Flatten immediately nested conditional types in the false position (#36583)

* Flatten immediately nested conditional types in the false position

* Add test

* Accept new baselines

* Handle nested distributive types with different checkType

* Allow deeply nested immediately resolving conditionals without any syntactic requirements or implementation contortions

Extract logic into function

Co-authored-by: Wesley Wigham <wewigham@microsoft.com>

Author: ahejlsberg

src/compiler/checker.ts

@@ -12889,65 +12889,82 @@ namespace ts {
         }
 
         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;
+            let result;
+            let extraTypes: Type[] | undefined;
+            // We loop here for an immediately nested conditional type in the false position, effectively treating
+            // types of the form 'A extends B ? X : C extends D ? Y : E extends F ? Z : ...' as a single construct for
+            // purposes of resolution. This means such types aren't subject to the instatiation depth limiter.
+            while (true) {
+                const checkType = instantiateType(root.checkType, mapper);
+                const checkTypeInstantiable = isGenericObjectType(checkType) || isGenericIndexType(checkType);
+                const extendsType = instantiateType(root.extendsType, mapper);
+                if (checkType === wildcardType || extendsType === wildcardType) {
+                    return wildcardType;
+                }
+                let combinedMapper: TypeMapper | undefined;
+                if (root.inferTypeParameters) {
+                    const context = createInferenceContext(root.inferTypeParameters, /*signature*/ undefined, InferenceFlags.None);
+                    // We skip inference of the possible `infer` types unles the `extendsType` _is_ an infer type
+                    // if it was, it's trivial to say that extendsType = checkType, however such a pattern is used to
+                    // "reset" the type being build up during constraint calculation and avoid making an apparently "infinite" constraint
+                    // so in those cases we refain from performing inference and retain the uninfered type parameter
+                    if (!checkTypeInstantiable || !some(root.inferTypeParameters, t => t === extendsType)) {
+                        // We don't want inferences from constraints as they may cause us to eagerly resolve the
+                        // conditional type instead of deferring resolution. Also, we always want strict function
+                        // types rules (i.e. proper contravariance) for inferences.
+                        inferTypes(context.inferences, checkType, extendsType, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
+                    }
+                    combinedMapper = mergeTypeMappers(mapper, context.mapper);
+                }
+                // Instantiate the extends type including inferences for 'infer T' type parameters
+                const inferredExtendsType = combinedMapper ? instantiateType(root.extendsType, combinedMapper) : extendsType;
+                // We attempt to resolve the conditional type only when the check and extends types are non-generic
+                if (!checkTypeInstantiable && !isGenericObjectType(inferredExtendsType) && !isGenericIndexType(inferredExtendsType)) {
+                    // Return falseType for a definitely false extends check. We check an instantiations of the two
+                    // types with type parameters mapped to the wildcard type, the most permissive instantiations
+                    // possible (the wildcard type is assignable to and from all types). If those are not related,
+                    // then no instantiations will be and we can just return the false branch type.
+                    if (!(inferredExtendsType.flags & TypeFlags.AnyOrUnknown) && (checkType.flags & TypeFlags.Any || !isTypeAssignableTo(getPermissiveInstantiation(checkType), getPermissiveInstantiation(inferredExtendsType)))) {
+                        // Return union of trueType and falseType for 'any' since it matches anything
+                        if (checkType.flags & TypeFlags.Any) {
+                            (extraTypes || (extraTypes = [])).push(instantiateTypeWithoutDepthIncrease(root.trueType, combinedMapper || mapper));
+                        }
+                        // If falseType is an immediately nested conditional type that isn't distributive or has an
+                        // identical checkType, switch to that type and loop.
+                        const falseType = root.falseType;
+                        if (falseType.flags & TypeFlags.Conditional) {
+                            const newRoot = (<ConditionalType>falseType).root;
+                            if (newRoot.node.parent === root.node && (!newRoot.isDistributive || newRoot.checkType === root.checkType)) {
+                                root = newRoot;
+                                continue;
+                            }
+                        }
+                        result = instantiateTypeWithoutDepthIncrease(falseType, mapper);
+                        break;
+                    }
+                    // Return trueType for a definitely true extends check. We check instantiations of the two
+                    // types with type parameters mapped to their restrictive form, i.e. a form of the type parameter
+                    // that has no constraint. This ensures that, for example, the type
+                    //   type Foo<T extends { x: any }> = T extends { x: string } ? string : number
+                    // doesn't immediately resolve to 'string' instead of being deferred.
+                    if (inferredExtendsType.flags & TypeFlags.AnyOrUnknown || isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(inferredExtendsType))) {
+                        result = instantiateTypeWithoutDepthIncrease(root.trueType, combinedMapper || mapper);
+                        break;
+                    }
+                }
+                // Return a deferred type for a check that is neither definitely true nor definitely false
+                const erasedCheckType = getActualTypeVariable(checkType);
+                result = <ConditionalType>createType(TypeFlags.Conditional);
+                result.root = root;
+                result.checkType = erasedCheckType;
+                result.extendsType = extendsType;
+                result.mapper = mapper;
+                result.combinedMapper = combinedMapper;
+                result.aliasSymbol = root.aliasSymbol;
+                result.aliasTypeArguments = instantiateTypes(root.aliasTypeArguments, mapper!); // TODO: GH#18217
+                break;
             }
-            const checkTypeInstantiable = isGenericObjectType(checkType) || isGenericIndexType(checkType);
-            let combinedMapper: TypeMapper | undefined;
-            if (root.inferTypeParameters) {
-                const context = createInferenceContext(root.inferTypeParameters, /*signature*/ undefined, InferenceFlags.None);
-                // We skip inference of the possible `infer` types unles the `extendsType` _is_ an infer type
-                // if it was, it's trivial to say that extendsType = checkType, however such a pattern is used to
-                // "reset" the type being build up during constraint calculation and avoid making an apparently "infinite" constraint
-                // so in those cases we refain from performing inference and retain the uninfered type parameter
-                if (!checkTypeInstantiable || !some(root.inferTypeParameters, t => t === extendsType)) {
-                    // We don't want inferences from constraints as they may cause us to eagerly resolve the
-                    // conditional type instead of deferring resolution. Also, we always want strict function
-                    // types rules (i.e. proper contravariance) for inferences.
-                    inferTypes(context.inferences, checkType, extendsType, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
-                }
-                combinedMapper = mergeTypeMappers(mapper, context.mapper);
-            }
-            // Instantiate the extends type including inferences for 'infer T' type parameters
-            const inferredExtendsType = combinedMapper ? instantiateType(root.extendsType, combinedMapper) : extendsType;
-            // We attempt to resolve the conditional type only when the check and extends types are non-generic
-            if (!checkTypeInstantiable && !isGenericObjectType(inferredExtendsType) && !isGenericIndexType(inferredExtendsType)) {
-                if (inferredExtendsType.flags & TypeFlags.AnyOrUnknown) {
-                    return instantiateType(root.trueType, combinedMapper || mapper);
-                }
-                // Return union of trueType and falseType for 'any' since it matches anything
-                if (checkType.flags & TypeFlags.Any) {
-                    return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), instantiateType(root.falseType, mapper)]);
-                }
-                // Return falseType for a definitely false extends check. We check an instantiations of the two
-                // types with type parameters mapped to the wildcard type, the most permissive instantiations
-                // possible (the wildcard type is assignable to and from all types). If those are not related,
-                // then no instantiations will be and we can just return the false branch type.
-                if (!isTypeAssignableTo(getPermissiveInstantiation(checkType), getPermissiveInstantiation(inferredExtendsType))) {
-                    return instantiateType(root.falseType, mapper);
-                }
-                // Return trueType for a definitely true extends check. We check instantiations of the two
-                // types with type parameters mapped to their restrictive form, i.e. a form of the type parameter
-                // that has no constraint. This ensures that, for example, the type
-                //   type Foo<T extends { x: any }> = T extends { x: string } ? string : number
-                // doesn't immediately resolve to 'string' instead of being deferred.
-                if (isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(inferredExtendsType))) {
-                    return instantiateType(root.trueType, combinedMapper || mapper);
-                }
-            }
-            // Return a deferred type for a check that is neither definitely true nor definitely false
-            const erasedCheckType = getActualTypeVariable(checkType);
-            const result = <ConditionalType>createType(TypeFlags.Conditional);
-            result.root = root;
-            result.checkType = erasedCheckType;
-            result.extendsType = extendsType;
-            result.mapper = mapper;
-            result.combinedMapper = combinedMapper;
-            result.aliasSymbol = root.aliasSymbol;
-            result.aliasTypeArguments = instantiateTypes(root.aliasTypeArguments, mapper!); // TODO: GH#18217
-            return result;
+            return extraTypes ? getUnionType(append(extraTypes, result)) : result;
         }
 
         function getTrueTypeFromConditionalType(type: ConditionalType) {
@@ -13929,6 +13946,17 @@ namespace ts {
             return result;
         }
 
+        /**
+         * This can be used to avoid the penalty on instantiation depth for types which result from immediate
+         * simplification. It essentially removes the depth increase done in `instantiateType`.
+         */
+        function instantiateTypeWithoutDepthIncrease(type: Type, mapper: TypeMapper | undefined) {
+            instantiationDepth--;
+            const result = instantiateType(type, mapper);
+            instantiationDepth++;
+            return result;
+        }
+
         function instantiateTypeWorker(type: Type, mapper: TypeMapper): Type {
             const flags = type.flags;
             if (flags & TypeFlags.TypeParameter) {

tests/baselines/reference/deeplyNestedConditionalTypes.js

@@ -0,0 +1,116 @@
+//// [deeplyNestedConditionalTypes.ts]
+type Foo<T> =
+    T extends 0 ? '0' :
+    T extends 1 ? '1' :
+    T extends 2 ? '2' :
+    T extends 3 ? '3' :
+    T extends 4 ? '4' :
+    T extends 5 ? '5' :
+    T extends 6 ? '6' :
+    T extends 7 ? '7' :
+    T extends 8 ? '8' :
+    T extends 9 ? '9' :
+    T extends 10 ? '10' :
+    T extends 11 ? '11' :
+    T extends 12 ? '12' :
+    T extends 13 ? '13' :
+    T extends 14 ? '14' :
+    T extends 15 ? '15' :
+    T extends 16 ? '16' :
+    T extends 17 ? '17' :
+    T extends 18 ? '18' :
+    T extends 19 ? '19' :
+    T extends 20 ? '20' :
+    T extends 21 ? '21' :
+    T extends 22 ? '22' :
+    T extends 23 ? '23' :
+    T extends 24 ? '24' :
+    T extends 25 ? '25' :
+    T extends 26 ? '26' :
+    T extends 27 ? '27' :
+    T extends 28 ? '28' :
+    T extends 29 ? '29' :
+    T extends 30 ? '30' :
+    T extends 31 ? '31' :
+    T extends 32 ? '32' :
+    T extends 33 ? '33' :
+    T extends 34 ? '34' :
+    T extends 35 ? '35' :
+    T extends 36 ? '36' :
+    T extends 37 ? '37' :
+    T extends 38 ? '38' :
+    T extends 39 ? '39' :
+    T extends 40 ? '40' :
+    T extends 41 ? '41' :
+    T extends 42 ? '42' :
+    T extends 43 ? '43' :
+    T extends 44 ? '44' :
+    T extends 45 ? '45' :
+    T extends 46 ? '46' :
+    T extends 47 ? '47' :
+    T extends 48 ? '48' :
+    T extends 49 ? '49' :
+    T extends 50 ? '50' :
+    T extends 51 ? '51' :
+    T extends 52 ? '52' :
+    T extends 53 ? '53' :
+    T extends 54 ? '54' :
+    T extends 55 ? '55' :
+    T extends 56 ? '56' :
+    T extends 57 ? '57' :
+    T extends 58 ? '58' :
+    T extends 59 ? '59' :
+    T extends 60 ? '60' :
+    T extends 61 ? '61' :
+    T extends 62 ? '62' :
+    T extends 63 ? '63' :
+    T extends 64 ? '64' :
+    T extends 65 ? '65' :
+    T extends 66 ? '66' :
+    T extends 67 ? '67' :
+    T extends 68 ? '68' :
+    T extends 69 ? '69' :
+    T extends 70 ? '70' :
+    T extends 71 ? '71' :
+    T extends 72 ? '72' :
+    T extends 73 ? '73' :
+    T extends 74 ? '74' :
+    T extends 75 ? '75' :
+    T extends 76 ? '76' :
+    T extends 77 ? '77' :
+    T extends 78 ? '78' :
+    T extends 79 ? '79' :
+    T extends 80 ? '80' :
+    T extends 81 ? '81' :
+    T extends 82 ? '82' :
+    T extends 83 ? '83' :
+    T extends 84 ? '84' :
+    T extends 85 ? '85' :
+    T extends 86 ? '86' :
+    T extends 87 ? '87' :
+    T extends 88 ? '88' :
+    T extends 89 ? '89' :
+    T extends 90 ? '90' :
+    T extends 91 ? '91' :
+    T extends 92 ? '92' :
+    T extends 93 ? '93' :
+    T extends 94 ? '94' :
+    T extends 95 ? '95' :
+    T extends 96 ? '96' :
+    T extends 97 ? '97' :
+    T extends 98 ? '98' :
+    T extends 99 ? '99' :
+    never;
+
+type T0 = Foo<99>;
+type T1 = Foo<any>;
+
+
+//// [deeplyNestedConditionalTypes.js]
+"use strict";
+
+
+//// [deeplyNestedConditionalTypes.d.ts]
+declare type Foo<T> = T extends 0 ? '0' : T extends 1 ? '1' : T extends 2 ? '2' : T extends 3 ? '3' : T extends 4 ? '4' : T extends 5 ? '5' : T extends 6 ? '6' : T extends 7 ? '7' : T extends 8 ? '8' : T extends 9 ? '9' : T extends 10 ? '10' : T extends 11 ? '11' : T extends 12 ? '12' : T extends 13 ? '13' : T extends 14 ? '14' : T extends 15 ? '15' : T extends 16 ? '16' : T extends 17 ? '17' : T extends 18 ? '18' : T extends 19 ? '19' : T extends 20 ? '20' : T extends 21 ? '21' : T extends 22 ? '22' : T extends 23 ? '23' : T extends 24 ? '24' : T extends 25 ? '25' : T extends 26 ? '26' : T extends 27 ? '27' : T extends 28 ? '28' : T extends 29 ? '29' : T extends 30 ? '30' : T extends 31 ? '31' : T extends 32 ? '32' : T extends 33 ? '33' : T extends 34 ? '34' : T extends 35 ? '35' : T extends 36 ? '36' : T extends 37 ? '37' : T extends 38 ? '38' : T extends 39 ? '39' : T extends 40 ? '40' : T extends 41 ? '41' : T extends 42 ? '42' : T extends 43 ? '43' : T extends 44 ? '44' : T extends 45 ? '45' : T extends 46 ? '46' : T extends 47 ? '47' : T extends 48 ? '48' : T extends 49 ? '49' : T extends 50 ? '50' : T extends 51 ? '51' : T extends 52 ? '52' : T extends 53 ? '53' : T extends 54 ? '54' : T extends 55 ? '55' : T extends 56 ? '56' : T extends 57 ? '57' : T extends 58 ? '58' : T extends 59 ? '59' : T extends 60 ? '60' : T extends 61 ? '61' : T extends 62 ? '62' : T extends 63 ? '63' : T extends 64 ? '64' : T extends 65 ? '65' : T extends 66 ? '66' : T extends 67 ? '67' : T extends 68 ? '68' : T extends 69 ? '69' : T extends 70 ? '70' : T extends 71 ? '71' : T extends 72 ? '72' : T extends 73 ? '73' : T extends 74 ? '74' : T extends 75 ? '75' : T extends 76 ? '76' : T extends 77 ? '77' : T extends 78 ? '78' : T extends 79 ? '79' : T extends 80 ? '80' : T extends 81 ? '81' : T extends 82 ? '82' : T extends 83 ? '83' : T extends 84 ? '84' : T extends 85 ? '85' : T extends 86 ? '86' : T extends 87 ? '87' : T extends 88 ? '88' : T extends 89 ? '89' : T extends 90 ? '90' : T extends 91 ? '91' : T extends 92 ? '92' : T extends 93 ? '93' : T extends 94 ? '94' : T extends 95 ? '95' : T extends 96 ? '96' : T extends 97 ? '97' : T extends 98 ? '98' : T extends 99 ? '99' : never;
+declare type T0 = Foo<99>;
+declare type T1 = Foo<any>;