Implement(60881): Added detailed diagnostic for parent to child assignment

src/compiler/checker.ts

@@ -21966,6 +21966,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                         typeToString(constraint),
                     );
                 }
+                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,
+                        targetType,
+                    );
+                }
                 else {
                     errorInfo = undefined;
                     reportError(

src/compiler/diagnosticMessages.json

@@ -3991,6 +3991,10 @@
         "category": "Error",
         "code": 2881
     },
+    "'{1}' is constrained to be a subtype of '{0}'.": {
+        "category": "Error",
+        "code": 2882
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/better_subType_assignable_to_superType_error_messsage.errors.txt

@@ -0,0 +1,22 @@
+better_subType_assignable_to_superType_error_messsage.ts(2,3): error TS2552: Cannot find name 'subtype'. Did you mean 'subType2'?
+better_subType_assignable_to_superType_error_messsage.ts(5,3): error TS2322: Type 'SuperType' is not assignable to type 'SubType2'.
+  'SubType2' is constrained to be a subtype of 'SuperType'.
+
+
+==== better_subType_assignable_to_superType_error_messsage.ts (2 errors) ====
+    function parameterExtendsOtherParameter<SuperType, SubType extends SuperType, SubType2 extends SubType>(superType: SuperType, subType2: SubType2) {
+      subtype = superType;//error
+      ~~~~~~~
+!!! error TS2552: Cannot find name 'subtype'. Did you mean 'subType2'?
+!!! related TS2728 better_subType_assignable_to_superType_error_messsage.ts:1:127: 'subType2' is declared here.
+
+      //ensures that supertypes are not assignable to 'grandchild' subtypes
+      subType2 = superType;//error
+      ~~~~~~~~
+!!! error TS2322: Type 'SuperType' is not assignable to type 'SubType2'.
+!!! error TS2322:   'SubType2' is constrained to be a subtype of 'SuperType'.
+!!! related TS2208 better_subType_assignable_to_superType_error_messsage.ts:1:41: This type parameter might need an `extends SubType2` constraint.
+
+      superType = subType2;//ok
+    }
+

tests/baselines/reference/better_subType_assignable_to_superType_error_messsage.js

@@ -0,0 +1,20 @@
+//// [tests/cases/compiler/better_subType_assignable_to_superType_error_messsage.ts] ////
+
+//// [better_subType_assignable_to_superType_error_messsage.ts]
+function parameterExtendsOtherParameter<SuperType, SubType extends SuperType, SubType2 extends SubType>(superType: SuperType, subType2: SubType2) {
+  subtype = superType;//error
+
+  //ensures that supertypes are not assignable to 'grandchild' subtypes
+  subType2 = superType;//error
+
+  superType = subType2;//ok
+}
+
+
+//// [better_subType_assignable_to_superType_error_messsage.js]
+function parameterExtendsOtherParameter(superType, subType2) {
+    subtype = superType; //error
+    //ensures that supertypes are not assignable to 'grandchild' subtypes
+    subType2 = superType; //error
+    superType = subType2; //ok
+}

tests/baselines/reference/better_subType_assignable_to_superType_error_messsage.symbols

@@ -0,0 +1,28 @@
+//// [tests/cases/compiler/better_subType_assignable_to_superType_error_messsage.ts] ////
+
+=== better_subType_assignable_to_superType_error_messsage.ts ===
+function parameterExtendsOtherParameter<SuperType, SubType extends SuperType, SubType2 extends SubType>(superType: SuperType, subType2: SubType2) {
+>parameterExtendsOtherParameter : Symbol(parameterExtendsOtherParameter, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 0))
+>SuperType : Symbol(SuperType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 40))
+>SubType : Symbol(SubType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 50))
+>SuperType : Symbol(SuperType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 40))
+>SubType2 : Symbol(SubType2, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 77))
+>SubType : Symbol(SubType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 50))
+>superType : Symbol(superType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 104))
+>SuperType : Symbol(SuperType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 40))
+>subType2 : Symbol(subType2, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 125))
+>SubType2 : Symbol(SubType2, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 77))
+
+  subtype = superType;//error
+>superType : Symbol(superType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 104))
+
+  //ensures that supertypes are not assignable to 'grandchild' subtypes
+  subType2 = superType;//error
+>subType2 : Symbol(subType2, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 125))
+>superType : Symbol(superType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 104))
+
+  superType = subType2;//ok
+>superType : Symbol(superType, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 104))
+>subType2 : Symbol(subType2, Decl(better_subType_assignable_to_superType_error_messsage.ts, 0, 125))
+}
+

tests/baselines/reference/better_subType_assignable_to_superType_error_messsage.types

@@ -0,0 +1,37 @@
+//// [tests/cases/compiler/better_subType_assignable_to_superType_error_messsage.ts] ////
+
+=== better_subType_assignable_to_superType_error_messsage.ts ===
+function parameterExtendsOtherParameter<SuperType, SubType extends SuperType, SubType2 extends SubType>(superType: SuperType, subType2: SubType2) {
+>parameterExtendsOtherParameter : <SuperType, SubType extends SuperType, SubType2 extends SubType>(superType: SuperType, subType2: SubType2) => void
+>                               : ^         ^^       ^^^^^^^^^         ^^        ^^^^^^^^^       ^^         ^^         ^^        ^^        ^^^^^^^^^
+>superType : SuperType
+>          : ^^^^^^^^^
+>subType2 : SubType2
+>         : ^^^^^^^^
+
+  subtype = superType;//error
+>subtype = superType : SuperType
+>                    : ^^^^^^^^^
+>subtype : any
+>        : ^^^
+>superType : SuperType
+>          : ^^^^^^^^^
+
+  //ensures that supertypes are not assignable to 'grandchild' subtypes
+  subType2 = superType;//error
+>subType2 = superType : SuperType
+>                     : ^^^^^^^^^
+>subType2 : SubType2
+>         : ^^^^^^^^
+>superType : SuperType
+>          : ^^^^^^^^^
+
+  superType = subType2;//ok
+>superType = subType2 : SubType2
+>                     : ^^^^^^^^
+>superType : SuperType
+>          : ^^^^^^^^^
+>subType2 : SubType2
+>         : ^^^^^^^^
+}
+

tests/baselines/reference/chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.errors.txt

@@ -1,7 +1,7 @@
 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts(7,49): error TS2322: Type 'T' is not assignable to type 'S'.
-  'S' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+  'S' is constrained to be a subtype of 'T'.
 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts(10,35): error TS2322: Type 'T' is not assignable to type 'S'.
-  'S' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+  'S' is constrained to be a subtype of 'T'.
 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts(32,9): error TS2322: Type 'string' is not assignable to type 'number'.
 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts(36,9): error TS2322: Type 'string' is not assignable to type 'number'.
 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts(37,9): error TS2322: Type 'string' is not assignable to type 'number'.
@@ -17,15 +17,15 @@ chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts(37,9): error TS
             (new Chain(t)).then(tt => s).then(ss => t);
                                                     ~
 !!! error TS2322: Type 'T' is not assignable to type 'S'.
-!!! error TS2322:   'S' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+!!! error TS2322:   'S' is constrained to be a subtype of 'T'.
 !!! related TS2208 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts:1:13: This type parameter might need an `extends S` constraint.
 !!! related TS6502 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts:3:27: The expected type comes from the return type of this signature.
 
             // But error to try to climb up the chain
             (new Chain(s)).then(ss => t);
                                       ~
 !!! error TS2322: Type 'T' is not assignable to type 'S'.
-!!! error TS2322:   'S' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+!!! error TS2322:   'S' is constrained to be a subtype of 'T'.
 !!! related TS2208 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts:1:13: This type parameter might need an `extends S` constraint.
 !!! related TS6502 chainedCallsWithTypeParameterConstrainedToOtherTypeParameter2.ts:3:27: The expected type comes from the return type of this signature.
 

tests/baselines/reference/conditionalTypes2.errors.txt

@@ -1,9 +1,9 @@
 conditionalTypes2.ts(15,5): error TS2322: Type 'Covariant<A>' is not assignable to type 'Covariant<B>'.
   Type 'A' is not assignable to type 'B'.
-    'B' could be instantiated with an arbitrary type which could be unrelated to 'A'.
+    'B' is constrained to be a subtype of 'A'.
 conditionalTypes2.ts(19,5): error TS2322: Type 'Contravariant<B>' is not assignable to type 'Contravariant<A>'.
   Type 'A' is not assignable to type 'B'.
-    'B' could be instantiated with an arbitrary type which could be unrelated to 'A'.
+    'B' is constrained to be a subtype of 'A'.
 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'.
@@ -23,7 +23,7 @@ conditionalTypes2.ts(25,5): error TS2322: Type 'Invariant<A>' is not assignable
       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'.
+            'B' is constrained to be a subtype of 'A'.
 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; }'.
   Type 'Extract<T, Bar>' is not assignable to type '{ foo: string; bat: string; }'.
     Property 'bat' is missing in type 'Bar & Foo' but required in type '{ foo: string; bat: string; }'.
@@ -53,7 +53,7 @@ conditionalTypes2.ts(75,12): error TS2345: Argument of type 'Extract2<T, Foo, Ba
         ~
 !!! error TS2322: Type 'Covariant<A>' is not assignable to type 'Covariant<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:     'B' is constrained to be a subtype of 'A'.
 !!! related TS2208 conditionalTypes2.ts:13:13: This type parameter might need an `extends B` constraint.
     }
 
@@ -62,7 +62,7 @@ conditionalTypes2.ts(75,12): error TS2345: Argument of type 'Extract2<T, Foo, Ba
         ~
 !!! error TS2322: Type 'Contravariant<B>' is not assignable to type 'Contravariant<A>'.
 !!! 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:     'B' is constrained to be a subtype of 'A'.
 !!! related TS2208 conditionalTypes2.ts:18:13: This type parameter might need an `extends B` constraint.
         b = a;
     }
@@ -91,7 +91,7 @@ conditionalTypes2.ts(75,12): error TS2345: Argument of type 'Extract2<T, Foo, Ba
 !!! 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'.
+!!! error TS2322:             'B' is constrained to be a subtype of 'A'.
 !!! related TS2208 conditionalTypes2.ts:23:13: This type parameter might need an `extends B` constraint.
 !!! related TS2208 conditionalTypes2.ts:23:13: This type parameter might need an `extends B extends string ? keyof B : B` constraint.
     }

tests/baselines/reference/genericCallWithObjectTypeArgsAndInitializers.errors.txt

@@ -1,9 +1,9 @@
 genericCallWithObjectTypeArgsAndInitializers.ts(5,33): error TS2322: Type 'number' is not assignable to type 'T'.
   'number' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Number'.
 genericCallWithObjectTypeArgsAndInitializers.ts(6,37): error TS2322: Type 'T' is not assignable to type 'U'.
-  'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+  'U' is constrained to be a subtype of 'T'.
 genericCallWithObjectTypeArgsAndInitializers.ts(8,56): error TS2322: Type 'U' is not assignable to type 'V'.
-  'V' could be instantiated with an arbitrary type which could be unrelated to 'U'.
+  'V' is constrained to be a subtype of 'U'.
 
 
 ==== genericCallWithObjectTypeArgsAndInitializers.ts (3 errors) ====
@@ -18,11 +18,11 @@ genericCallWithObjectTypeArgsAndInitializers.ts(8,56): error TS2322: Type 'U' is
     function foo4<T, U extends T>(x: T, y: U = x) { } // error
                                         ~~~~~~~~
 !!! error TS2322: Type 'T' is not assignable to type 'U'.
-!!! error TS2322:   'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+!!! error TS2322:   'U' is constrained to be a subtype of 'T'.
 !!! related TS2208 genericCallWithObjectTypeArgsAndInitializers.ts:6:15: This type parameter might need an `extends U` constraint.
     function foo5<T, U extends T>(x: U, y: T = x) { } // ok
     function foo6<T, U extends T, V extends U>(x: T, y: U, z: V = y) { } // error
                                                            ~~~~~~~~
 !!! error TS2322: Type 'U' is not assignable to type 'V'.
-!!! error TS2322:   'V' could be instantiated with an arbitrary type which could be unrelated to 'U'.
+!!! error TS2322:   'V' is constrained to be a subtype of 'U'.
     function foo7<T, U extends T, V extends U>(x: V, y: U = x) { } // should be ok

tests/baselines/reference/keyofAndIndexedAccessErrors.errors.txt

@@ -68,16 +68,16 @@ keyofAndIndexedAccessErrors.ts(105,9): error TS2322: Type 'T[Extract<keyof T, st
         'string' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'string'.
 keyofAndIndexedAccessErrors.ts(108,5): error TS2322: Type 'T[K]' is not assignable to type 'U[K]'.
   Type 'T' is not assignable to type 'U'.
-    'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+    'U' is constrained to be a subtype of 'T'.
 keyofAndIndexedAccessErrors.ts(111,5): error TS2322: Type 'T[J]' is not assignable to type 'U[J]'.
   Type 'T' is not assignable to type 'U'.
-    'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+    'U' is constrained to be a subtype of 'T'.
 keyofAndIndexedAccessErrors.ts(114,5): error TS2322: Type 'T[K]' is not assignable to type 'T[J]'.
   Type 'K' is not assignable to type 'J'.
     'K' is assignable to the constraint of type 'J', but 'J' could be instantiated with a different subtype of constraint 'string'.
 keyofAndIndexedAccessErrors.ts(117,5): error TS2322: Type 'T[K]' is not assignable to type 'U[J]'.
   Type 'T' is not assignable to type 'U'.
-    'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+    'U' is constrained to be a subtype of 'T'.
 keyofAndIndexedAccessErrors.ts(122,5): error TS2322: Type 'number' is not assignable to type 'keyof T'.
 keyofAndIndexedAccessErrors.ts(123,5): error TS2322: Type 'string' is not assignable to type 'keyof T'.
 keyofAndIndexedAccessErrors.ts(140,5): error TS2322: Type 'number' is not assignable to type 'T[K]'.
@@ -301,15 +301,15 @@ keyofAndIndexedAccessErrors.ts(165,5): error TS2322: Type 'number' is not assign
         ~~
 !!! error TS2322: Type 'T[K]' is not assignable to type 'U[K]'.
 !!! error TS2322:   Type 'T' is not assignable to type 'U'.
-!!! error TS2322:     'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+!!! error TS2322:     'U' is constrained to be a subtype of 'T'.
 !!! related TS2208 keyofAndIndexedAccessErrors.ts:99:13: This type parameter might need an `extends U` constraint.
 
         tj = uj;
         uj = tj; // error
         ~~
 !!! error TS2322: Type 'T[J]' is not assignable to type 'U[J]'.
 !!! error TS2322:   Type 'T' is not assignable to type 'U'.
-!!! error TS2322:     'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+!!! error TS2322:     'U' is constrained to be a subtype of 'T'.
 !!! related TS2208 keyofAndIndexedAccessErrors.ts:99:13: This type parameter might need an `extends U` constraint.
 
         tk = tj;
@@ -324,7 +324,7 @@ keyofAndIndexedAccessErrors.ts(165,5): error TS2322: Type 'number' is not assign
         ~~
 !!! error TS2322: Type 'T[K]' is not assignable to type 'U[J]'.
 !!! error TS2322:   Type 'T' is not assignable to type 'U'.
-!!! error TS2322:     'U' could be instantiated with an arbitrary type which could be unrelated to 'T'.
+!!! error TS2322:     'U' is constrained to be a subtype of 'T'.
 !!! related TS2208 keyofAndIndexedAccessErrors.ts:99:13: This type parameter might need an `extends U` constraint.
     }