Improved undefined/null handling for relational operators

Author: ahejlsberg

src/compiler/checker.ts

@@ -14585,7 +14585,6 @@ namespace ts {
             }
             // NOTE: do not raise error if right is unknown as related error was already reported
             if (!(isTypeAny(rightType) ||
-                  rightType.flags & TypeFlags.Nullable ||
                   getSignaturesOfType(rightType, SignatureKind.Call).length ||
                   getSignaturesOfType(rightType, SignatureKind.Construct).length ||
                   isTypeSubtypeOf(rightType, globalFunctionType))) {
@@ -14598,6 +14597,8 @@ namespace ts {
             if (leftType === silentNeverType || rightType === silentNeverType) {
                 return silentNeverType;
             }
+            leftType = checkNonNullType(leftType, left);
+            rightType = checkNonNullType(rightType, right);
             // TypeScript 1.0 spec (April 2014): 4.15.5
             // The in operator requires the left operand to be of type Any, the String primitive type, or the Number primitive type,
             // and the right operand to be of type Any, an object type, or a type parameter type.
@@ -14952,8 +14953,8 @@ namespace ts {
                 case SyntaxKind.LessThanEqualsToken:
                 case SyntaxKind.GreaterThanEqualsToken:
                     if (checkForDisallowedESSymbolOperand(operator)) {
-                        leftType = getBaseTypeOfLiteralType(leftType);
-                        rightType = getBaseTypeOfLiteralType(rightType);
+                        leftType = getBaseTypeOfLiteralType(checkNonNullType(leftType, left));
+                        rightType = getBaseTypeOfLiteralType(checkNonNullType(rightType, right));
                         if (!isTypeComparableTo(leftType, rightType) && !isTypeComparableTo(rightType, leftType)) {
                             reportOperatorError();
                         }