Remove simplification logic from getConditionalType + simplify substitution types

Author: ahejlsberg

src/compiler/checker.ts

@@ -10196,6 +10196,7 @@ namespace ts {
         function getSimplifiedType(type: Type, writing: boolean): Type {
             return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(<IndexedAccessType>type, writing) :
                 type.flags & TypeFlags.Conditional ? getSimplifiedConditionalType(<ConditionalType>type, writing) :
+                type.flags & TypeFlags.Substitution ? writing ? (<SubstitutionType>type).typeVariable : (<SubstitutionType>type).substitute :
                 type;
         }
 
@@ -10260,10 +10261,10 @@ namespace ts {
         }
 
         function getSimplifiedConditionalType(type: ConditionalType, writing: boolean) {
-            const falseType = getFalseTypeFromConditionalType(type);
-            const trueType = getTrueTypeFromConditionalType(type);
             const checkType = type.checkType;
             const extendsType = type.extendsType;
+            const trueType = getTrueTypeFromConditionalType(type);
+            const falseType = getFalseTypeFromConditionalType(type);
             // Simplifications for types of the form `T extends U ? T : never` and `T extends U ? never : T`.
             if (falseType.flags & TypeFlags.Never && getActualTypeVariable(trueType) === getActualTypeVariable(checkType)) {
                 if (checkType.flags & TypeFlags.Any || isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
@@ -10281,10 +10282,16 @@ namespace ts {
                     return getSimplifiedType(falseType, writing);
                 }
             }
-
             return type;
         }
 
+        /**
+         * Invokes union simplification logic to determine if an intersection is considered empty as a union constituent
+         */
+        function isIntersectionEmpty(type1: Type, type2: Type) {
+            return !!(getUnionType([intersectTypes(type1, type2), neverType]).flags & TypeFlags.Never);
+        }
+
         function substituteIndexedMappedType(objectType: MappedType, index: Type) {
             const mapper = createTypeMapper([getTypeParameterFromMappedType(objectType)], [index]);
             const templateMapper = combineTypeMappers(objectType.mapper, mapper);
@@ -10391,36 +10398,12 @@ namespace ts {
             return type;
         }
 
-        /**
-         * Invokes union simplification logic to determine if an intersection is considered empty as a union constituent
-         */
-        function isIntersectionEmpty(type1: Type, type2: Type) {
-            return !!(getUnionType([intersectTypes(type1, type2), neverType]).flags & TypeFlags.Never);
-        }
-
         function getConditionalType(root: ConditionalRoot, mapper: TypeMapper | undefined): Type {
             const checkType = instantiateType(root.checkType, mapper);
             const extendsType = instantiateType(root.extendsType, mapper);
             if (checkType === wildcardType || extendsType === wildcardType) {
                 return wildcardType;
             }
-            // Simplifications for types of the form `T extends U ? T : never` and `T extends U ? never : T`.
-            if (root.falseType.flags & TypeFlags.Never && getActualTypeVariable(root.trueType) === getActualTypeVariable(root.checkType)) {
-                if (checkType.flags & TypeFlags.Any || isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
-                    return checkType;
-                }
-                else if (isIntersectionEmpty(checkType, extendsType)) { // Always false
-                    return neverType;
-                }
-            }
-            else if (root.trueType.flags & TypeFlags.Never && getActualTypeVariable(root.falseType) === getActualTypeVariable(root.checkType)) {
-                if (!(checkType.flags & TypeFlags.Any) && isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
-                    return neverType;
-                }
-                else if (checkType.flags & TypeFlags.Any || isIntersectionEmpty(checkType, extendsType)) { // Always false
-                    return checkType;
-                }
-            }
             const checkTypeInstantiable = maybeTypeOfKind(checkType, TypeFlags.Instantiable | TypeFlags.GenericMappedType);
             let combinedMapper: TypeMapper | undefined;
             if (root.inferTypeParameters) {
@@ -10461,10 +10444,6 @@ namespace ts {
                 }
             }
             // Return a deferred type for a check that is neither definitely true nor definitely false
-            return getDeferredConditionalType(root, mapper, combinedMapper, checkType, extendsType);
-        }
-
-        function getDeferredConditionalType(root: ConditionalRoot, mapper: TypeMapper | undefined, combinedMapper: TypeMapper | undefined, checkType: Type, extendsType: Type) {
             const erasedCheckType = getActualTypeVariable(checkType);
             const result = <ConditionalType>createType(TypeFlags.Conditional);
             result.root = root;
@@ -12453,12 +12432,6 @@ namespace ts {
                 if (isFreshLiteralType(target)) {
                     target = (<FreshableType>target).regularType;
                 }
-                if (source.flags & TypeFlags.Substitution) {
-                    source = (<SubstitutionType>source).substitute;
-                }
-                if (target.flags & TypeFlags.Substitution) {
-                    target = (<SubstitutionType>target).typeVariable;
-                }
                 if (source.flags & TypeFlags.Simplifiable) {
                     source = getSimplifiedType(source, /*writing*/ false);
                 }

src/compiler/types.ts

@@ -3967,7 +3967,7 @@ namespace ts {
         /* @internal */
         ObjectFlagsType = Nullable | Never | Object | Union | Intersection,
         /* @internal */
-        Simplifiable = IndexedAccess | Conditional,
+        Simplifiable = IndexedAccess | Conditional | Substitution,
         // 'Narrowable' types are types where narrowing actually narrows.
         // This *should* be every type other than null, undefined, void, and never
         Narrowable = Any | Unknown | StructuredOrInstantiable | StringLike | NumberLike | BigIntLike | BooleanLike | ESSymbol | UniqueESSymbol | NonPrimitive,