Merge trivially mergeable intersection types for identity comparison

src/compiler/checker.ts

@@ -14485,17 +14485,22 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             return getAugmentedPropertiesOfType(unionType);
         }
 
+        const props = getMembersOfUnionOrIntersection(unionType as UnionType);
+        return arrayFrom(props.values());
+    }
+
+    function getMembersOfUnionOrIntersection(type: UnionOrIntersectionType): SymbolTable {
         const props = createSymbolTable();
-        for (const memberType of types) {
+        for (const memberType of type.types) {
             for (const { escapedName } of getAugmentedPropertiesOfType(memberType)) {
                 if (!props.has(escapedName)) {
-                    const prop = createUnionOrIntersectionProperty(unionType as UnionType, escapedName);
+                    const prop = createUnionOrIntersectionProperty(type, escapedName);
                     // May be undefined if the property is private
                     if (prop) props.set(escapedName, prop);
                 }
             }
         }
-        return arrayFrom(props.values());
+        return props;
     }
 
     function getConstraintOfType(type: InstantiableType | UnionOrIntersectionType): Type | undefined {
@@ -21730,6 +21735,22 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             hasSubstitution ||= isNarrowingSubstitutionType(t); // This avoids displaying error messages with types like `T & T` when narrowing a return type
             if (hasInstantiable && hasNullableOrEmpty || hasSubstitution) return true;
         }
+
+        return false;
+    }
+
+    function isTypeMergeableIntersectionConstituent(type: Type) {
+        if (
+            type.flags === TypeFlags.Object &&
+            !!((type as ObjectType).objectFlags & ObjectFlags.Anonymous) &&
+            !((type as ObjectType).objectFlags & ObjectFlags.Instantiated)
+        ) {
+            if ((type as ObjectType).objectFlags & ObjectFlags.ReverseMapped) {
+                return isTypeMergeableIntersectionConstituent((type as ReverseMappedType).source);
+            }
+
+            return !typeHasCallOrConstructSignatures(type) && getIndexInfosOfType(type).length === 0;
+        }
         return false;
     }
 
@@ -22151,12 +22172,19 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             // turn deferred type references into regular type references, simplify indexed access and
             // conditional types, and resolve substitution types to either the substitution (on the source
             // side) or the type variable (on the target side).
-            const source = getNormalizedType(originalSource, /*writing*/ false);
+            let source = getNormalizedType(originalSource, /*writing*/ false);
             let target = getNormalizedType(originalTarget, /*writing*/ true);
 
             if (source === target) return Ternary.True;
 
             if (relation === identityRelation) {
+                if (source.flags & TypeFlags.Intersection) {
+                    source = mergeIntersectionTypeIfPossible(source as IntersectionType, /*writing*/ false);
+                }
+                if (target.flags & TypeFlags.Intersection) {
+                    target = mergeIntersectionTypeIfPossible(target as IntersectionType, /*writing*/ true);
+                }
+
                 if (source.flags !== target.flags) return Ternary.False;
                 if (source.flags & TypeFlags.Singleton) return Ternary.True;
                 traceUnionsOrIntersectionsTooLarge(source, target);
@@ -22244,6 +22272,21 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             return Ternary.False;
         }
 
+        function mergeIntersectionTypeIfPossible(type: IntersectionType, writing: boolean) {
+            if (every(type.types, isTypeMergeableIntersectionConstituent)) {
+                const reduced = getReducedType(type);
+                if (reduced.flags & TypeFlags.Intersection) {
+                    type = reduced as IntersectionType;
+                    const members = getMembersOfUnionOrIntersection(type);
+                    const intersection = createAnonymousType(/*symbol*/ undefined, members, emptyArray, emptyArray, emptyArray);
+                    intersection.aliasSymbol = type.aliasSymbol;
+                    intersection.aliasTypeArguments = type.aliasTypeArguments;
+                    return getNormalizedType(intersection, writing)
+                }
+            }
+            return type;
+        }
+
         function reportErrorResults(originalSource: Type, originalTarget: Type, source: Type, target: Type, headMessage: DiagnosticMessage | undefined) {
             const sourceHasBase = !!getSingleBaseForNonAugmentingSubtype(originalSource);
             const targetHasBase = !!getSingleBaseForNonAugmentingSubtype(originalTarget);

tests/baselines/reference/identityRelationIntersectionTypes.js

@@ -0,0 +1,70 @@
+//// [tests/cases/compiler/identityRelationIntersectionTypes.ts] ////
+
+//// [identityRelationIntersectionTypes.ts]
+namespace identityRelationIntersectionTypes {
+    type Equals<A, B> = (<T>() => T extends B ? 1 : 0) extends (<T>() => T extends A ? 1 : 0) ? true : false;
+
+    type GoodIntersection = Equals<{a: 1} & {b: 2}, {a: 1; b: 2}>;  // true
+
+    // Interfaces aren't mergeable
+    interface I {i: 3};
+    type BadIntersection1 = Equals<{a: 1} & I, {a: 1; i: 3}>;  // false
+
+    // Objects with call or constructor signatures aren't mergeable
+    type BadIntersection2 = Equals<{a: 1} & {b: 2; (): void}, {a: 1; b: 2; (): void}>;  // false
+    type BadIntersection3 = Equals<{a: 1} & {b: 2; new (): void}, {a: 1; b: 2; new (): void}>;  // false
+
+    // Objects with index signatures aren't mergeable
+    type BadIntersection4 = Equals<{a: 1} & {b: 2; [key: string]: number}, {a: 1; b: 2; [key: string]: number}>;  // false
+
+    // Shouldn't merge intersection if any constituents aren't mergeable
+    type StillBadIntersection1 = Equals<{a: 1} & {b: 2} & I, {a: 1; b: 2; i: 3}>;  // false
+    type StillBadIntersection2 = Equals<{a: 1} & {b: 2} & I, {a: 1; b: 2} & I>;  // false
+
+    // Parentheses don't matter because intersections are flattened
+    type StillBadIntersection3 = Equals<({a: 1} & {b: 2}) & I, {a: 1; b: 2; i: 3}>;  // false
+    type StillBadIntersection4 = Equals<({a: 1} & {b: 2}) & I, {a: 1; b: 2} & I>;  // false
+
+    // Type aliases also don't prevent flattening
+    type AB = {a: 1} & {b: 2};
+    type StillBadIntersection5 = Equals<AB & I, {a: 1; b: 2; i: 3}>;  // false
+    type StillBadIntersection6 = Equals<AB & I, {a: 1; b: 2} & I>;  // false
+
+    type GoodDeepIntersection1 = Equals<{a: 0 | 1} & {a: 1 | 2}, {a: 1}>;  // true
+    type GoodDeepIntersection2 = Equals<{a: {x: 1}} & {a: {y: 2}}, {a: {x: 1; y: 2}}>;  // true
+
+    type GoodShallowBadDeepIntersection1 = Equals<{a: {x: 1}} & {a: {y: 2} & I}, {a: {x: 1; y: 2} & I}>;  // false
+    type GoodShallowBadDeepIntersection2 = Equals<{a: {x: 1}} & {a: {y: 2} & I}, {a: {x: 1} & {y: 2} & I}>;  // true
+
+    // Reduction applies to nested intersections
+    type DeepReduction = Equals<{a: {x: 1}} & {a: {x: 2}}, {a: never}>;  // true
+
+    // Intersections are distributed and merged if possible with union constituents
+    type Distributed = Equals<
+        {a: 1} & {b: 2} & ({c: 3} | {d: 4} | I),
+        {a: 1; b: 2; c: 3} | {a: 1; b: 2; d: 4} | {a: 1} & {b: 2} & I
+    >;  // true
+
+    // Should work with recursive types
+    type R1 = {a: R1; x: 1};
+    type R2 = {a: R2; y: 1};
+    type R = R1 & R2;
+
+    type Recursive1 = Equals<R, {a: R1 & R2; x: 1; y: 1}>;  // true
+    type Recursive2 = Equals<R, {a: {a: R1 & R2; x: 1; y: 1}; x: 1; y: 1}>;  // true
+    type Recursive3 = Equals<R, {a: {a: {a: R1 & R2; x: 1; y: 1}; x: 1; y: 1}; x: 1; y: 1}>;  // true
+    type Recursive4 = Equals<R, {a: {a: {a: R1 & R2; x: 1; y: 0}; x: 1; y: 1}; x: 1; y: 1}>;  // false
+}
+
+
+//// [identityRelationIntersectionTypes.js]
+"use strict";
+var identityRelationIntersectionTypes;
+(function (identityRelationIntersectionTypes) {
+    ;
+})(identityRelationIntersectionTypes || (identityRelationIntersectionTypes = {}));
+
+
+//// [identityRelationIntersectionTypes.d.ts]
+declare namespace identityRelationIntersectionTypes {
+}