {}.
+                if (constraint && everyType(constraint, t => !!(t.flags & (TypeFlags.Primitive | TypeFlags.NonPrimitive)) || isEmptyAnonymousObjectType(t))) {
+                    // If T's constraint is a subtype of P, simply return T. For example, given `T extends "a" | "b"`,
+                    // the intersection `T & string` reduces to just T.
+                    if (isTypeStrictSubtypeOf(constraint, primitiveType)) {
+                        return typeVariable;
+                    }
+                    if (!(constraint.flags & TypeFlags.Union && someType(constraint, c => isTypeStrictSubtypeOf(c, primitiveType)))) {
+                        // No constituent of T's constraint is a subtype of P. If P is also not a subtype of T's constraint,
+                        // then the constraint and P are unrelated, and the intersection reduces to never. For example, given
+                        // `T extends "a" | "b"`, the intersection `T & number` reduces to never.
+                        if (!isTypeStrictSubtypeOf(primitiveType, constraint)) {
+                            return neverType;
+                        }
+                    }
+                    // Some constituent of T's constraint is a subtype of P, or P is a subtype of T's constraint. Thus,
+                    // the intersection further constrains the type variable. For example, given `T extends string | number`,
+                    // the intersection `T & "a"` is marked as a constrained type variable. Likewise, given `T extends "a" | 1`,
+                    // the intersection `T & number` is marked as a constrained type variable.
+                    objectFlags = ObjectFlags.IsConstrainedTypeVariable;
+                }
+            }
+        }
         const id = getTypeListId(typeSet) + getAliasId(aliasSymbol, aliasTypeArguments);
         let result = intersectionTypes.get(id);
         if (!result) {
@@ -17385,7 +17463,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
             }
             else {
-                result = createIntersectionType(typeSet, aliasSymbol, aliasTypeArguments);
+                result = createIntersectionType(typeSet, objectFlags, aliasSymbol, aliasTypeArguments);
             }
             intersectionTypes.set(id, result);
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 9b0d53efce953..b7e88a65d64dd 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -6153,6 +6153,8 @@ export const enum TypeFlags {
     /** @internal */
     IncludesInstantiable = Substitution,
     /** @internal */
+    IncludesConstrainedTypeVariable = StringMapping,
+    /** @internal */
     NotPrimitiveUnion = Any | Unknown | Void | Never | Object | Intersection | IncludesInstantiable,
 }
 
@@ -6313,6 +6315,8 @@ export const enum ObjectFlags {
     IsNeverIntersectionComputed = 1 << 24, // IsNeverLike flag has been computed
     /** @internal */
     IsNeverIntersection = 1 << 25, // Intersection reduces to never
+    /** @internal */
+    IsConstrainedTypeVariable = 1 << 26, // T & C, where T's constraint and C are primitives, object, or {}
 }
 
 /** @internal */

From 4bf6ae78b8aa9184b7b886c5665899566b363b47 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 22 Nov 2023 14:02:12 -0800
Subject: [PATCH 2/3] Accept new baselines

---
 .../reference/conditionalTypes1.errors.txt    |  1 +
 .../reference/inKeywordAndUnknown.types       | 10 +++----
 .../inKeywordTypeguard(strict=false).types    |  4 +--
 .../inKeywordTypeguard(strict=true).types     |  4 +--
 .../reference/indexSignatures1.errors.txt     |  8 +++++-
 .../reference/indexSignatures1.types          |  2 +-
 .../intersectionWithUnionConstraint.types     |  4 +--
 .../reference/spreadObjectOrFalsy.js          |  2 +-
 .../reference/spreadObjectOrFalsy.types       | 14 +++++-----
 .../reference/unknownControlFlow.errors.txt   |  8 +++++-
 .../reference/unknownControlFlow.types        | 28 +++++++++----------
 11 files changed, 49 insertions(+), 36 deletions(-)

diff --git a/tests/baselines/reference/conditionalTypes1.errors.txt b/tests/baselines/reference/conditionalTypes1.errors.txt
index c3b5f377180ff..ea66f14d92783 100644
--- a/tests/baselines/reference/conditionalTypes1.errors.txt
+++ b/tests/baselines/reference/conditionalTypes1.errors.txt
@@ -88,6 +88,7 @@ conditionalTypes1.ts(288,43): error TS2322: Type 'T95<U>' is not assignable to t
 !!! error TS2322: Type 'T' is not assignable to type 'NonNullable<T>'.
 !!! error TS2322:   Type 'T' is not assignable to type '{}'.
 !!! related TS2208 conditionalTypes1.ts:10:13: This type parameter might need an `extends {}` constraint.
+!!! related TS2208 conditionalTypes1.ts:10:13: This type parameter might need an `extends NonNullable<T>` constraint.
     }
     
     function f2<T extends string | undefined>(x: T, y: NonNullable<T>) {
diff --git a/tests/baselines/reference/inKeywordAndUnknown.types b/tests/baselines/reference/inKeywordAndUnknown.types
index a45017bd52d28..81a24b272970f 100644
--- a/tests/baselines/reference/inKeywordAndUnknown.types
+++ b/tests/baselines/reference/inKeywordAndUnknown.types
@@ -138,12 +138,12 @@ function f5<T>(x: T & {}) {
 
 function f6<T extends {}>(x: T & {}) {
 >f6 : <T extends {}>(x: T & {}) => boolean
->x : T & {}
+>x : T
 
     return x instanceof Object && 'a' in x;
 >x instanceof Object && 'a' in x : boolean
 >x instanceof Object : boolean
->x : T & {}
+>x : T
 >Object : ObjectConstructor
 >'a' in x : boolean
 >'a' : "a"
@@ -152,15 +152,15 @@ function f6<T extends {}>(x: T & {}) {
 
 function f7<T extends object>(x: T & {}) {
 >f7 : <T extends object>(x: T & {}) => boolean
->x : T & {}
+>x : T
 
     return x instanceof Object && 'a' in x;
 >x instanceof Object && 'a' in x : boolean
 >x instanceof Object : boolean
->x : T & {}
+>x : T
 >Object : ObjectConstructor
 >'a' in x : boolean
 >'a' : "a"
->x : T & {}
+>x : T
 }
 
diff --git a/tests/baselines/reference/inKeywordTypeguard(strict=false).types b/tests/baselines/reference/inKeywordTypeguard(strict=false).types
index 66ce2de68eee0..2caf941dedaac 100644
--- a/tests/baselines/reference/inKeywordTypeguard(strict=false).types
+++ b/tests/baselines/reference/inKeywordTypeguard(strict=false).types
@@ -1078,12 +1078,12 @@ function isHTMLTable<T extends object | null>(table: T): boolean {
 const f = <P extends object>(a: P & {}) => {
 >f : <P extends object>(a: P & {}) => void
 ><P extends object>(a: P & {}) => {    "foo" in a;} : <P extends object>(a: P & {}) => void
->a : P & {}
+>a : P
 
     "foo" in a;
 >"foo" in a : boolean
 >"foo" : "foo"
->a : P & {}
+>a : P
 
 };
 
diff --git a/tests/baselines/reference/inKeywordTypeguard(strict=true).types b/tests/baselines/reference/inKeywordTypeguard(strict=true).types
index 4a2a2094181b7..004ea8e7e2038 100644
--- a/tests/baselines/reference/inKeywordTypeguard(strict=true).types
+++ b/tests/baselines/reference/inKeywordTypeguard(strict=true).types
@@ -1078,12 +1078,12 @@ function isHTMLTable<T extends object | null>(table: T): boolean {
 const f = <P extends object>(a: P & {}) => {
 >f : <P extends object>(a: P & {}) => void
 ><P extends object>(a: P & {}) => {    "foo" in a;} : <P extends object>(a: P & {}) => void
->a : P & {}
+>a : P
 
     "foo" in a;
 >"foo" in a : boolean
 >"foo" : "foo"
->a : P & {}
+>a : P
 
 };
 
diff --git a/tests/baselines/reference/indexSignatures1.errors.txt b/tests/baselines/reference/indexSignatures1.errors.txt
index ae9b07b65cec9..05d0436ca43c1 100644
--- a/tests/baselines/reference/indexSignatures1.errors.txt
+++ b/tests/baselines/reference/indexSignatures1.errors.txt
@@ -15,8 +15,10 @@ indexSignatures1.ts(73,5): error TS2374: Duplicate index signature for type '`fo
 indexSignatures1.ts(81,5): error TS2413: '`a${string}a`' index type '"c"' is not assignable to '`${string}a`' index type '"b"'.
 indexSignatures1.ts(81,5): error TS2413: '`a${string}a`' index type '"c"' is not assignable to '`a${string}`' index type '"a"'.
 indexSignatures1.ts(87,6): error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
+indexSignatures1.ts(88,5): error TS2374: Duplicate index signature for type 'T'.
 indexSignatures1.ts(88,6): error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
 indexSignatures1.ts(89,6): error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
+indexSignatures1.ts(90,5): error TS2374: Duplicate index signature for type 'T'.
 indexSignatures1.ts(90,6): error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
 indexSignatures1.ts(117,1): error TS7053: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'I1'.
   No index signature with a parameter of type 'string' was found on type 'I1'.
@@ -69,7 +71,7 @@ indexSignatures1.ts(289,7): error TS2322: Type 'number' is not assignable to typ
 indexSignatures1.ts(312,43): error TS2353: Object literal may only specify known properties, and '[sym]' does not exist in type '{ [key: number]: string; }'.
 
 
-==== indexSignatures1.ts (50 errors) ====
+==== indexSignatures1.ts (52 errors) ====
     // Symbol index signature checking
     
     const sym = Symbol();
@@ -188,12 +190,16 @@ indexSignatures1.ts(312,43): error TS2353: Object literal may only specify known
          ~~~
 !!! error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
         [key: T | number]: string;  // Error
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2374: Duplicate index signature for type 'T'.
          ~~~
 !!! error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
         [key: Error]: string;  // Error
          ~~~
 !!! error TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.
         [key: T & string]: string;  // Error
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2374: Duplicate index signature for type 'T'.
          ~~~
 !!! error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.
     }
diff --git a/tests/baselines/reference/indexSignatures1.types b/tests/baselines/reference/indexSignatures1.types
index 4f7e248192fb8..144face6af93b 100644
--- a/tests/baselines/reference/indexSignatures1.types
+++ b/tests/baselines/reference/indexSignatures1.types
@@ -261,7 +261,7 @@ type Invalid<T extends string> = {
 >key : Error
 
     [key: T & string]: string;  // Error
->key : T & string
+>key : T
 }
 
 // Intersections in index signatures
diff --git a/tests/baselines/reference/intersectionWithUnionConstraint.types b/tests/baselines/reference/intersectionWithUnionConstraint.types
index d687036196c4a..86c318fe89bb1 100644
--- a/tests/baselines/reference/intersectionWithUnionConstraint.types
+++ b/tests/baselines/reference/intersectionWithUnionConstraint.types
@@ -45,7 +45,7 @@ type T1 = (string | number | undefined) & (string | null | undefined);  // strin
 
 function f3<T extends string | number | undefined>(x: T & (number | object | undefined)) {
 >f3 : <T extends string | number | undefined>(x: T & (number | object | undefined)) => void
->x : T & (number | object | undefined)
+>x : (T & undefined) | (T & number)
 
     const y: number | undefined = x;
 >y : number | undefined
@@ -54,7 +54,7 @@ function f3<T extends string | number | undefined>(x: T & (number | object | und
 
 function f4<T extends string | number>(x: T & (number | object)) {
 >f4 : <T extends string | number>(x: T & (number | object)) => void
->x : T & (number | object)
+>x : T & number
 
     const y: number = x;
 >y : number
diff --git a/tests/baselines/reference/spreadObjectOrFalsy.js b/tests/baselines/reference/spreadObjectOrFalsy.js
index c5ee59ce93a9b..979b98f14d411 100644
--- a/tests/baselines/reference/spreadObjectOrFalsy.js
+++ b/tests/baselines/reference/spreadObjectOrFalsy.js
@@ -113,7 +113,7 @@ declare function f5<S, T extends undefined>(a: S | T): S | T;
 declare function f6<T extends object | undefined>(a: T): T;
 declare function g1<T extends {}, A extends {
     z: (T | undefined) & T;
-}>(a: A): T | (undefined & T);
+}>(a: A): T;
 interface DatafulFoo<T> {
     data: T;
 }
diff --git a/tests/baselines/reference/spreadObjectOrFalsy.types b/tests/baselines/reference/spreadObjectOrFalsy.types
index b984a20079a27..3f57c796bdf7b 100644
--- a/tests/baselines/reference/spreadObjectOrFalsy.types
+++ b/tests/baselines/reference/spreadObjectOrFalsy.types
@@ -58,19 +58,19 @@ function f6<T extends object | undefined>(a: T) {
 // Repro from #46976
 
 function g1<T extends {}, A extends { z: (T | undefined) & T }>(a: A) {
->g1 : <T extends {}, A extends { z: (T | undefined) & T; }>(a: A) => T | (undefined & T)
->z : T | (undefined & T)
+>g1 : <T extends {}, A extends { z: (T | undefined) & T; }>(a: A) => T
+>z : T
 >a : A
 
     const { z } = a;
->z : T | (undefined & T)
+>z : T
 >a : A
 
     return {
->{        ...z    } : T | (undefined & T)
+>{        ...z    } : T
 
         ...z
->z : T | (undefined & T)
+>z : T
 
     };
 }
@@ -100,9 +100,9 @@ class Foo<T extends string> {
             this.data.toLocaleLowerCase();
 >this.data.toLocaleLowerCase() : string
 >this.data.toLocaleLowerCase : (locales?: string | string[] | undefined) => string
->this.data : T | (undefined & T)
+>this.data : T
 >this : this & DatafulFoo<T>
->data : T | (undefined & T)
+>data : T
 >toLocaleLowerCase : (locales?: string | string[] | undefined) => string
         }
     }
diff --git a/tests/baselines/reference/unknownControlFlow.errors.txt b/tests/baselines/reference/unknownControlFlow.errors.txt
index 2fa0730617da2..e8f4f198a505a 100644
--- a/tests/baselines/reference/unknownControlFlow.errors.txt
+++ b/tests/baselines/reference/unknownControlFlow.errors.txt
@@ -3,9 +3,11 @@ unknownControlFlow.ts(283,5): error TS2536: Type 'keyof (T & {})' cannot be used
 unknownControlFlow.ts(290,11): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
 unknownControlFlow.ts(291,5): error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
 unknownControlFlow.ts(293,5): error TS2345: Argument of type 'null' is not assignable to parameter of type 'never'.
+unknownControlFlow.ts(323,9): error TS2367: This comparison appears to be unintentional because the types 'T' and 'number' have no overlap.
+unknownControlFlow.ts(341,9): error TS2367: This comparison appears to be unintentional because the types 'T' and 'number' have no overlap.
 
 
-==== unknownControlFlow.ts (5 errors) ====
+==== unknownControlFlow.ts (7 errors) ====
     type T01 = {} & string;  // {} & string
     type T02 = {} & 'a';  // 'a'
     type T03 = {} & object;  // object
@@ -339,6 +341,8 @@ unknownControlFlow.ts(293,5): error TS2345: Argument of type 'null' is not assig
     
     function fx2<T extends {}>(value: T & ({} | null)) {
         if (value === 42) {
+            ~~~~~~~~~~~~
+!!! error TS2367: This comparison appears to be unintentional because the types 'T' and 'number' have no overlap.
             value;  // T & {}
         }
         else {
@@ -357,6 +361,8 @@ unknownControlFlow.ts(293,5): error TS2345: Argument of type 'null' is not assig
     
     function fx4<T extends {} | null>(value: T & ({} | null)) {
         if (value === 42) {
+            ~~~~~~~~~~~~
+!!! error TS2367: This comparison appears to be unintentional because the types 'T' and 'number' have no overlap.
             value;  // T & {}
         }
         else {
diff --git a/tests/baselines/reference/unknownControlFlow.types b/tests/baselines/reference/unknownControlFlow.types
index 4f452904c7105..7b320964b838a 100644
--- a/tests/baselines/reference/unknownControlFlow.types
+++ b/tests/baselines/reference/unknownControlFlow.types
@@ -819,55 +819,55 @@ function fx1<T extends unknown>(value: T & ({} | null)) {
 
 function fx2<T extends {}>(value: T & ({} | null)) {
 >fx2 : <T extends {}>(value: T & ({} | null)) => void
->value : T & ({} | null)
+>value : T
 
     if (value === 42) {
 >value === 42 : boolean
->value : T & ({} | null)
+>value : T
 >42 : 42
 
         value;  // T & {}
->value : T & ({} | null)
+>value : never
     }
     else {
         value;  // T & ({} | null)
->value : T & ({} | null)
+>value : T
     }
 }
 
 function fx3<T extends {} | undefined>(value: T & ({} | null)) {
 >fx3 : <T extends {} | undefined>(value: T & ({} | null)) => void
->value : T & ({} | null)
+>value : T & {}
 
     if (value === 42) {
 >value === 42 : boolean
->value : T & ({} | null)
+>value : T & {}
 >42 : 42
 
         value;  // T & {}
->value : T & ({} | null)
+>value : T & {}
     }
     else {
         value;  // T & ({} | null)
->value : T & ({} | null)
+>value : T & {}
     }
 }
 
 function fx4<T extends {} | null>(value: T & ({} | null)) {
 >fx4 : <T extends {} | null>(value: T & ({} | null)) => void
->value : T & ({} | null)
+>value : T
 
     if (value === 42) {
 >value === 42 : boolean
->value : T & ({} | null)
+>value : T
 >42 : 42
 
         value;  // T & {}
->value : T & {}
+>value : never
     }
     else {
         value;  // T & ({} | null)
->value : T & ({} | null)
+>value : T
     }
 }
 
@@ -1036,12 +1036,12 @@ type AB = "A" | "B";
 
 function x<T_AB extends AB>(x: T_AB & undefined, y: any) {
 >x : <T_AB extends AB>(x: T_AB & undefined, y: any) => void
->x : T_AB & undefined
+>x : never
 >y : any
 
     let r2: never = y as T_AB & undefined;
 >r2 : never
->y as T_AB & undefined : T_AB & undefined
+>y as T_AB & undefined : never
 >y : any
 } 
 

From 5b6f969cd8ef0b794449c5bf47c726ea0705faee Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 22 Nov 2023 14:14:10 -0800
Subject: [PATCH 3/3] Add tests

---
 ...ypeVariableConstraintIntersections.symbols | 335 ++++++++++++++++++
 .../typeVariableConstraintIntersections.types | 258 ++++++++++++++
 .../typeVariableConstraintIntersections.ts    |  95 +++++
 3 files changed, 688 insertions(+)
 create mode 100644 tests/baselines/reference/typeVariableConstraintIntersections.symbols
 create mode 100644 tests/baselines/reference/typeVariableConstraintIntersections.types
 create mode 100644 tests/cases/compiler/typeVariableConstraintIntersections.ts

diff --git a/tests/baselines/reference/typeVariableConstraintIntersections.symbols b/tests/baselines/reference/typeVariableConstraintIntersections.symbols
new file mode 100644
index 0000000000000..ab22dba71173d
--- /dev/null
+++ b/tests/baselines/reference/typeVariableConstraintIntersections.symbols
@@ -0,0 +1,335 @@
+//// [tests/cases/compiler/typeVariableConstraintIntersections.ts] ////
+
+=== typeVariableConstraintIntersections.ts ===
+type T00<K extends "a" | "b"> = K & "a";
+>T00 : Symbol(T00, Decl(typeVariableConstraintIntersections.ts, 0, 0))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 0, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 0, 9))
+
+type T01<K extends "a" | "b"> = K & "c";
+>T01 : Symbol(T01, Decl(typeVariableConstraintIntersections.ts, 0, 40))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 1, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 1, 9))
+
+type T02<K extends "a" | "b"> = K & string;
+>T02 : Symbol(T02, Decl(typeVariableConstraintIntersections.ts, 1, 40))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 2, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 2, 9))
+
+type T10<K extends string> = K & "a";
+>T10 : Symbol(T10, Decl(typeVariableConstraintIntersections.ts, 2, 43))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 4, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 4, 9))
+
+type T11<K extends string> = K & "c";
+>T11 : Symbol(T11, Decl(typeVariableConstraintIntersections.ts, 4, 37))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 5, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 5, 9))
+
+type T12<K extends string> = K & string;
+>T12 : Symbol(T12, Decl(typeVariableConstraintIntersections.ts, 5, 37))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 6, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 6, 9))
+
+type T20<K extends "a" | "b" | "c"> = K & ("a" | "b" | "c");
+>T20 : Symbol(T20, Decl(typeVariableConstraintIntersections.ts, 6, 40))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 8, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 8, 9))
+
+type T21<K extends "a" | "b" | "c"> = ("a" | "b" | "c") & K;
+>T21 : Symbol(T21, Decl(typeVariableConstraintIntersections.ts, 8, 60))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 9, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 9, 9))
+
+type T22<K extends "a" | "b" | "c"> = K & ("a" | "b");
+>T22 : Symbol(T22, Decl(typeVariableConstraintIntersections.ts, 9, 60))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 10, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 10, 9))
+
+type T23<K extends "a" | "b" | "c"> = ("a" | "b") & K;
+>T23 : Symbol(T23, Decl(typeVariableConstraintIntersections.ts, 10, 54))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 11, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 11, 9))
+
+type T30<K extends "a" | "b"> = K & ("a" | "b" | "c");
+>T30 : Symbol(T30, Decl(typeVariableConstraintIntersections.ts, 11, 54))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 13, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 13, 9))
+
+type T31<K extends "a" | "b"> = ("a" | "b" | "c") & K;
+>T31 : Symbol(T31, Decl(typeVariableConstraintIntersections.ts, 13, 54))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 14, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 14, 9))
+
+type T32<K extends "a" | "b"> = K & ("a" | "b");
+>T32 : Symbol(T32, Decl(typeVariableConstraintIntersections.ts, 14, 54))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 15, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 15, 9))
+
+type T33<K extends "a" | "b"> = ("a" | "b") & K;
+>T33 : Symbol(T33, Decl(typeVariableConstraintIntersections.ts, 15, 48))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 16, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 16, 9))
+
+type T40<K extends {}> = K & undefined;
+>T40 : Symbol(T40, Decl(typeVariableConstraintIntersections.ts, 16, 48))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 18, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 18, 9))
+
+type T41<K extends {}> = K & null;
+>T41 : Symbol(T41, Decl(typeVariableConstraintIntersections.ts, 18, 39))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 19, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 19, 9))
+
+type T42<K extends {}> = K & object;
+>T42 : Symbol(T42, Decl(typeVariableConstraintIntersections.ts, 19, 34))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 20, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 20, 9))
+
+type T43<K extends {}> = K & {};
+>T43 : Symbol(T43, Decl(typeVariableConstraintIntersections.ts, 20, 36))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 21, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 21, 9))
+
+type T50<K extends "a" | 0> = K & "a";
+>T50 : Symbol(T50, Decl(typeVariableConstraintIntersections.ts, 21, 32))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 23, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 23, 9))
+
+type T51<K extends "a" | 0> = K & "b";
+>T51 : Symbol(T51, Decl(typeVariableConstraintIntersections.ts, 23, 38))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 24, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 24, 9))
+
+type T52<K extends "a" | 0> = K & string;
+>T52 : Symbol(T52, Decl(typeVariableConstraintIntersections.ts, 24, 38))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 25, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 25, 9))
+
+type T53<K extends "a" | 0> = K & 0;
+>T53 : Symbol(T53, Decl(typeVariableConstraintIntersections.ts, 25, 41))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 26, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 26, 9))
+
+type T54<K extends "a" | 0> = K & 1;
+>T54 : Symbol(T54, Decl(typeVariableConstraintIntersections.ts, 26, 36))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 27, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 27, 9))
+
+type T55<K extends "a" | 0> = K & number;
+>T55 : Symbol(T55, Decl(typeVariableConstraintIntersections.ts, 27, 36))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 28, 9))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 28, 9))
+
+type T60<T extends "a" | "b", U extends T> = U & "a";
+>T60 : Symbol(T60, Decl(typeVariableConstraintIntersections.ts, 28, 41))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 30, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 30, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 30, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 30, 29))
+
+type T61<T extends "a" | "b", U extends T> = U & ("a" | "b");
+>T61 : Symbol(T61, Decl(typeVariableConstraintIntersections.ts, 30, 53))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 31, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 31, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 31, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 31, 29))
+
+type T62<T extends "a" | "b", U extends T> = U & ("a" | "b" | "c");
+>T62 : Symbol(T62, Decl(typeVariableConstraintIntersections.ts, 31, 61))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 32, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 32, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 32, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 32, 29))
+
+type T63<T extends "a" | "b", U extends T> = U & string;
+>T63 : Symbol(T63, Decl(typeVariableConstraintIntersections.ts, 32, 67))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 33, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 33, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 33, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 33, 29))
+
+type T70<T extends "a" | "b", U extends T | "c"> = U & "a";
+>T70 : Symbol(T70, Decl(typeVariableConstraintIntersections.ts, 33, 56))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 35, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 35, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 35, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 35, 29))
+
+type T71<T extends "a" | "b", U extends T | "c"> = U & ("a" | "b");
+>T71 : Symbol(T71, Decl(typeVariableConstraintIntersections.ts, 35, 59))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 36, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 36, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 36, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 36, 29))
+
+type T72<T extends "a" | "b", U extends T | "c"> = U & ("a" | "b" | "c");
+>T72 : Symbol(T72, Decl(typeVariableConstraintIntersections.ts, 36, 67))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 37, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 37, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 37, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 37, 29))
+
+type T73<T extends "a" | "b", U extends T | "c"> = U & string;
+>T73 : Symbol(T73, Decl(typeVariableConstraintIntersections.ts, 37, 73))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 38, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 38, 29))
+>T : Symbol(T, Decl(typeVariableConstraintIntersections.ts, 38, 9))
+>U : Symbol(U, Decl(typeVariableConstraintIntersections.ts, 38, 29))
+
+declare function isA(x: any): x is "a";
+>isA : Symbol(isA, Decl(typeVariableConstraintIntersections.ts, 38, 62))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 40, 21))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 40, 21))
+
+declare function isB(x: any): x is "b";
+>isB : Symbol(isB, Decl(typeVariableConstraintIntersections.ts, 40, 39))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 41, 21))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 41, 21))
+
+declare function isC(x: any): x is "c";
+>isC : Symbol(isC, Decl(typeVariableConstraintIntersections.ts, 41, 39))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 42, 21))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 42, 21))
+
+function foo<K extends "a" | "b">(x: K) {
+>foo : Symbol(foo, Decl(typeVariableConstraintIntersections.ts, 42, 39))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 44, 13))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 44, 13))
+
+  if (isA(x)) {
+>isA : Symbol(isA, Decl(typeVariableConstraintIntersections.ts, 38, 62))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+
+    x;  // K & "a"
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+  }
+  if (isB(x)) {
+>isB : Symbol(isB, Decl(typeVariableConstraintIntersections.ts, 40, 39))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+
+    x;  // K & "b"
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+  }
+  if (isC(x)) {
+>isC : Symbol(isC, Decl(typeVariableConstraintIntersections.ts, 41, 39))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+
+    x;  // never
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+  }
+  if (isA(x) || isB(x)) {
+>isA : Symbol(isA, Decl(typeVariableConstraintIntersections.ts, 38, 62))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+>isB : Symbol(isB, Decl(typeVariableConstraintIntersections.ts, 40, 39))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+
+    x;  // K
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+  }
+  if (!(isA(x) || isB(x))) {
+>isA : Symbol(isA, Decl(typeVariableConstraintIntersections.ts, 38, 62))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+>isB : Symbol(isB, Decl(typeVariableConstraintIntersections.ts, 40, 39))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+
+    return;
+  }
+  x;  // K
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 44, 34))
+}
+
+// Example from #30581
+
+type OptionOne = {
+>OptionOne : Symbol(OptionOne, Decl(typeVariableConstraintIntersections.ts, 61, 1))
+
+  kind: "one";
+>kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 65, 18))
+
+  s: string;
+>s : Symbol(s, Decl(typeVariableConstraintIntersections.ts, 66, 14))
+
+};
+
+type OptionTwo = {
+>OptionTwo : Symbol(OptionTwo, Decl(typeVariableConstraintIntersections.ts, 68, 2))
+
+  kind: "two";
+>kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 70, 18))
+
+  x: number;
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 71, 14))
+
+  y: number;
+>y : Symbol(y, Decl(typeVariableConstraintIntersections.ts, 72, 12))
+
+};
+
+type Options = OptionOne | OptionTwo;
+>Options : Symbol(Options, Decl(typeVariableConstraintIntersections.ts, 74, 2))
+>OptionOne : Symbol(OptionOne, Decl(typeVariableConstraintIntersections.ts, 61, 1))
+>OptionTwo : Symbol(OptionTwo, Decl(typeVariableConstraintIntersections.ts, 68, 2))
+
+type OptionHandlers = {
+>OptionHandlers : Symbol(OptionHandlers, Decl(typeVariableConstraintIntersections.ts, 76, 37))
+
+  [K in Options['kind']]: (option: Options & { kind: K }) => string;
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 79, 3))
+>Options : Symbol(Options, Decl(typeVariableConstraintIntersections.ts, 74, 2))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 79, 27))
+>Options : Symbol(Options, Decl(typeVariableConstraintIntersections.ts, 74, 2))
+>kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 79, 46))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 79, 3))
+}
+
+const optionHandlers: OptionHandlers = {
+>optionHandlers : Symbol(optionHandlers, Decl(typeVariableConstraintIntersections.ts, 82, 5))
+>OptionHandlers : Symbol(OptionHandlers, Decl(typeVariableConstraintIntersections.ts, 76, 37))
+
+  "one": option => option.s,
+>"one" : Symbol("one", Decl(typeVariableConstraintIntersections.ts, 82, 40))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 83, 8))
+>option.s : Symbol(s, Decl(typeVariableConstraintIntersections.ts, 66, 14))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 83, 8))
+>s : Symbol(s, Decl(typeVariableConstraintIntersections.ts, 66, 14))
+
+  "two": option => option.x + "," + option.y,
+>"two" : Symbol("two", Decl(typeVariableConstraintIntersections.ts, 83, 28))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 84, 8))
+>option.x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 71, 14))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 84, 8))
+>x : Symbol(x, Decl(typeVariableConstraintIntersections.ts, 71, 14))
+>option.y : Symbol(y, Decl(typeVariableConstraintIntersections.ts, 72, 12))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 84, 8))
+>y : Symbol(y, Decl(typeVariableConstraintIntersections.ts, 72, 12))
+
+};
+
+function handleOption<K extends Options['kind']>(option: Options & { kind: K }): string {
+>handleOption : Symbol(handleOption, Decl(typeVariableConstraintIntersections.ts, 85, 2))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 87, 22))
+>Options : Symbol(Options, Decl(typeVariableConstraintIntersections.ts, 74, 2))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 87, 49))
+>Options : Symbol(Options, Decl(typeVariableConstraintIntersections.ts, 74, 2))
+>kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 87, 68))
+>K : Symbol(K, Decl(typeVariableConstraintIntersections.ts, 87, 22))
+
+  const kind = option.kind;
+>kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 88, 7))
+>option.kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 65, 18), Decl(typeVariableConstraintIntersections.ts, 87, 68), Decl(typeVariableConstraintIntersections.ts, 70, 18), Decl(typeVariableConstraintIntersections.ts, 87, 68))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 87, 49))
+>kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 65, 18), Decl(typeVariableConstraintIntersections.ts, 87, 68), Decl(typeVariableConstraintIntersections.ts, 70, 18), Decl(typeVariableConstraintIntersections.ts, 87, 68))
+
+  const handler = optionHandlers[kind];
+>handler : Symbol(handler, Decl(typeVariableConstraintIntersections.ts, 89, 7))
+>optionHandlers : Symbol(optionHandlers, Decl(typeVariableConstraintIntersections.ts, 82, 5))
+>kind : Symbol(kind, Decl(typeVariableConstraintIntersections.ts, 88, 7))
+
+  return handler(option);
+>handler : Symbol(handler, Decl(typeVariableConstraintIntersections.ts, 89, 7))
+>option : Symbol(option, Decl(typeVariableConstraintIntersections.ts, 87, 49))
+
+};
+
diff --git a/tests/baselines/reference/typeVariableConstraintIntersections.types b/tests/baselines/reference/typeVariableConstraintIntersections.types
new file mode 100644
index 0000000000000..6d89a64f8a0b8
--- /dev/null
+++ b/tests/baselines/reference/typeVariableConstraintIntersections.types
@@ -0,0 +1,258 @@
+//// [tests/cases/compiler/typeVariableConstraintIntersections.ts] ////
+
+=== typeVariableConstraintIntersections.ts ===
+type T00<K extends "a" | "b"> = K & "a";
+>T00 : T00<K>
+
+type T01<K extends "a" | "b"> = K & "c";
+>T01 : never
+
+type T02<K extends "a" | "b"> = K & string;
+>T02 : K
+
+type T10<K extends string> = K & "a";
+>T10 : T10<K>
+
+type T11<K extends string> = K & "c";
+>T11 : T11<K>
+
+type T12<K extends string> = K & string;
+>T12 : K
+
+type T20<K extends "a" | "b" | "c"> = K & ("a" | "b" | "c");
+>T20 : K
+
+type T21<K extends "a" | "b" | "c"> = ("a" | "b" | "c") & K;
+>T21 : K
+
+type T22<K extends "a" | "b" | "c"> = K & ("a" | "b");
+>T22 : T22<K>
+
+type T23<K extends "a" | "b" | "c"> = ("a" | "b") & K;
+>T23 : T23<K>
+
+type T30<K extends "a" | "b"> = K & ("a" | "b" | "c");
+>T30 : K
+
+type T31<K extends "a" | "b"> = ("a" | "b" | "c") & K;
+>T31 : K
+
+type T32<K extends "a" | "b"> = K & ("a" | "b");
+>T32 : K
+
+type T33<K extends "a" | "b"> = ("a" | "b") & K;
+>T33 : K
+
+type T40<K extends {}> = K & undefined;
+>T40 : never
+
+type T41<K extends {}> = K & null;
+>T41 : never
+
+type T42<K extends {}> = K & object;
+>T42 : T42<K>
+
+type T43<K extends {}> = K & {};
+>T43 : K
+
+type T50<K extends "a" | 0> = K & "a";
+>T50 : T50<K>
+
+type T51<K extends "a" | 0> = K & "b";
+>T51 : never
+
+type T52<K extends "a" | 0> = K & string;
+>T52 : T52<K>
+
+type T53<K extends "a" | 0> = K & 0;
+>T53 : T53<K>
+
+type T54<K extends "a" | 0> = K & 1;
+>T54 : never
+
+type T55<K extends "a" | 0> = K & number;
+>T55 : T55<K>
+
+type T60<T extends "a" | "b", U extends T> = U & "a";
+>T60 : T60<T, U>
+
+type T61<T extends "a" | "b", U extends T> = U & ("a" | "b");
+>T61 : U
+
+type T62<T extends "a" | "b", U extends T> = U & ("a" | "b" | "c");
+>T62 : U
+
+type T63<T extends "a" | "b", U extends T> = U & string;
+>T63 : U
+
+type T70<T extends "a" | "b", U extends T | "c"> = U & "a";
+>T70 : T70<T, U>
+
+type T71<T extends "a" | "b", U extends T | "c"> = U & ("a" | "b");
+>T71 : T71<T, U>
+
+type T72<T extends "a" | "b", U extends T | "c"> = U & ("a" | "b" | "c");
+>T72 : U
+
+type T73<T extends "a" | "b", U extends T | "c"> = U & string;
+>T73 : U
+
+declare function isA(x: any): x is "a";
+>isA : (x: any) => x is "a"
+>x : any
+
+declare function isB(x: any): x is "b";
+>isB : (x: any) => x is "b"
+>x : any
+
+declare function isC(x: any): x is "c";
+>isC : (x: any) => x is "c"
+>x : any
+
+function foo<K extends "a" | "b">(x: K) {
+>foo : <K extends "a" | "b">(x: K) => void
+>x : K
+
+  if (isA(x)) {
+>isA(x) : boolean
+>isA : (x: any) => x is "a"
+>x : "a" | "b"
+
+    x;  // K & "a"
+>x : K & "a"
+  }
+  if (isB(x)) {
+>isB(x) : boolean
+>isB : (x: any) => x is "b"
+>x : "a" | "b"
+
+    x;  // K & "b"
+>x : K & "b"
+  }
+  if (isC(x)) {
+>isC(x) : boolean
+>isC : (x: any) => x is "c"
+>x : "a" | "b"
+
+    x;  // never
+>x : never
+  }
+  if (isA(x) || isB(x)) {
+>isA(x) || isB(x) : boolean
+>isA(x) : boolean
+>isA : (x: any) => x is "a"
+>x : "a" | "b"
+>isB(x) : boolean
+>isB : (x: any) => x is "b"
+>x : "b"
+
+    x;  // K
+>x : K
+  }
+  if (!(isA(x) || isB(x))) {
+>!(isA(x) || isB(x)) : boolean
+>(isA(x) || isB(x)) : boolean
+>isA(x) || isB(x) : boolean
+>isA(x) : boolean
+>isA : (x: any) => x is "a"
+>x : "a" | "b"
+>isB(x) : boolean
+>isB : (x: any) => x is "b"
+>x : "b"
+
+    return;
+  }
+  x;  // K
+>x : K
+}
+
+// Example from #30581
+
+type OptionOne = {
+>OptionOne : { kind: "one"; s: string; }
+
+  kind: "one";
+>kind : "one"
+
+  s: string;
+>s : string
+
+};
+
+type OptionTwo = {
+>OptionTwo : { kind: "two"; x: number; y: number; }
+
+  kind: "two";
+>kind : "two"
+
+  x: number;
+>x : number
+
+  y: number;
+>y : number
+
+};
+
+type Options = OptionOne | OptionTwo;
+>Options : OptionOne | OptionTwo
+
+type OptionHandlers = {
+>OptionHandlers : { one: (option: OptionOne & { kind: "one"; }) => string; two: (option: OptionTwo & { kind: "two"; }) => string; }
+
+  [K in Options['kind']]: (option: Options & { kind: K }) => string;
+>option : Options & { kind: K; }
+>kind : K
+}
+
+const optionHandlers: OptionHandlers = {
+>optionHandlers : OptionHandlers
+>{  "one": option => option.s,  "two": option => option.x + "," + option.y,} : { one: (option: OptionOne & { kind: "one"; }) => string; two: (option: OptionTwo & { kind: "two"; }) => string; }
+
+  "one": option => option.s,
+>"one" : (option: OptionOne & { kind: "one"; }) => string
+>option => option.s : (option: OptionOne & { kind: "one"; }) => string
+>option : OptionOne & { kind: "one"; }
+>option.s : string
+>option : OptionOne & { kind: "one"; }
+>s : string
+
+  "two": option => option.x + "," + option.y,
+>"two" : (option: OptionTwo & { kind: "two"; }) => string
+>option => option.x + "," + option.y : (option: OptionTwo & { kind: "two"; }) => string
+>option : OptionTwo & { kind: "two"; }
+>option.x + "," + option.y : string
+>option.x + "," : string
+>option.x : number
+>option : OptionTwo & { kind: "two"; }
+>x : number
+>"," : ","
+>option.y : number
+>option : OptionTwo & { kind: "two"; }
+>y : number
+
+};
+
+function handleOption<K extends Options['kind']>(option: Options & { kind: K }): string {
+>handleOption : <K extends "one" | "two">(option: Options & { kind: K; }) => string
+>option : Options & { kind: K; }
+>kind : K
+
+  const kind = option.kind;
+>kind : K
+>option.kind : K
+>option : Options & { kind: K; }
+>kind : K
+
+  const handler = optionHandlers[kind];
+>handler : OptionHandlers[K]
+>optionHandlers[kind] : OptionHandlers[K]
+>optionHandlers : OptionHandlers
+>kind : K
+
+  return handler(option);
+>handler(option) : string
+>handler : OptionHandlers[K]
+>option : Options & { kind: K; }
+
+};
+
diff --git a/tests/cases/compiler/typeVariableConstraintIntersections.ts b/tests/cases/compiler/typeVariableConstraintIntersections.ts
new file mode 100644
index 0000000000000..3d5d69fe3223d
--- /dev/null
+++ b/tests/cases/compiler/typeVariableConstraintIntersections.ts
@@ -0,0 +1,95 @@
+// @strict: true
+// @noEmit: true
+
+type T00<K extends "a" | "b"> = K & "a";
+type T01<K extends "a" | "b"> = K & "c";
+type T02<K extends "a" | "b"> = K & string;
+
+type T10<K extends string> = K & "a";
+type T11<K extends string> = K & "c";
+type T12<K extends string> = K & string;
+
+type T20<K extends "a" | "b" | "c"> = K & ("a" | "b" | "c");
+type T21<K extends "a" | "b" | "c"> = ("a" | "b" | "c") & K;
+type T22<K extends "a" | "b" | "c"> = K & ("a" | "b");
+type T23<K extends "a" | "b" | "c"> = ("a" | "b") & K;
+
+type T30<K extends "a" | "b"> = K & ("a" | "b" | "c");
+type T31<K extends "a" | "b"> = ("a" | "b" | "c") & K;
+type T32<K extends "a" | "b"> = K & ("a" | "b");
+type T33<K extends "a" | "b"> = ("a" | "b") & K;
+
+type T40<K extends {}> = K & undefined;
+type T41<K extends {}> = K & null;
+type T42<K extends {}> = K & object;
+type T43<K extends {}> = K & {};
+
+type T50<K extends "a" | 0> = K & "a";
+type T51<K extends "a" | 0> = K & "b";
+type T52<K extends "a" | 0> = K & string;
+type T53<K extends "a" | 0> = K & 0;
+type T54<K extends "a" | 0> = K & 1;
+type T55<K extends "a" | 0> = K & number;
+
+type T60<T extends "a" | "b", U extends T> = U & "a";
+type T61<T extends "a" | "b", U extends T> = U & ("a" | "b");
+type T62<T extends "a" | "b", U extends T> = U & ("a" | "b" | "c");
+type T63<T extends "a" | "b", U extends T> = U & string;
+
+type T70<T extends "a" | "b", U extends T | "c"> = U & "a";
+type T71<T extends "a" | "b", U extends T | "c"> = U & ("a" | "b");
+type T72<T extends "a" | "b", U extends T | "c"> = U & ("a" | "b" | "c");
+type T73<T extends "a" | "b", U extends T | "c"> = U & string;
+
+declare function isA(x: any): x is "a";
+declare function isB(x: any): x is "b";
+declare function isC(x: any): x is "c";
+
+function foo<K extends "a" | "b">(x: K) {
+  if (isA(x)) {
+    x;  // K & "a"
+  }
+  if (isB(x)) {
+    x;  // K & "b"
+  }
+  if (isC(x)) {
+    x;  // never
+  }
+  if (isA(x) || isB(x)) {
+    x;  // K
+  }
+  if (!(isA(x) || isB(x))) {
+    return;
+  }
+  x;  // K
+}
+
+// Example from #30581
+
+type OptionOne = {
+  kind: "one";
+  s: string;
+};
+
+type OptionTwo = {
+  kind: "two";
+  x: number;
+  y: number;
+};
+
+type Options = OptionOne | OptionTwo;
+
+type OptionHandlers = {
+  [K in Options['kind']]: (option: Options & { kind: K }) => string;
+}
+
+const optionHandlers: OptionHandlers = {
+  "one": option => option.s,
+  "two": option => option.x + "," + option.y,
+};
+
+function handleOption<K extends Options['kind']>(option: Options & { kind: K }): string {
+  const kind = option.kind;
+  const handler = optionHandlers[kind];
+  return handler(option);
+};