Fix type parameter comparability to consistently allow comparisons on unconstrained type parameters

Author: weswigham

src/compiler/checker.ts

@@ -19338,6 +19338,20 @@ namespace ts {
                             }
                         }
                     }
+                    if (relation === comparableRelation && sourceFlags & TypeFlags.TypeParameter) {
+                        // This is a carve-out in comparability to essentially forbid comparing a type parameter
+                        // with another type parameter unless one extends the other. (Remember: comparability is mostly bidirectional!)
+                        let constraint = getConstraintOfTypeParameter(source);
+                        if (constraint && hasNonCircularBaseConstraint(source)) {
+                            while (constraint && constraint.flags & TypeFlags.TypeParameter) {
+                                if (result = isRelatedTo(constraint, target, RecursionFlags.Source, /*reportErrors*/ false)) {
+                                    return result;
+                                }
+                                constraint = getConstraintOfTypeParameter(constraint);
+                            }
+                        }
+                        return Ternary.False;
+                    }
                 }
                 else if (targetFlags & TypeFlags.Index) {
                     const targetType = (target as IndexType).type;
@@ -19549,7 +19563,7 @@ namespace ts {
                 if (sourceFlags & TypeFlags.TypeVariable) {
                     // IndexedAccess comparisons are handled above in the `targetFlags & TypeFlage.IndexedAccess` branch
                     if (!(sourceFlags & TypeFlags.IndexedAccess && targetFlags & TypeFlags.IndexedAccess)) {
-                        const constraint = getConstraintOfType(source as TypeVariable);
+                        const constraint = getConstraintOfType(source as TypeVariable) || unknownType;
                         if (!strictNullChecks && (!constraint || (sourceFlags & 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), RecursionFlags.Both)) {
@@ -19558,12 +19572,12 @@ namespace ts {
                             }
                         }
                         // 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 (constraint && (result = isRelatedTo(constraint, target, RecursionFlags.Source, /*reportErrors*/ false, /*headMessage*/ undefined, intersectionState))) {
+                        else if (result = isRelatedTo(constraint, target, RecursionFlags.Source, /*reportErrors*/ false, /*headMessage*/ undefined, intersectionState)) {
                             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 (constraint && (result = isRelatedTo(getTypeWithThisArgument(constraint, source), target, RecursionFlags.Source, reportErrors && !(targetFlags & sourceFlags & TypeFlags.TypeParameter), /*headMessage*/ undefined, intersectionState))) {
+                        else if (result = isRelatedTo(getTypeWithThisArgument(constraint, source), target, RecursionFlags.Source, reportErrors && !(targetFlags & sourceFlags & TypeFlags.TypeParameter), /*headMessage*/ undefined, intersectionState)) {
                             resetErrorInfo(saveErrorInfo);
                             return result;
                         }

tests/baselines/reference/assignmentCompatWithCallSignatures3.errors.txt

@@ -1,15 +1,19 @@
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(47,1): error TS2322: Type '(x: number) => number[]' is not assignable to type '<T>(x: T) => T[]'.
   Types of parameters 'x' and 'x' are incompatible.
     Type 'T' is not assignable to type 'number'.
+      Type 'unknown' is not assignable to type 'number'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(50,1): error TS2322: Type '(x: number) => string[]' is not assignable to type '<T>(x: T) => string[]'.
   Types of parameters 'x' and 'x' are incompatible.
     Type 'T' is not assignable to type 'number'.
+      Type 'unknown' is not assignable to type 'number'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(53,1): error TS2322: Type '(x: number) => void' is not assignable to type '<T>(x: T) => T'.
   Types of parameters 'x' and 'x' are incompatible.
     Type 'T' is not assignable to type 'number'.
+      Type 'unknown' is not assignable to type 'number'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(56,1): error TS2322: Type '(x: string, y: number) => string' is not assignable to type '<T, U>(x: T, y: U) => T'.
   Types of parameters 'x' and 'x' are incompatible.
     Type 'T' is not assignable to type 'string'.
+      Type 'unknown' is not assignable to type 'string'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts(59,1): error TS2322: Type '(x: (arg: string) => number) => string' is not assignable to type '<T, U>(x: (arg: T) => U) => T'.
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
@@ -60,6 +64,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
     Type '{ a: T; b: T; }' is not assignable to type '{ a: string; b: number; }'.
       Types of property 'a' are incompatible.
         Type 'T' is not assignable to type 'string'.
+          Type 'unknown' is not assignable to type 'string'.
 
 
 ==== tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures3.ts (15 errors) ====
@@ -114,27 +119,31 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322: Type '(x: number) => number[]' is not assignable to type '<T>(x: T) => T[]'.
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Type 'T' is not assignable to type 'number'.
+!!! error TS2322:       Type 'unknown' is not assignable to type 'number'.
     var b2: <T>(x: T) => string[]; 
     a2 = b2; // ok 
     b2 = a2; // ok
     ~~
 !!! error TS2322: Type '(x: number) => string[]' is not assignable to type '<T>(x: T) => string[]'.
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Type 'T' is not assignable to type 'number'.
+!!! error TS2322:       Type 'unknown' is not assignable to type 'number'.
     var b3: <T>(x: T) => T; 
     a3 = b3; // ok
     b3 = a3; // ok
     ~~
 !!! error TS2322: Type '(x: number) => void' is not assignable to type '<T>(x: T) => T'.
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Type 'T' is not assignable to type 'number'.
+!!! error TS2322:       Type 'unknown' is not assignable to type 'number'.
     var b4: <T, U>(x: T, y: U) => T; 
     a4 = b4; // ok
     b4 = a4; // ok
     ~~
 !!! error TS2322: Type '(x: string, y: number) => string' is not assignable to type '<T, U>(x: T, y: U) => T'.
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Type 'T' is not assignable to type 'string'.
+!!! error TS2322:       Type 'unknown' is not assignable to type 'string'.
     var b5: <T, U>(x: (arg: T) => U) => T; 
     a5 = b5; // ok
     b5 = a5; // ok
@@ -227,6 +236,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:     Type '{ a: T; b: T; }' is not assignable to type '{ a: string; b: number; }'.
 !!! error TS2322:       Types of property 'a' are incompatible.
 !!! error TS2322:         Type 'T' is not assignable to type 'string'.
+!!! error TS2322:           Type 'unknown' is not assignable to type 'string'.
     var b15: <T>(x: T) => T[]; 
     a15 = b15; // ok
     b15 = a15; // ok

tests/baselines/reference/assignmentCompatWithCallSignatures4.errors.txt

@@ -1,6 +1,7 @@
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(45,9): error TS2322: Type '(x: number) => string[]' is not assignable to type '<T, U>(x: T) => U[]'.
   Types of parameters 'x' and 'x' are incompatible.
     Type 'T' is not assignable to type 'number'.
+      Type 'unknown' is not assignable to type 'number'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(49,9): error TS2322: Type '(x: (arg: Base) => Derived) => (r: Base) => Derived2' is not assignable to type '<T extends Base, U extends Derived, V extends Derived2>(x: (arg: T) => U) => (r: T) => V'.
   Types of parameters 'x' and 'x' are incompatible.
     Types of parameters 'arg' and 'arg' are incompatible.
@@ -36,6 +37,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
     Type '{ a: T; b: T; }' is not assignable to type '{ a: string; b: number; }'.
       Types of property 'a' are incompatible.
         Type 'T' is not assignable to type 'string'.
+          Type 'unknown' is not assignable to type 'string'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(73,9): error TS2322: Type '<T extends Base>(x: { a: T; b: T; }) => number' 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: Base; b: Base; }'.
@@ -54,9 +56,11 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 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'.
+      Type 'unknown' is not assignable to type 'string'.
 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures4.ts(95,9): 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'.
+      Type 'unknown' is not assignable to type 'string'.
 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'.
@@ -113,6 +117,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322: Type '(x: number) => string[]' is not assignable to type '<T, U>(x: T) => U[]'.
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Type 'T' is not assignable to type 'number'.
+!!! error TS2322:       Type 'unknown' is not assignable to type 'number'.
 
             var b7: <T extends Base, U extends Derived, V extends Derived2>(x: (arg: T) => U) => (r: T) => V;
             a7 = b7;
@@ -181,6 +186,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:     Type '{ a: T; b: T; }' is not assignable to type '{ a: string; b: number; }'.
 !!! error TS2322:       Types of property 'a' are incompatible.
 !!! error TS2322:         Type 'T' is not assignable to type 'string'.
+!!! error TS2322:           Type 'unknown' is not assignable to type 'string'.
 
             var b15a: <T extends Base>(x: { a: T; b: T }) => number; 
             a15 = b15a;
@@ -223,6 +229,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322: Type '<T>(x: T) => T[]' is not assignable to type '<T>(x: T) => string[]'.
 !!! error TS2322:   Type 'T[]' is not assignable to type 'string[]'.
 !!! error TS2322:     Type 'T' is not assignable to type 'string'.
+!!! error TS2322:       Type 'unknown' is not assignable to type 'string'.
 
             // target type has generic call signature
             var a3: <T>(x: T) => string[];
@@ -232,6 +239,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322: Type '<T>(x: T) => T[]' is not assignable to type '<T>(x: T) => string[]'.
 !!! error TS2322:   Type 'T[]' is not assignable to type 'string[]'.
 !!! error TS2322:     Type 'T' is not assignable to type 'string'.
+!!! error TS2322:       Type 'unknown' is not assignable to type 'string'.
             b3 = a3;
             ~~
 !!! error TS2322: Type '<T>(x: T) => string[]' is not assignable to type '<T>(x: T) => T[]'.

tests/baselines/reference/assignmentCompatWithCallSignatures5.errors.txt

@@ -18,6 +18,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
     Type '{ a: U; b: V; }' is not assignable to type '{ a: Base; b: Base; }'.
       Types of property 'a' are incompatible.
         Type 'U' is not assignable to type 'Base'.
+          Property 'foo' is missing in type '{}' but required in type 'Base'.
 
 
 ==== tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures5.ts (4 errors) ====
@@ -103,6 +104,8 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:     Type '{ a: U; b: V; }' is not assignable to type '{ a: Base; b: Base; }'.
 !!! error TS2322:       Types of property 'a' are incompatible.
 !!! error TS2322:         Type 'U' is not assignable to type 'Base'.
+!!! error TS2322:           Property 'foo' is missing in type '{}' but required in type 'Base'.
+!!! related TS2728 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures5.ts:3:14: 'foo' is declared here.
     var b17: <T>(x: (a: T) => T) => T[]; 
     a17 = b17; // ok
     b17 = a17; // ok

tests/baselines/reference/assignmentCompatWithCallSignatures6.errors.txt

@@ -12,6 +12,7 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
     Type '{ a: T; b: T; }' is not assignable to type '{ a: Base; b: Base; }'.
       Types of property 'a' are incompatible.
         Type 'T' is not assignable to type 'Base'.
+          Property 'foo' is missing in type '{}' but required in type 'Base'.
 
 
 ==== tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures6.ts (3 errors) ====
@@ -73,4 +74,6 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
 !!! error TS2322:   Types of parameters 'x' and 'x' are incompatible.
 !!! error TS2322:     Type '{ a: T; b: T; }' is not assignable to type '{ a: Base; b: Base; }'.
 !!! error TS2322:       Types of property 'a' are incompatible.
-!!! error TS2322:         Type 'T' is not assignable to type 'Base'.
+!!! error TS2322:         Type 'T' is not assignable to type 'Base'.
+!!! error TS2322:           Property 'foo' is missing in type '{}' but required in type 'Base'.
+!!! related TS2728 tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithCallSignatures6.ts:3:14: 'foo' is declared here.