Improve checks for infinitely expanding recursive conditional types (#46326)

* Improve checks for infinitely expanding recursive conditional types

* Accept new baselines

* Add regression tests

* Remove 'export' modifier

* Accept new baselines

Author: ahejlsberg

src/compiler/checker.ts

@@ -19024,6 +19024,12 @@ namespace ts {
                     }
                 }
                 else if (target.flags & TypeFlags.Conditional) {
+                    // If we reach 10 levels of nesting for the same conditional type, assume it is an infinitely expanding recursive
+                    // conditional type and bail out with a Ternary.Maybe result.
+                    if (isDeeplyNestedType(target, targetStack, targetDepth, 10)) {
+                        resetErrorInfo(saveErrorInfo);
+                        return Ternary.Maybe;
+                    }
                     const c = target as ConditionalType;
                     // Check if the conditional is always true or always false but still deferred for distribution purposes
                     const skipTrue = !isTypeAssignableTo(getPermissiveInstantiation(c.checkType), getPermissiveInstantiation(c.extendsType));
@@ -19123,33 +19129,34 @@ namespace ts {
                     }
                 }
                 else if (source.flags & TypeFlags.Conditional) {
+                    // If we reach 10 levels of nesting for the same conditional type, assume it is an infinitely expanding recursive
+                    // conditional type and bail out with a Ternary.Maybe result.
+                    if (isDeeplyNestedType(source, sourceStack, sourceDepth, 10)) {
+                        resetErrorInfo(saveErrorInfo);
+                        return Ternary.Maybe;
+                    }
                     if (target.flags & TypeFlags.Conditional) {
-                        // If one of the conditionals under comparison seems to be infinitely expanding, stop comparing it - back out, try
-                        // the constraint, and failing that, give up trying to relate the two. This is the only way we can handle recursive conditional
-                        // types, which might expand forever.
-                        if (!isDeeplyNestedType(source, sourceStack, sourceDepth) && !isDeeplyNestedType(target, targetStack, targetDepth)) {
-                            // Two conditional types 'T1 extends U1 ? X1 : Y1' and 'T2 extends U2 ? X2 : Y2' are related if
-                            // one of T1 and T2 is related to the other, U1 and U2 are identical types, X1 is related to X2,
-                            // and Y1 is related to Y2.
-                            const sourceParams = (source as ConditionalType).root.inferTypeParameters;
-                            let sourceExtends = (source as ConditionalType).extendsType;
-                            let mapper: TypeMapper | undefined;
-                            if (sourceParams) {
-                                // If the source has infer type parameters, we instantiate them in the context of the target
-                                const ctx = createInferenceContext(sourceParams, /*signature*/ undefined, InferenceFlags.None, isRelatedToWorker);
-                                inferTypes(ctx.inferences, (target as ConditionalType).extendsType, sourceExtends, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
-                                sourceExtends = instantiateType(sourceExtends, ctx.mapper);
-                                mapper = ctx.mapper;
-                            }
-                            if (isTypeIdenticalTo(sourceExtends, (target as ConditionalType).extendsType) &&
-                                (isRelatedTo((source as ConditionalType).checkType, (target as ConditionalType).checkType, RecursionFlags.Both) || isRelatedTo((target as ConditionalType).checkType, (source as ConditionalType).checkType, RecursionFlags.Both))) {
-                                if (result = isRelatedTo(instantiateType(getTrueTypeFromConditionalType(source as ConditionalType), mapper), getTrueTypeFromConditionalType(target as ConditionalType), RecursionFlags.Both, reportErrors)) {
-                                    result &= isRelatedTo(getFalseTypeFromConditionalType(source as ConditionalType), getFalseTypeFromConditionalType(target as ConditionalType), RecursionFlags.Both, reportErrors);
-                                }
-                                if (result) {
-                                    resetErrorInfo(saveErrorInfo);
-                                    return result;
-                                }
+                        // Two conditional types 'T1 extends U1 ? X1 : Y1' and 'T2 extends U2 ? X2 : Y2' are related if
+                        // one of T1 and T2 is related to the other, U1 and U2 are identical types, X1 is related to X2,
+                        // and Y1 is related to Y2.
+                        const sourceParams = (source as ConditionalType).root.inferTypeParameters;
+                        let sourceExtends = (source as ConditionalType).extendsType;
+                        let mapper: TypeMapper | undefined;
+                        if (sourceParams) {
+                            // If the source has infer type parameters, we instantiate them in the context of the target
+                            const ctx = createInferenceContext(sourceParams, /*signature*/ undefined, InferenceFlags.None, isRelatedToWorker);
+                            inferTypes(ctx.inferences, (target as ConditionalType).extendsType, sourceExtends, InferencePriority.NoConstraints | InferencePriority.AlwaysStrict);
+                            sourceExtends = instantiateType(sourceExtends, ctx.mapper);
+                            mapper = ctx.mapper;
+                        }
+                        if (isTypeIdenticalTo(sourceExtends, (target as ConditionalType).extendsType) &&
+                            (isRelatedTo((source as ConditionalType).checkType, (target as ConditionalType).checkType, RecursionFlags.Both) || isRelatedTo((target as ConditionalType).checkType, (source as ConditionalType).checkType, RecursionFlags.Both))) {
+                            if (result = isRelatedTo(instantiateType(getTrueTypeFromConditionalType(source as ConditionalType), mapper), getTrueTypeFromConditionalType(target as ConditionalType), RecursionFlags.Both, reportErrors)) {
+                                result &= isRelatedTo(getFalseTypeFromConditionalType(source as ConditionalType), getFalseTypeFromConditionalType(target as ConditionalType), RecursionFlags.Both, reportErrors);
+                            }
+                            if (result) {
+                                resetErrorInfo(saveErrorInfo);
+                                return result;
                             }
                         }
                     }
@@ -19165,18 +19172,13 @@ namespace ts {
                         }
                     }
 
-
-                    // We'll repeatedly decompose source side conditionals if they're recursive - check if we've already recured on the constraint a lot and, if so, bail
-                    // on the comparison.
-                    if (!isDeeplyNestedType(source, sourceStack, sourceDepth)) {
-                        // conditionals _can_ be related to one another via normal constraint, as, eg, `A extends B ? O : never` should be assignable to `O`
-                        // when `O` is a conditional (`never` is trivially assignable to `O`, as is `O`!).
-                        const defaultConstraint = getDefaultConstraintOfConditionalType(source as ConditionalType);
-                        if (defaultConstraint) {
-                            if (result = isRelatedTo(defaultConstraint, target, RecursionFlags.Source, reportErrors)) {
-                                resetErrorInfo(saveErrorInfo);
-                                return result;
-                            }
+                    // conditionals _can_ be related to one another via normal constraint, as, eg, `A extends B ? O : never` should be assignable to `O`
+                    // when `O` is a conditional (`never` is trivially assignable to `O`, as is `O`!).
+                    const defaultConstraint = getDefaultConstraintOfConditionalType(source as ConditionalType);
+                    if (defaultConstraint) {
+                        if (result = isRelatedTo(defaultConstraint, target, RecursionFlags.Source, reportErrors)) {
+                            resetErrorInfo(saveErrorInfo);
+                            return result;
                         }
                     }
                 }
@@ -20322,14 +20324,14 @@ namespace ts {
         // `type A<T> = null extends T ? [A<NonNullable<T>>] : [T]`
         // has expanded into `[A<NonNullable<NonNullable<NonNullable<NonNullable<NonNullable<T>>>>>>]`
         // in such cases we need to terminate the expansion, and we do so here.
-        function isDeeplyNestedType(type: Type, stack: Type[], depth: number): boolean {
-            if (depth >= 5) {
+        function isDeeplyNestedType(type: Type, stack: Type[], depth: number, maxDepth = 5): boolean {
+            if (depth >= maxDepth) {
                 const identity = getRecursionIdentity(type);
                 let count = 0;
                 for (let i = 0; i < depth; i++) {
                     if (getRecursionIdentity(stack[i]) === identity) {
                         count++;
-                        if (count >= 5) {
+                        if (count >= maxDepth) {
                             return true;
                         }
                     }

tests/baselines/reference/genericConditionalConstrainedToUnknownNotAssignableToConcreteObject.errors.txt

@@ -8,7 +8,8 @@ tests/cases/compiler/genericConditionalConstrainedToUnknownNotAssignableToConcre
               Type 'ReturnType<T[string]>' is not assignable to type 'A'.
                 Type 'unknown' is not assignable to type 'A'.
                   Type 'ReturnType<FunctionsObj<T>[string]>' is not assignable to type 'A'.
-                    Property 'x' is missing in type '{}' but required in type 'A'.
+                    Type 'unknown' is not assignable to type 'A'.
+                      Property 'x' is missing in type '{}' but required in type 'A'.
 
 
 ==== tests/cases/compiler/genericConditionalConstrainedToUnknownNotAssignableToConcreteObject.ts (1 errors) ====
@@ -36,7 +37,8 @@ tests/cases/compiler/genericConditionalConstrainedToUnknownNotAssignableToConcre
 !!! error TS2322:               Type 'ReturnType<T[string]>' is not assignable to type 'A'.
 !!! error TS2322:                 Type 'unknown' is not assignable to type 'A'.
 !!! error TS2322:                   Type 'ReturnType<FunctionsObj<T>[string]>' is not assignable to type 'A'.
-!!! error TS2322:                     Property 'x' is missing in type '{}' but required in type 'A'.
+!!! error TS2322:                     Type 'unknown' is not assignable to type 'A'.
+!!! error TS2322:                       Property 'x' is missing in type '{}' but required in type 'A'.
 !!! related TS2728 tests/cases/compiler/genericConditionalConstrainedToUnknownNotAssignableToConcreteObject.ts:1:15: 'x' is declared here.
     }
 

tests/baselines/reference/infiniteConstraints.errors.txt

@@ -2,10 +2,9 @@ tests/cases/compiler/infiniteConstraints.ts(4,37): error TS2536: Type '"val"' ca
 tests/cases/compiler/infiniteConstraints.ts(31,43): error TS2322: Type 'Value<"dup">' is not assignable to type 'never'.
 tests/cases/compiler/infiniteConstraints.ts(31,63): error TS2322: Type 'Value<"dup">' is not assignable to type 'never'.
 tests/cases/compiler/infiniteConstraints.ts(36,71): error TS2536: Type '"foo"' cannot be used to index type 'T[keyof T]'.
-tests/cases/compiler/infiniteConstraints.ts(48,27): error TS2321: Excessive stack depth comparing types 'Conv<ExactExtract<U, T>, ExactExtract<U, T>>' and 'unknown[]'.
 
 
-==== tests/cases/compiler/infiniteConstraints.ts (5 errors) ====
+==== tests/cases/compiler/infiniteConstraints.ts (4 errors) ====
     // Both of the following types trigger the recursion limiter in getImmediateBaseConstraint
 
     type T1<B extends { [K in keyof B]: Extract<B[Exclude<keyof B, K>], { val: string }>["val"] }> = B;
@@ -64,6 +63,4 @@ tests/cases/compiler/infiniteConstraints.ts(48,27): error TS2321: Excessive stac
 
     type Conv<T, U = T> =
       { 0: [T]; 1: Prepend<T, Conv<ExactExtract<U, T>>>;}[U extends T ? 0 : 1];
-                              ~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2321: Excessive stack depth comparing types 'Conv<ExactExtract<U, T>, ExactExtract<U, T>>' and 'unknown[]'.
 

tests/baselines/reference/recursiveConditionalTypes.errors.txt

@@ -23,9 +23,10 @@ tests/cases/compiler/recursiveConditionalTypes.ts(117,9): error TS2345: Argument
           Type '[string]' is not assignable to type 'Grow1<[number], T>'.
             Type '[string]' is not assignable to type '[number]'.
               Type 'string' is not assignable to type 'number'.
+tests/cases/compiler/recursiveConditionalTypes.ts(169,5): error TS2322: Type 'number' is not assignable to type 'Enumerate<T["length"]>'.
 
 
-==== tests/cases/compiler/recursiveConditionalTypes.ts (9 errors) ====
+==== tests/cases/compiler/recursiveConditionalTypes.ts (10 errors) ====
     // Awaiting promises
 
     type __Awaited<T> =
@@ -198,4 +199,38 @@ tests/cases/compiler/recursiveConditionalTypes.ts(117,9): error TS2345: Argument
     type Helper<T> = T extends ParseSuccess<infer R> ? ParseSuccess<R> : null
 
     type TP2 = ParseManyWhitespace2<" foo">;
+    
+    // Repro from #46183
+    
+    type NTuple<N extends number, Tup extends unknown[] = []> =
+        Tup['length'] extends N ? Tup : NTuple<N, [...Tup, unknown]>;
+    
+    type Add<A extends number, B extends number> =
+        [...NTuple<A>, ...NTuple<B>]['length'];
+    
+    let five: Add<2, 3>;
+    
+    // Repro from #46316
+    
+    type _PrependNextNum<A extends Array<unknown>> = A['length'] extends infer T
+        ? [T, ...A] extends [...infer X] 
+            ? X
+            : never
+        : never;
+    
+    type _Enumerate<A extends Array<unknown>, N extends number> = N extends A['length']
+        ? A
+        : _Enumerate<_PrependNextNum<A>, N> & number;
+    
+    type Enumerate<N extends number> = number extends N
+        ? number
+        : _Enumerate<[], N> extends (infer E)[]
+        ? E
+        : never;
+    
+    function foo2<T extends unknown[]>(value: T): Enumerate<T['length']> {
+        return value.length;  // Error
+        ~~~~~~~~~~~~~~~~~~~~
+!!! error TS2322: Type 'number' is not assignable to type 'Enumerate<T["length"]>'.
+    }
 

tests/baselines/reference/recursiveConditionalTypes.js

@@ -137,6 +137,38 @@ type ParseManyWhitespace2<S extends string> =
 type Helper<T> = T extends ParseSuccess<infer R> ? ParseSuccess<R> : null
 
 type TP2 = ParseManyWhitespace2<" foo">;
+
+// Repro from #46183
+
+type NTuple<N extends number, Tup extends unknown[] = []> =
+    Tup['length'] extends N ? Tup : NTuple<N, [...Tup, unknown]>;
+
+type Add<A extends number, B extends number> =
+    [...NTuple<A>, ...NTuple<B>]['length'];
+
+let five: Add<2, 3>;
+
+// Repro from #46316
+
+type _PrependNextNum<A extends Array<unknown>> = A['length'] extends infer T
+    ? [T, ...A] extends [...infer X] 
+        ? X
+        : never
+    : never;
+
+type _Enumerate<A extends Array<unknown>, N extends number> = N extends A['length']
+    ? A
+    : _Enumerate<_PrependNextNum<A>, N> & number;
+
+type Enumerate<N extends number> = number extends N
+    ? number
+    : _Enumerate<[], N> extends (infer E)[]
+    ? E
+    : never;
+
+function foo2<T extends unknown[]>(value: T): Enumerate<T['length']> {
+    return value.length;  // Error
+}
 
 
 //// [recursiveConditionalTypes.js]
@@ -169,6 +201,10 @@ function f20(x, y) {
 function f21(x, y) {
     f21(y, x); // Error
 }
+let five;
+function foo2(value) {
+    return value.length; // Error
+}
 
 
 //// [recursiveConditionalTypes.d.ts]
@@ -244,3 +280,13 @@ declare type TP1 = ParseManyWhitespace<" foo">;
 declare type ParseManyWhitespace2<S extends string> = S extends ` ${infer R0}` ? Helper<ParseManyWhitespace2<R0>> : ParseSuccess<S>;
 declare type Helper<T> = T extends ParseSuccess<infer R> ? ParseSuccess<R> : null;
 declare type TP2 = ParseManyWhitespace2<" foo">;
+declare type NTuple<N extends number, Tup extends unknown[] = []> = Tup['length'] extends N ? Tup : NTuple<N, [...Tup, unknown]>;
+declare type Add<A extends number, B extends number> = [
+    ...NTuple<A>,
+    ...NTuple<B>
+]['length'];
+declare let five: Add<2, 3>;
+declare type _PrependNextNum<A extends Array<unknown>> = A['length'] extends infer T ? [T, ...A] extends [...infer X] ? X : never : never;
+declare type _Enumerate<A extends Array<unknown>, N extends number> = N extends A['length'] ? A : _Enumerate<_PrependNextNum<A>, N> & number;
+declare type Enumerate<N extends number> = number extends N ? number : _Enumerate<[], N> extends (infer E)[] ? E : never;
+declare function foo2<T extends unknown[]>(value: T): Enumerate<T['length']>;