Allow nested conditionals to be related via constraints (#37208)

* Allow nested conditionals to be related via constraints

* Delete word in comment

Author: weswigham

src/compiler/checker.ts

@@ -16040,19 +16040,23 @@ namespace ts {
                         }
                     }
                     else {
+                        // conditionals aren't related to one another via distributive constraint as it is much too inaccurate and allows way
+                        // more assignments than are desirable (since it maps the source check type to its constraint, it loses information)
                         const distributiveConstraint = getConstraintOfDistributiveConditionalType(<ConditionalType>source);
                         if (distributiveConstraint) {
                             if (result = isRelatedTo(distributiveConstraint, target, reportErrors)) {
                                 resetErrorInfo(saveErrorInfo);
                                 return result;
                             }
                         }
-                        const defaultConstraint = getDefaultConstraintOfConditionalType(<ConditionalType>source);
-                        if (defaultConstraint) {
-                            if (result = isRelatedTo(defaultConstraint, target, 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 aissgnable to `O`, as is `O`!).
+                    const defaultConstraint = getDefaultConstraintOfConditionalType(<ConditionalType>source);
+                    if (defaultConstraint) {
+                        if (result = isRelatedTo(defaultConstraint, target, reportErrors)) {
+                            resetErrorInfo(saveErrorInfo);
+                            return result;
                         }
                     }
                 }

tests/baselines/reference/circularlyConstrainedMappedTypeContainingConditionalNoInfiniteInstantiationDepth.errors.txt

@@ -25,14 +25,16 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(114,5): error TS2
   Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
     Type 'string' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(115,5): error TS2322: Type 'T[keyof T] extends Function ? never : keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-  Type 'keyof T' is not assignable to type 'never'.
-    Type 'string | number | symbol' is not assignable to type 'never'.
-      Type 'string' is not assignable to type 'never'.
+  Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+    Type 'keyof T' is not assignable to type 'never'.
+      Type 'string | number | symbol' is not assignable to type 'never'.
+        Type 'string' is not assignable to type 'never'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(116,5): error TS2322: Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
   Type 'string | number | symbol' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
     Type 'string' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(117,5): error TS2322: Type 'T[keyof T] extends Function ? keyof T : never' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-  Type 'keyof T' is not assignable to type 'never'.
+  Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+    Type 'keyof T' is not assignable to type 'never'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(134,10): error TS2540: Cannot assign to 'id' because it is a read-only property.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(135,5): error TS2542: Index signature in type 'DeepReadonlyArray<Part>' only permits reading.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(136,22): error TS2540: Cannot assign to 'id' because it is a read-only property.
@@ -52,7 +54,9 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(160,5): error TS2
     Type 'string' is not assignable to type 'ZeroOf<T>'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(263,9): error TS2403: Subsequent variable declarations must have the same type.  Variable 'z' must be of type 'T1', but here has type 'Foo<T & U>'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS2322: Type 'T95<U>' is not assignable to type 'T94<U>'.
-  Type 'boolean' is not assignable to type 'true'.
+  Type 'number | boolean' is not assignable to type 'T94<U>'.
+    Type 'number' is not assignable to type 'T94<U>'.
+      Type 'boolean' is not assignable to type 'true'.
 
 
 ==== tests/cases/conformance/types/conditional/conditionalTypes1.ts (22 errors) ====
@@ -210,9 +214,10 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
         y = z;  // Error
         ~
 !!! error TS2322: Type 'T[keyof T] extends Function ? never : keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
-!!! error TS2322:   Type 'keyof T' is not assignable to type 'never'.
-!!! error TS2322:     Type 'string | number | symbol' is not assignable to type 'never'.
-!!! error TS2322:       Type 'string' is not assignable to type 'never'.
+!!! error TS2322:   Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? keyof T : never'.
+!!! error TS2322:     Type 'keyof T' is not assignable to type 'never'.
+!!! error TS2322:       Type 'string | number | symbol' is not assignable to type 'never'.
+!!! error TS2322:         Type 'string' is not assignable to type 'never'.
         z = x;  // Error
         ~
 !!! error TS2322: Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
@@ -221,7 +226,8 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
         z = y;  // Error
         ~
 !!! error TS2322: Type 'T[keyof T] extends Function ? keyof T : never' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
-!!! error TS2322:   Type 'keyof T' is not assignable to type 'never'.
+!!! error TS2322:   Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
+!!! error TS2322:     Type 'keyof T' is not assignable to type 'never'.
     }
 
     type DeepReadonly<T> =
@@ -421,7 +427,9 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
     const f45 = <U>(value: T95<U>): T94<U> => value;  // Error
                                               ~~~~~
 !!! error TS2322: Type 'T95<U>' is not assignable to type 'T94<U>'.
-!!! error TS2322:   Type 'boolean' is not assignable to type 'true'.
+!!! error TS2322:   Type 'number | boolean' is not assignable to type 'T94<U>'.
+!!! error TS2322:     Type 'number' is not assignable to type 'T94<U>'.
+!!! error TS2322:       Type 'boolean' is not assignable to type 'true'.
 
     // Repro from #21863
 

tests/baselines/reference/conditionalTypes1.errors.txt

@@ -7,21 +7,26 @@ tests/cases/conformance/types/conditional/conditionalTypes2.ts(19,5): error TS23
 tests/cases/conformance/types/conditional/conditionalTypes2.ts(24,5): error TS2322: Type 'Invariant<B>' is not assignable to type 'Invariant<A>'.
   Types of property 'foo' are incompatible.
     Type 'B extends string ? keyof B : B' is not assignable to type 'A extends string ? keyof A : A'.
-      Type 'keyof B' is not assignable to type 'keyof A'.
-        Type 'string | number | symbol' is not assignable to type 'keyof A'.
-          Type 'string' is not assignable to type 'keyof A'.
-            Type 'string' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
-              Type 'keyof B' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
-                Type 'string | number | symbol' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+      Type 'B | keyof B' is not assignable to type 'A extends string ? keyof A : A'.
+        Type 'B' is not assignable to type 'A extends string ? keyof A : A'.
+          Type 'A' is not assignable to type 'A extends string ? keyof A : A'.
+            Type 'keyof B' is not assignable to type 'keyof A'.
+              Type 'string | number | symbol' is not assignable to type 'keyof A'.
+                Type 'string' is not assignable to type 'keyof A'.
                   Type 'string' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
-                    Type 'keyof B' is not assignable to type 'number'.
-                      Type 'string | number | symbol' is not assignable to type 'number'.
-                        Type 'string' is not assignable to type 'number'.
+                    Type 'keyof B' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+                      Type 'string | number | symbol' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+                        Type 'string' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+                          Type 'keyof B' is not assignable to type 'number'.
+                            Type 'string | number | symbol' is not assignable to type 'number'.
+                              Type 'string' is not assignable to type 'number'.
 tests/cases/conformance/types/conditional/conditionalTypes2.ts(25,5): error TS2322: Type 'Invariant<A>' is not assignable to type 'Invariant<B>'.
   Types of property 'foo' are incompatible.
     Type 'A extends string ? keyof A : A' is not assignable to type 'B extends string ? keyof B : B'.
-      Type 'A' is not assignable to type 'B'.
-        'B' could be instantiated with an arbitrary type which could be unrelated to 'A'.
+      Type 'A | keyof A' is not assignable to type 'B extends string ? keyof B : B'.
+        Type 'A' is not assignable to type 'B extends string ? keyof B : B'.
+          Type 'A' is not assignable to type 'B'.
+            'B' could be instantiated with an arbitrary type which could be unrelated to 'A'.
 tests/cases/conformance/types/conditional/conditionalTypes2.ts(73,12): error TS2345: Argument of type 'Extract<Extract<T, Foo>, Bar>' is not assignable to parameter of type '{ foo: string; bat: string; }'.
   Property 'bat' is missing in type 'Bar & Foo' but required in type '{ foo: string; bat: string; }'.
     Type 'Extract<T, Bar>' is not assignable to type '{ foo: string; bat: string; }'.
@@ -70,23 +75,28 @@ tests/cases/conformance/types/conditional/conditionalTypes2.ts(75,12): error TS2
 !!! error TS2322: Type 'Invariant<B>' is not assignable to type 'Invariant<A>'.
 !!! error TS2322:   Types of property 'foo' are incompatible.
 !!! error TS2322:     Type 'B extends string ? keyof B : B' is not assignable to type 'A extends string ? keyof A : A'.
-!!! error TS2322:       Type 'keyof B' is not assignable to type 'keyof A'.
-!!! error TS2322:         Type 'string | number | symbol' is not assignable to type 'keyof A'.
-!!! error TS2322:           Type 'string' is not assignable to type 'keyof A'.
-!!! error TS2322:             Type 'string' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
-!!! error TS2322:               Type 'keyof B' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
-!!! error TS2322:                 Type 'string | number | symbol' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+!!! error TS2322:       Type 'B | keyof B' is not assignable to type 'A extends string ? keyof A : A'.
+!!! error TS2322:         Type 'B' is not assignable to type 'A extends string ? keyof A : A'.
+!!! error TS2322:           Type 'A' is not assignable to type 'A extends string ? keyof A : A'.
+!!! error TS2322:             Type 'keyof B' is not assignable to type 'keyof A'.
+!!! error TS2322:               Type 'string | number | symbol' is not assignable to type 'keyof A'.
+!!! error TS2322:                 Type 'string' is not assignable to type 'keyof A'.
 !!! error TS2322:                   Type 'string' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
-!!! error TS2322:                     Type 'keyof B' is not assignable to type 'number'.
-!!! error TS2322:                       Type 'string | number | symbol' is not assignable to type 'number'.
-!!! error TS2322:                         Type 'string' is not assignable to type 'number'.
+!!! error TS2322:                     Type 'keyof B' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+!!! error TS2322:                       Type 'string | number | symbol' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+!!! error TS2322:                         Type 'string' is not assignable to type 'number | "toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" | "lastIndexOf" | "localeCompare" | "match" | "replace" | "search" | "slice" | "split" | "substring" | "toLowerCase" | "toLocaleLowerCase" | "toUpperCase" | "toLocaleUpperCase" | "trim" | "length" | "substr" | "valueOf"'.
+!!! error TS2322:                           Type 'keyof B' is not assignable to type 'number'.
+!!! error TS2322:                             Type 'string | number | symbol' is not assignable to type 'number'.
+!!! error TS2322:                               Type 'string' is not assignable to type 'number'.
         b = a;  // Error
         ~
 !!! error TS2322: Type 'Invariant<A>' is not assignable to type 'Invariant<B>'.
 !!! error TS2322:   Types of property 'foo' are incompatible.
 !!! error TS2322:     Type 'A extends string ? keyof A : A' is not assignable to type 'B extends string ? keyof B : B'.
-!!! error TS2322:       Type 'A' is not assignable to type 'B'.
-!!! error TS2322:         'B' could be instantiated with an arbitrary type which could be unrelated to 'A'.
+!!! error TS2322:       Type 'A | keyof A' is not assignable to type 'B extends string ? keyof B : B'.
+!!! error TS2322:         Type 'A' is not assignable to type 'B extends string ? keyof B : B'.
+!!! error TS2322:           Type 'A' is not assignable to type 'B'.
+!!! error TS2322:             'B' could be instantiated with an arbitrary type which could be unrelated to 'A'.
     }
 
     // Extract<T, Function> is a T that is known to be a Function