Delete the {} and unconstrained type parameter assignability rule

Author: weswigham

src/compiler/checker.ts

@@ -13678,22 +13678,17 @@ namespace ts {
                     }
                     else {
                         const constraint = getConstraintOfType(<TypeVariable>source);
-                        if (!constraint || (source.flags & TypeFlags.TypeParameter && constraint.flags & TypeFlags.Any)) {
-                            // A type variable with no constraint is not related to the non-primitive object type.
-                            if (result = isRelatedTo(emptyObjectType, extractTypesOfKind(target, ~TypeFlags.NonPrimitive))) {
+                        if (constraint) {
+                            // hi-speed no-this-instantiation check (less accurate, but avoids costly `this`-instantiation when the constraint will suffice), see #28231 for report on why this is needed
+                            if (result = isRelatedTo(constraint, target, /*reportErrors*/ false, /*headMessage*/ undefined, isIntersectionConstituent)) {
+                                resetErrorInfo(saveErrorInfo);
+                                return result;
+                            }
+                            // slower, fuller, this-instantiated check (necessary when comparing raw `this` types from base classes), see `subclassWithPolymorphicThisIsAssignable.ts` test for example
+                            else if (result = isRelatedTo(getTypeWithThisArgument(constraint, source), target, reportErrors, /*headMessage*/ undefined, isIntersectionConstituent)) {
                                 resetErrorInfo(saveErrorInfo);
                                 return result;
                             }
-                        }
-                        // hi-speed no-this-instantiation check (less accurate, but avoids costly `this`-instantiation when the constraint will suffice), see #28231 for report on why this is needed
-                        else if (result = isRelatedTo(constraint, target, /*reportErrors*/ false, /*headMessage*/ undefined, isIntersectionConstituent)) {
-                            resetErrorInfo(saveErrorInfo);
-                            return result;
-                        }
-                        // slower, fuller, this-instantiated check (necessary when comparing raw `this` types from base classes), see `subclassWithPolymorphicThisIsAssignable.ts` test for example
-                        else if (result = isRelatedTo(getTypeWithThisArgument(constraint, source), target, reportErrors, /*headMessage*/ undefined, isIntersectionConstituent)) {
-                            resetErrorInfo(saveErrorInfo);
-                            return result;
                         }
                     }
                 }

tests/baselines/reference/assignmentCompatWithCallSignatures5.errors.txt

@@ -5,13 +5,11 @@ 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; }'.
@@ -81,7 +79,6 @@ 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
@@ -91,7 +88,6 @@ 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

@@ -5,7 +5,6 @@ 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; }'.
@@ -62,7 +61,6 @@ 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;

tests/baselines/reference/assignmentCompatWithConstructSignatures5.errors.txt

@@ -5,13 +5,11 @@ 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/assignmentCompatWithConstructSignatures5.ts(55,1): error TS2322: Type 'new <T>(x: { a: T; b: T; }) => T[]' is not assignable to type 'new <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/assignmentCompatWithConstructSignatures5.ts(58,1): error TS2322: Type 'new <T extends Base>(x: { a: T; b: T; }) => T[]' is not assignable to type 'new <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; }'.
@@ -81,7 +79,6 @@ 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: new <U, V>(x: { a: U; b: V; }) => U[]; 
     a15 = b15; // ok
     b15 = a15; // ok
@@ -91,7 +88,6 @@ 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: new <T>(x: { a: T; b: T }) => T[]; 
     a15 = b16; // ok
     b15 = a16; // ok

tests/baselines/reference/assignmentCompatWithConstructSignatures6.errors.txt

@@ -5,7 +5,6 @@ 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/assignmentCompatWithConstructSignatures6.ts(42,1): error TS2322: Type 'new <T extends Base>(x: { a: T; b: T; }) => T[]' is not assignable to type 'new <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; }'.
@@ -62,7 +61,6 @@ 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: new <T>(x: { a: T; b: T }) => T[]; 
     x.a16 = b16;
     b16 = x.a16;