Fixed distributive conditional types with never intersections (#57345)

Author: Andarist

src/compiler/checker.ts

@@ -19877,12 +19877,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (!result) {
                 const newMapper = createTypeMapper(root.outerTypeParameters, typeArguments);
                 const checkType = root.checkType;
-                const distributionType = root.isDistributive ? getMappedType(checkType, newMapper) : undefined;
+                const distributionType = root.isDistributive ? getReducedType(getMappedType(checkType, newMapper)) : undefined;
                 // Distributive conditional types are distributed over union types. For example, when the
                 // distributive conditional type T extends U ? X : Y is instantiated with A | B for T, the
                 // result is (A extends U ? X : Y) | (B extends U ? X : Y).
                 result = distributionType && checkType !== distributionType && distributionType.flags & (TypeFlags.Union | TypeFlags.Never) ?
-                    mapTypeWithAlias(getReducedType(distributionType), t => getConditionalType(root, prependTypeMapping(checkType, t, newMapper), forConstraint), aliasSymbol, aliasTypeArguments) :
+                    mapTypeWithAlias(distributionType, t => getConditionalType(root, prependTypeMapping(checkType, t, newMapper), forConstraint), aliasSymbol, aliasTypeArguments) :
                     getConditionalType(root, newMapper, forConstraint, aliasSymbol, aliasTypeArguments);
                 root.instantiations!.set(id, result);
             }

tests/baselines/reference/distributiveConditionalTypeNeverIntersection1.symbols

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/distributiveConditionalTypeNeverIntersection1.ts] ////
+
+=== distributiveConditionalTypeNeverIntersection1.ts ===
+// https://github.com/microsoft/TypeScript/issues/57343
+
+type IsNumber<T> = T extends number ? true : false;
+>IsNumber : Symbol(IsNumber, Decl(distributiveConditionalTypeNeverIntersection1.ts, 0, 0))
+>T : Symbol(T, Decl(distributiveConditionalTypeNeverIntersection1.ts, 2, 14))
+>T : Symbol(T, Decl(distributiveConditionalTypeNeverIntersection1.ts, 2, 14))
+
+type Conflicted = { x: true } & { x: false };
+>Conflicted : Symbol(Conflicted, Decl(distributiveConditionalTypeNeverIntersection1.ts, 2, 51))
+>x : Symbol(x, Decl(distributiveConditionalTypeNeverIntersection1.ts, 4, 19))
+>x : Symbol(x, Decl(distributiveConditionalTypeNeverIntersection1.ts, 4, 33))
+
+type Ex1 = IsNumber<Conflicted>; // never
+>Ex1 : Symbol(Ex1, Decl(distributiveConditionalTypeNeverIntersection1.ts, 4, 45))
+>IsNumber : Symbol(IsNumber, Decl(distributiveConditionalTypeNeverIntersection1.ts, 0, 0))
+>Conflicted : Symbol(Conflicted, Decl(distributiveConditionalTypeNeverIntersection1.ts, 2, 51))
+
+type Ex2 = IsNumber<"OEEE" | Conflicted>; // false
+>Ex2 : Symbol(Ex2, Decl(distributiveConditionalTypeNeverIntersection1.ts, 6, 32))
+>IsNumber : Symbol(IsNumber, Decl(distributiveConditionalTypeNeverIntersection1.ts, 0, 0))
+>Conflicted : Symbol(Conflicted, Decl(distributiveConditionalTypeNeverIntersection1.ts, 2, 51))
+
+type Ex3 = IsNumber<1 | Conflicted>; // true
+>Ex3 : Symbol(Ex3, Decl(distributiveConditionalTypeNeverIntersection1.ts, 7, 41))
+>IsNumber : Symbol(IsNumber, Decl(distributiveConditionalTypeNeverIntersection1.ts, 0, 0))
+>Conflicted : Symbol(Conflicted, Decl(distributiveConditionalTypeNeverIntersection1.ts, 2, 51))
+

tests/baselines/reference/distributiveConditionalTypeNeverIntersection1.types

@@ -0,0 +1,26 @@
+//// [tests/cases/compiler/distributiveConditionalTypeNeverIntersection1.ts] ////
+
+=== distributiveConditionalTypeNeverIntersection1.ts ===
+// https://github.com/microsoft/TypeScript/issues/57343
+
+type IsNumber<T> = T extends number ? true : false;
+>IsNumber : IsNumber<T>
+>true : true
+>false : false
+
+type Conflicted = { x: true } & { x: false };
+>Conflicted : never
+>x : true
+>true : true
+>x : false
+>false : false
+
+type Ex1 = IsNumber<Conflicted>; // never
+>Ex1 : never
+
+type Ex2 = IsNumber<"OEEE" | Conflicted>; // false
+>Ex2 : false
+
+type Ex3 = IsNumber<1 | Conflicted>; // true
+>Ex3 : true
+

tests/cases/compiler/distributiveConditionalTypeNeverIntersection1.ts

@@ -0,0 +1,12 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/57343
+
+type IsNumber<T> = T extends number ? true : false;
+
+type Conflicted = { x: true } & { x: false };
+
+type Ex1 = IsNumber<Conflicted>; // never
+type Ex2 = IsNumber<"OEEE" | Conflicted>; // false
+type Ex3 = IsNumber<1 | Conflicted>; // true