Added custom error message when trying to assign constraint type to generic type parameter

src/compiler/checker.ts

@@ -12260,6 +12260,15 @@ namespace ts {
                     targetType = typeToString(target, /*enclosingDeclaration*/ undefined, TypeFormatFlags.UseFullyQualifiedType);
                 }
 
+                if (target.flags & TypeFlags.TypeParameter && target.immediateBaseConstraint !== undefined && isTypeAssignableTo(source, target.immediateBaseConstraint)) {
+                    reportError(
+                        Diagnostics._0_is_assignable_to_the_constraint_of_type_1_but_1_could_be_instantiated_with_a_different_subtype_of_constraint_2,
+                        sourceType,
+                        targetType,
+                        typeToString(target.immediateBaseConstraint),
+                    );
+                }
+
                 if (!message) {
                     if (relation === comparableRelation) {
                         message = Diagnostics.Type_0_is_not_comparable_to_type_1;

src/compiler/diagnosticMessages.json

@@ -3096,6 +3096,10 @@
         "category": "Error",
         "code": 5074
     },
+    "'{0}' is assignable to the constraint of type '{1}', but '{1}' could be instantiated with a different subtype of constraint '{2}'.": {
+        "category": "Error",
+        "code": 5075
+    },
 
     "Generates a sourcemap for each corresponding '.d.ts' file.": {
         "category": "Message",

tests/baselines/reference/assignmentCompatWithCallSignatures3.errors.txt

@@ -14,24 +14,30 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
       Type 'string' is not assignable to type 'T'.
+        'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(62,1): error TS2322: Type '(x: (arg: Base) => Derived) => Base' is not assignable to type '<T extends Base, U extends Derived>(x: (arg: T) => U) => T'.
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
       Type 'Base' is not assignable to type 'T'.
+        'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(65,1): error TS2322: Type '(x: (arg: Base) => Derived) => (r: Base) => Derived' is not assignable to type '<T extends Base, U extends Derived>(x: (arg: T) => U) => (r: T) => U'.
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
       Type 'Base' is not assignable to type 'T'.
+        'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(68,1): error TS2322: Type '(x: (arg: Base) => Derived, y: (arg2: Base) => Derived) => (r: Base) => Derived' is not assignable to type '<T extends Base, U extends Derived>(x: (arg: T) => U, y: (arg2: T) => U) => (r: T) => U'.
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
       Type 'Base' is not assignable to type 'T'.
+        'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(71,1): error TS2322: Type '(x: (arg: Base) => Derived, y: (arg2: Base) => Derived) => (r: Base) => Derived' is not assignable to type '<T extends Base, U extends Derived>(x: (arg: T) => U, y: (arg2: { foo: string; bing: number; }) => U) => (r: T) => U'.
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
       Type 'Base' is not assignable to type 'T'.
+        'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(74,1): error TS2322: Type '(...x: Derived[]) => Derived' is not assignable to type '<T extends Derived>(...x: T[]) => T'.
   Type 'Derived' is not assignable to type 'T'.
+    'Derived' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Derived'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(77,1): error TS2322: Type '(x: { foo: string; }, y: { foo: string; bar: string; }) => Base' is not assignable to type '<T extends Base>(x: T, y: T) => T'.
   Types of parameters 'y' and 'y' are incompatible.
     Type 'T' is not assignable to type '{ foo: string; bar: string; }'.
@@ -43,6 +49,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
         Type 'Base' is missing the following properties from type 'Derived2': baz, bar
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(83,1): error TS2322: Type '(x: Base[], y: Derived[]) => Derived[]' is not assignable to type '<T extends Derived[]>(x: Base[], y: T) => T'.
   Type 'Derived[]' is not assignable to type 'T'.
+    'Derived[]' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Derived[]'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(85,1): error TS2322: Type '<T>(x: { a: T; b: T; }) => T' is not assignable to type '(x: { a: string; b: number; }) => Object'.
   Types of parameters 'x' and 'x' are incompatible.
     Type '{ a: string; b: number; }' is not assignable to type '{ a: string; b: string; }'.
@@ -136,6 +143,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Types of parameters 'arg' and 'arg' are incompatible.
 !!! error TS2322:       Type 'string' is not assignable to type 'T'.
+!!! error TS2322:         'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
     var b6: <T extends Base, U extends Derived>(x: (arg: T) => U) => T; 
     a6 = b6; // ok
     b6 = a6; // ok
@@ -144,6 +152,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Types of parameters 'arg' and 'arg' are incompatible.
 !!! error TS2322:       Type 'Base' is not assignable to type 'T'.
+!!! error TS2322:         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
     var b7: <T extends Base, U extends Derived>(x: (arg: T) => U) => (r: T) => U; 
     a7 = b7; // ok
     b7 = a7; // ok
@@ -152,6 +161,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Types of parameters 'arg' and 'arg' are incompatible.
 !!! error TS2322:       Type 'Base' is not assignable to type 'T'.
+!!! error TS2322:         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
     var b8: <T extends Base, U extends Derived>(x: (arg: T) => U, y: (arg2: T) => U) => (r: T) => U;
     a8 = b8; // ok
     b8 = a8; // ok
@@ -160,6 +170,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Types of parameters 'arg' and 'arg' are incompatible.
 !!! error TS2322:       Type 'Base' is not assignable to type 'T'.
+!!! error TS2322:         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
     var b9: <T extends Base, U extends Derived>(x: (arg: T) => U, y: (arg2: { foo: string; bing: number }) => U) => (r: T) => U; 
     a9 = b9; // ok
     b9 = a9; // ok
@@ -168,12 +179,14 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Types of parameters 'arg' and 'arg' are incompatible.
 !!! error TS2322:       Type 'Base' is not assignable to type 'T'.
+!!! error TS2322:         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
     var b10: <T extends Derived>(...x: T[]) => T; 
     a10 = b10; // ok
     b10 = a10; // ok
     ~~~
 !!! error TS2322: Type '(...x: Derived[]) => Derived' is not assignable to type '<T extends Derived>(...x: T[]) => T'.
 !!! error TS2322:   Type 'Derived' is not assignable to type 'T'.
+!!! error TS2322:     'Derived' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Derived'.
     var b11: <T extends Base>(x: T, y: T) => T; 
     a11 = b11; // ok
     b11 = a11; // ok
@@ -198,6 +211,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
     ~~~
 !!! error TS2322: Type '(x: Base[], y: Derived[]) => Derived[]' is not assignable to type '<T extends Derived[]>(x: Base[], y: T) => T'.
 !!! error TS2322:   Type 'Derived[]' is not assignable to type 'T'.
+!!! error TS2322:     'Derived[]' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Derived[]'.
     var b14: <T>(x: { a: T; b: T }) => T; 
     a14 = b14; // ok
     ~~~

tests/baselines/reference/assignmentCompatWithCallSignatures4.errors.txt

@@ -5,6 +5,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
       Type 'Base' is not assignable to type 'T'.
+        'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(52,9): error TS2322: Type '<T extends Base, U extends Derived>(x: (arg: T) => U, y: (arg2: { foo: number; }) => U) => (r: T) => U' is not assignable to type '(x: (arg: Base) => Derived, y: (arg2: Base) => Derived) => (r: Base) => Derived'.
   Types of parameters 'y' and 'y' are incompatible.
     Types of parameters 'arg2' and 'arg2' are incompatible.
@@ -15,6 +16,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
       Type 'Base' is not assignable to type 'T'.
+        'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/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'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/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'.
@@ -45,6 +47,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(89,9): error TS2322: Type '<T>(x: T) => string[]' is not assignable to type '<T>(x: T) => T[]'.
   Type 'string[]' is not assignable to type 'T[]'.
     Type 'string' is not assignable to type 'T'.
+      'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(90,9): error TS2322: Type '<T>(x: T) => T[]' is not assignable to type '<T>(x: T) => string[]'.
   Type 'T[]' is not assignable to type 'string[]'.
     Type 'T' is not assignable to type 'string'.
@@ -54,6 +57,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(96,9): error TS2322: Type '<T>(x: T) => string[]' is not assignable to type '<T>(x: T) => T[]'.
   Type 'string[]' is not assignable to type 'T[]'.
     Type 'string' is not assignable to type 'T'.
+      'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
 
 
 ==== tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts (15 errors) ====
@@ -115,6 +119,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Types of parameters 'arg' and 'arg' are incompatible.
 !!! error TS2322:       Type 'Base' is not assignable to type 'T'.
+!!! error TS2322:         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 
             var b8: <T extends Base, U extends Derived>(x: (arg: T) => U, y: (arg2: { foo: number; }) => U) => (r: T) => U; 
             a8 = b8; // error, { foo: number } and Base are incompatible
@@ -131,6 +136,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Types of parameters 'arg' and 'arg' are incompatible.
 !!! error TS2322:       Type 'Base' is not assignable to type 'T'.
+!!! error TS2322:         'Base' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'Base'.
 
 
             var b10: <T extends Derived>(...x: T[]) => T; 
@@ -205,6 +211,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322: Type '<T>(x: T) => string[]' is not assignable to type '<T>(x: T) => T[]'.
 !!! error TS2322:   Type 'string[]' is not assignable to type 'T[]'.
 !!! error TS2322:     Type 'string' is not assignable to type 'T'.
+!!! error TS2322:       'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
             b2 = a2;
             ~~
 !!! error TS2322: Type '<T>(x: T) => T[]' is not assignable to type '<T>(x: T) => string[]'.
@@ -224,5 +231,6 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322: Type '<T>(x: T) => string[]' is not assignable to type '<T>(x: T) => T[]'.
 !!! error TS2322:   Type 'string[]' is not assignable to type 'T[]'.
 !!! error TS2322:     Type 'string' is not assignable to type 'T'.
+!!! error TS2322:       'string' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
         }
     }

tests/baselines/reference/assignmentCompatWithCallSignatures5.errors.txt

@@ -5,11 +5,13 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
     Type '{ foo: U; bar: U; }' is not assignable to type '{ foo: T; bar: T; }'.
       Types of property 'foo' are incompatible.
         Type 'U' is not assignable to type 'T'.
+          'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures5.ts(55,1): error TS2322: Type '<T>(x: { a: T; b: T; }) => T[]' is not assignable to type '<U, V>(x: { a: U; b: V; }) => U[]'.
   Types of parameters 'x' and 'x' are incompatible.
     Type '{ a: U; b: V; }' is not assignable to type '{ a: U; b: U; }'.
       Types of property 'b' are incompatible.
         Type 'V' is not assignable to type 'U'.
+          'V' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint '{}'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures5.ts(58,1): error TS2322: Type '<T extends Base>(x: { a: T; b: T; }) => T[]' is not assignable to type '<U, V>(x: { a: U; b: V; }) => U[]'.
   Types of parameters 'x' and 'x' are incompatible.
     Type '{ a: U; b: V; }' is not assignable to type '{ a: Base; b: Base; }'.
@@ -79,6 +81,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:     Type '{ foo: U; bar: U; }' is not assignable to type '{ foo: T; bar: T; }'.
 !!! error TS2322:       Types of property 'foo' are incompatible.
 !!! error TS2322:         Type 'U' is not assignable to type 'T'.
+!!! error TS2322:           'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
     var b15: <U, V>(x: { a: U; b: V; }) => U[]; 
     a15 = b15; // ok, T = U, T = V
     b15 = a15; // ok
@@ -88,6 +91,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:     Type '{ a: U; b: V; }' is not assignable to type '{ a: U; b: U; }'.
 !!! error TS2322:       Types of property 'b' are incompatible.
 !!! error TS2322:         Type 'V' is not assignable to type 'U'.
+!!! error TS2322:           'V' is assignable to the constraint of type 'U', but 'U' could be instantiated with a different subtype of constraint '{}'.
     var b16: <T>(x: { a: T; b: T }) => T[]; 
     a15 = b16; // ok
     b15 = a16; // ok

tests/baselines/reference/assignmentCompatWithCallSignatures6.errors.txt

@@ -5,6 +5,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
     Type '{ foo: U; bar: U; }' is not assignable to type '{ foo: T; bar: T; }'.
       Types of property 'foo' are incompatible.
         Type 'U' is not assignable to type 'T'.
+          'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures6.ts(42,1): error TS2322: Type '<T extends Base>(x: { a: T; b: T; }) => T[]' is not assignable to type '<T>(x: { a: T; b: T; }) => T[]'.
   Types of parameters 'x' and 'x' are incompatible.
     Type '{ a: T; b: T; }' is not assignable to type '{ a: Base; b: Base; }'.
@@ -61,6 +62,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:     Type '{ foo: U; bar: U; }' is not assignable to type '{ foo: T; bar: T; }'.
 !!! error TS2322:       Types of property 'foo' are incompatible.
 !!! error TS2322:         Type 'U' is not assignable to type 'T'.
+!!! error TS2322:           'U' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint '{}'.
     var b16: <T>(x: { a: T; b: T }) => T[]; 
     x.a16 = b16;
     b16 = x.a16;