new approach and all changed tests reviewed

src/compiler/checker.ts

@@ -21965,8 +21965,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         typeToString(constraint),
                     );
                 }
-                // check if target is a descendant of the source
-                else if ((isGenericType(target) && isGenericType(source)) &&( isTypeAssignableTo(target, getBaseConstraintOrType(generalizedSource)) || (needsOriginalSource = isTypeAssignableTo(target, getBaseConstraintOrType(source))))) {
+
+                else if ((source.flags & TypeFlags.TypeParameter) && (isTypeAssignableTo(target, getBaseConstraintOrType(generalizedSource)) || (needsOriginalSource = isTypeAssignableTo(target, getBaseConstraintOrType(source))))) {
                     reportError(
                         Diagnostics._1_is_constrained_to_be_a_subtype_of_0,
                         needsOriginalSource ? sourceType : generalizedSourceType,

tests/baselines/reference/assignmentCompatWithCallSignatures4.errors.txt

@@ -19,13 +19,13 @@ assignmentCompatWithCallSignatures4.ts(53,9): error TS2322: Type '(x: (arg: Base
         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 assignmentCompatWithCallSignatures4.ts(58,9): error TS2322: Type '(...x: Base[]) => Base' is not assignable to type '<T extends Derived>(...x: T[]) => T'.
   Type 'Base' is not assignable to type 'T'.
-    'T' is constrained to be a subtype of 'Base'.
+    'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 assignmentCompatWithCallSignatures4.ts(62,9): error TS2322: Type '(x: { foo: string; }, y: { foo: string; bar: string; }) => Base' is not assignable to type '<T extends Derived>(x: T, y: T) => T'.
   Type 'Base' is not assignable to type 'T'.
-    'T' is constrained to be a subtype of 'Base'.
+    'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 assignmentCompatWithCallSignatures4.ts(66,9): error TS2322: Type '(x: Base[], y: Derived2[]) => Derived[]' is not assignable to type '<T extends Array<Derived2>>(x: Base[], y: Base[]) => T'.
   Type 'Derived[]' is not assignable to type 'T'.
-    'T' is constrained to be a subtype of 'Derived[]'.
+    'T' could be instantiated with an arbitrary type which could be unrelated to 'Derived[]'.
 assignmentCompatWithCallSignatures4.ts(69,9): error TS2322: Type '<T>(x: { a: T; b: T; }) => T' is not assignable to type '(x: { a: string; b: number; }) => number'.
   Types of parameters 'x' and 'x' are incompatible.
     Type '{ a: string; b: number; }' is not assignable to type '{ a: string; b: string; }'.
@@ -149,23 +149,23 @@ assignmentCompatWithCallSignatures4.ts(96,9): error TS2322: Type '<T>(x: T) => s
             ~~~
 !!! error TS2322: Type '(...x: Base[]) => Base' is not assignable to type '<T extends Derived>(...x: T[]) => T'.
 !!! error TS2322:   Type 'Base' is not assignable to type 'T'.
-!!! error TS2322:     'T' is constrained to be a subtype of 'Base'.
+!!! error TS2322:     'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 
             var b11: <T extends Derived>(x: T, y: T) => T; 
             a11 = b11;
             b11 = a11;
             ~~~
 !!! error TS2322: Type '(x: { foo: string; }, y: { foo: string; bar: string; }) => Base' is not assignable to type '<T extends Derived>(x: T, y: T) => T'.
 !!! error TS2322:   Type 'Base' is not assignable to type 'T'.
-!!! error TS2322:     'T' is constrained to be a subtype of 'Base'.
+!!! error TS2322:     'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 
             var b12: <T extends Array<Derived2>>(x: Array<Base>, y: Array<Base>) => T; 
             a12 = b12;
             b12 = a12;
             ~~~
 !!! error TS2322: Type '(x: Base[], y: Derived2[]) => Derived[]' is not assignable to type '<T extends Array<Derived2>>(x: Base[], y: Base[]) => T'.
 !!! error TS2322:   Type 'Derived[]' is not assignable to type 'T'.
-!!! error TS2322:     'T' is constrained to be a subtype of 'Derived[]'.
+!!! error TS2322:     'T' could be instantiated with an arbitrary type which could be unrelated to 'Derived[]'.
 
             var b15: <T>(x: { a: T; b: T }) => T; 
             a15 = b15;

tests/baselines/reference/assignmentCompatWithConstructSignatures4.errors.txt

@@ -19,13 +19,13 @@ assignmentCompatWithConstructSignatures4.ts(53,9): error TS2322: Type 'new (x: (
         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 assignmentCompatWithConstructSignatures4.ts(58,9): error TS2322: Type 'new (...x: Base[]) => Base' is not assignable to type 'new <T extends Derived>(...x: T[]) => T'.
   Type 'Base' is not assignable to type 'T'.
-    'T' is constrained to be a subtype of 'Base'.
+    'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 assignmentCompatWithConstructSignatures4.ts(62,9): error TS2322: Type 'new (x: { foo: string; }, y: { foo: string; bar: string; }) => Base' is not assignable to type 'new <T extends Derived>(x: T, y: T) => T'.
   Type 'Base' is not assignable to type 'T'.
-    'T' is constrained to be a subtype of 'Base'.
+    'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 assignmentCompatWithConstructSignatures4.ts(66,9): error TS2322: Type 'new (x: Base[], y: Derived2[]) => Derived[]' is not assignable to type 'new <T extends Array<Derived2>>(x: Base[], y: Base[]) => T'.
   Type 'Derived[]' is not assignable to type 'T'.
-    'T' is constrained to be a subtype of 'Derived[]'.
+    'T' could be instantiated with an arbitrary type which could be unrelated to 'Derived[]'.
 assignmentCompatWithConstructSignatures4.ts(69,9): error TS2322: Type 'new <T>(x: { a: T; b: T; }) => T' is not assignable to type 'new (x: { a: string; b: number; }) => number'.
   Types of parameters 'x' and 'x' are incompatible.
     Type '{ a: string; b: number; }' is not assignable to type '{ a: string; b: string; }'.
@@ -165,23 +165,23 @@ assignmentCompatWithConstructSignatures4.ts(96,9): error TS2322: Type 'new <T>(x
             ~~~
 !!! error TS2322: Type 'new (...x: Base[]) => Base' is not assignable to type 'new <T extends Derived>(...x: T[]) => T'.
 !!! error TS2322:   Type 'Base' is not assignable to type 'T'.
-!!! error TS2322:     'T' is constrained to be a subtype of 'Base'.
+!!! error TS2322:     'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 
             var b11: new <T extends Derived>(x: T, y: T) => T; 
             a11 = b11; // ok
             b11 = a11; // ok
             ~~~
 !!! error TS2322: Type 'new (x: { foo: string; }, y: { foo: string; bar: string; }) => Base' is not assignable to type 'new <T extends Derived>(x: T, y: T) => T'.
 !!! error TS2322:   Type 'Base' is not assignable to type 'T'.
-!!! error TS2322:     'T' is constrained to be a subtype of 'Base'.
+!!! error TS2322:     'T' could be instantiated with an arbitrary type which could be unrelated to 'Base'.
 
             var b12: new <T extends Array<Derived2>>(x: Array<Base>, y: Array<Base>) => T; 
             a12 = b12; // ok
             b12 = a12; // ok
             ~~~
 !!! error TS2322: Type 'new (x: Base[], y: Derived2[]) => Derived[]' is not assignable to type 'new <T extends Array<Derived2>>(x: Base[], y: Base[]) => T'.
 !!! error TS2322:   Type 'Derived[]' is not assignable to type 'T'.
-!!! error TS2322:     'T' is constrained to be a subtype of 'Derived[]'.
+!!! error TS2322:     'T' could be instantiated with an arbitrary type which could be unrelated to 'Derived[]'.
 
             var b15: new <T>(x: { a: T; b: T }) => T; 
             a15 = b15; // ok

tests/baselines/reference/awaitedTypeNoLib.errors.txt

@@ -13,7 +13,7 @@ awaitedTypeNoLib.ts(18,27): error TS2345: Argument of type 'NotPromise<TResult>
     Type 'TResult | (TResult extends PromiseLike<unknown> ? never : TResult)' is not assignable to type 'Thenable<TResult>'.
       Type 'Thenable<unknown> & TResult' is not assignable to type 'Thenable<TResult>'.
         Type 'unknown' is not assignable to type 'TResult'.
-          'TResult' is constrained to be a subtype of 'unknown'.
+          'TResult' could be instantiated with an arbitrary type which could be unrelated to 'unknown'.
 
 
 !!! error TS2318: Cannot find global type 'Array'.
@@ -52,7 +52,7 @@ awaitedTypeNoLib.ts(18,27): error TS2345: Argument of type 'NotPromise<TResult>
 !!! error TS2345:     Type 'TResult | (TResult extends PromiseLike<unknown> ? never : TResult)' is not assignable to type 'Thenable<TResult>'.
 !!! error TS2345:       Type 'Thenable<unknown> & TResult' is not assignable to type 'Thenable<TResult>'.
 !!! error TS2345:         Type 'unknown' is not assignable to type 'TResult'.
-!!! error TS2345:           'TResult' is constrained to be a subtype of 'unknown'.
+!!! error TS2345:           'TResult' could be instantiated with an arbitrary type which could be unrelated to 'unknown'.
         }
       }
 

tests/baselines/reference/conditionalTypes1.errors.txt

@@ -20,9 +20,9 @@ conditionalTypes1.ts(29,5): error TS2322: Type 'T["x"]' is not assignable to typ
           Type 'string | undefined' is not assignable to type '{}'.
             Type 'undefined' is not assignable to type '{}'.
 conditionalTypes1.ts(103,5): error TS2322: Type 'FunctionProperties<T>' is not assignable to type 'T'.
-  'T' is constrained to be a subtype of 'FunctionProperties<T>'.
+  'T' could be instantiated with an arbitrary type which could be unrelated to 'FunctionProperties<T>'.
 conditionalTypes1.ts(104,5): error TS2322: Type 'NonFunctionProperties<T>' is not assignable to type 'T'.
-  'T' is constrained to be a subtype of 'NonFunctionProperties<T>'.
+  'T' could be instantiated with an arbitrary type which could be unrelated to 'NonFunctionProperties<T>'.
 conditionalTypes1.ts(106,5): error TS2322: Type 'NonFunctionProperties<T>' is not assignable to type 'FunctionProperties<T>'.
   Type 'FunctionPropertyNames<T>' is not assignable to type 'NonFunctionPropertyNames<T>'.
     Type 'keyof T' is not assignable to type 'T[keyof T] extends Function ? never : keyof T'.
@@ -212,11 +212,11 @@ conditionalTypes1.ts(288,43): error TS2322: Type 'T95<U>' is not assignable to t
         x = y;  // Error
         ~
 !!! error TS2322: Type 'FunctionProperties<T>' is not assignable to type 'T'.
-!!! error TS2322:   'T' is constrained to be a subtype of 'FunctionProperties<T>'.
+!!! error TS2322:   'T' could be instantiated with an arbitrary type which could be unrelated to 'FunctionProperties<T>'.
         x = z;  // Error
         ~
 !!! error TS2322: Type 'NonFunctionProperties<T>' is not assignable to type 'T'.
-!!! error TS2322:   'T' is constrained to be a subtype of 'NonFunctionProperties<T>'.
+!!! error TS2322:   'T' could be instantiated with an arbitrary type which could be unrelated to 'NonFunctionProperties<T>'.
         y = x;
         y = z;  // Error
         ~

tests/baselines/reference/deeplyNestedMappedTypes.errors.txt

@@ -9,7 +9,7 @@ deeplyNestedMappedTypes.ts(69,5): error TS2322: Type '{ level1: { level2: { foo:
     The types of 'level1.level2' are incompatible between these types.
       Property 'bar' is missing in type '{ foo: string; }' but required in type '{ foo: string; bar: string; }'.
 deeplyNestedMappedTypes.ts(73,5): error TS2322: Type '{ level1: { level2: { foo: string; }; }; }[]' is not assignable to type 'T'.
-  'T' is constrained to be a subtype of '{ level1: { level2: { foo: string; }; }; }[]'.
+  'T' could be instantiated with an arbitrary type which could be unrelated to '{ level1: { level2: { foo: string; }; }; }[]'.
 deeplyNestedMappedTypes.ts(77,5): error TS2322: Type '{ level1: { level2: { foo: string; }; }; }[]' is not assignable to type '{ level1: { level2: { foo: string; bar: string; }; }; }[]'.
   Type '{ level1: { level2: { foo: string; }; }; }' is not assignable to type '{ level1: { level2: { foo: string; bar: string; }; }; }'.
     The types of 'level1.level2' are incompatible between these types.
@@ -106,7 +106,7 @@ deeplyNestedMappedTypes.ts(77,5): error TS2322: Type '{ level1: { level2: { foo:
         return ors;  // Error
         ~~~~~~
 !!! error TS2322: Type '{ level1: { level2: { foo: string; }; }; }[]' is not assignable to type 'T'.
-!!! error TS2322:   'T' is constrained to be a subtype of '{ level1: { level2: { foo: string; }; }; }[]'.
+!!! error TS2322:   'T' could be instantiated with an arbitrary type which could be unrelated to '{ level1: { level2: { foo: string; }; }; }[]'.
     }
 
     function problematicFunction3(ors: (typeof Input.static)[]): Output[] {

tests/baselines/reference/incompatibleAssignmentOfIdenticallyNamedTypes.errors.txt

@@ -1,5 +1,5 @@
 incompatibleAssignmentOfIdenticallyNamedTypes.ts(6,9): error TS2719: Type 'T' is not assignable to type 'T'. Two different types with this name exist, but they are unrelated.
-  'T' is constrained to be a subtype of 'T'.
+  'T' could be instantiated with an arbitrary type which could be unrelated to 'T'.
 
 
 ==== incompatibleAssignmentOfIdenticallyNamedTypes.ts (1 errors) ====
@@ -11,7 +11,7 @@ incompatibleAssignmentOfIdenticallyNamedTypes.ts(6,9): error TS2719: Type 'T' is
             this.x = a;
             ~~~~~~
 !!! error TS2719: Type 'T' is not assignable to type 'T'. Two different types with this name exist, but they are unrelated.
-!!! error TS2719:   'T' is constrained to be a subtype of 'T'.
+!!! error TS2719:   'T' could be instantiated with an arbitrary type which could be unrelated to 'T'.
         }
     }
 

tests/baselines/reference/indexedAccessRelation.errors.txt

@@ -3,11 +3,7 @@ indexedAccessRelation.ts(16,23): error TS2345: Argument of type '{ a: T; }' is n
     Type 'T' is not assignable to type 'S["a"] & T'.
       Type 'Foo' is not assignable to type 'S["a"] & T'.
         Type 'Foo' is not assignable to type 'S["a"]'.
-          'S["a"]' is constrained to be a subtype of 'Foo'.
-            Type 'T' is not assignable to type 'S["a"]'.
-              'S["a"]' is constrained to be a subtype of 'T'.
-                Type 'Foo' is not assignable to type 'S["a"]'.
-                  'S["a"]' is constrained to be a subtype of 'Foo'.
+          'S["a"]' could be instantiated with an arbitrary type which could be unrelated to 'Foo'.
 
 
 ==== indexedAccessRelation.ts (1 errors) ====
@@ -33,11 +29,7 @@ indexedAccessRelation.ts(16,23): error TS2345: Argument of type '{ a: T; }' is n
 !!! error TS2345:     Type 'T' is not assignable to type 'S["a"] & T'.
 !!! error TS2345:       Type 'Foo' is not assignable to type 'S["a"] & T'.
 !!! error TS2345:         Type 'Foo' is not assignable to type 'S["a"]'.
-!!! error TS2345:           'S["a"]' is constrained to be a subtype of 'Foo'.
-!!! error TS2345:             Type 'T' is not assignable to type 'S["a"]'.
-!!! error TS2345:               'S["a"]' is constrained to be a subtype of 'T'.
-!!! error TS2345:                 Type 'Foo' is not assignable to type 'S["a"]'.
-!!! error TS2345:                   'S["a"]' is constrained to be a subtype of 'Foo'.
+!!! error TS2345:           'S["a"]' could be instantiated with an arbitrary type which could be unrelated to 'Foo'.
         }
     }
 

tests/baselines/reference/mappedTypeRelationships.errors.txt

@@ -9,13 +9,9 @@ mappedTypeRelationships.ts(21,12): error TS2536: Type 'keyof U' cannot be used t
 mappedTypeRelationships.ts(25,5): error TS2536: Type 'K' cannot be used to index type 'T'.
 mappedTypeRelationships.ts(26,12): error TS2536: Type 'K' cannot be used to index type 'T'.
 mappedTypeRelationships.ts(30,5): error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
-  'T[keyof T]' is constrained to be a subtype of 'T[keyof T] | undefined'.
-    Type 'undefined' is not assignable to type 'T[keyof T]'.
-      'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
+  'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'T[keyof T] | undefined'.
 mappedTypeRelationships.ts(35,5): error TS2322: Type 'T[K] | undefined' is not assignable to type 'T[K]'.
-  'T[K]' is constrained to be a subtype of 'T[K] | undefined'.
-    Type 'undefined' is not assignable to type 'T[K]'.
-      'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
+  'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'T[K] | undefined'.
 mappedTypeRelationships.ts(40,5): error TS2322: Type 'U[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
   'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'U[keyof T] | undefined'.
 mappedTypeRelationships.ts(41,5): error TS2322: Type 'T[keyof T]' is not assignable to type 'U[keyof T] | undefined'.
@@ -38,7 +34,7 @@ mappedTypeRelationships.ts(66,5): error TS2322: Type 'T[K]' is not assignable to
     'U' is constrained to be a subtype of 'T'.
 mappedTypeRelationships.ts(66,5): error TS2542: Index signature in type 'Readonly<U>' only permits reading.
 mappedTypeRelationships.ts(72,5): error TS2322: Type 'Partial<T>' is not assignable to type 'T'.
-  'T' is constrained to be a subtype of 'Partial<T>'.
+  'T' could be instantiated with an arbitrary type which could be unrelated to 'Partial<T>'.
 mappedTypeRelationships.ts(78,5): error TS2322: Type 'Partial<Thing>' is not assignable to type 'Partial<T>'.
 mappedTypeRelationships.ts(88,5): error TS2322: Type 'Readonly<Thing>' is not assignable to type 'Readonly<T>'.
 mappedTypeRelationships.ts(127,5): error TS2322: Type 'Partial<U>' is not assignable to type 'Identity<U>'.
@@ -128,19 +124,15 @@ mappedTypeRelationships.ts(168,5): error TS2322: Type '{ [P in K]: T[P]; }' is n
         x[k] = y[k];  // Error
         ~~~~
 !!! error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
-!!! error TS2322:   'T[keyof T]' is constrained to be a subtype of 'T[keyof T] | undefined'.
-!!! error TS2322:     Type 'undefined' is not assignable to type 'T[keyof T]'.
-!!! error TS2322:       'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
+!!! error TS2322:   'T[keyof T]' could be instantiated with an arbitrary type which could be unrelated to 'T[keyof T] | undefined'.
         y[k] = x[k];
     }
 
     function f11<T, K extends keyof T>(x: T, y: Partial<T>, k: K) {
         x[k] = y[k];  // Error
         ~~~~
 !!! error TS2322: Type 'T[K] | undefined' is not assignable to type 'T[K]'.
-!!! error TS2322:   'T[K]' is constrained to be a subtype of 'T[K] | undefined'.
-!!! error TS2322:     Type 'undefined' is not assignable to type 'T[K]'.
-!!! error TS2322:       'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
+!!! error TS2322:   'T[K]' could be instantiated with an arbitrary type which could be unrelated to 'T[K] | undefined'.
         y[k] = x[k];
     }
 
@@ -213,7 +205,7 @@ mappedTypeRelationships.ts(168,5): error TS2322: Type '{ [P in K]: T[P]; }' is n
         x = y;  // Error
         ~
 !!! error TS2322: Type 'Partial<T>' is not assignable to type 'T'.
-!!! error TS2322:   'T' is constrained to be a subtype of 'Partial<T>'.
+!!! error TS2322:   'T' could be instantiated with an arbitrary type which could be unrelated to 'Partial<T>'.
         y = x;
     }