Allow case comparison to undefined and null in strict null checking mode

src/compiler/checker.ts

@@ -15106,7 +15106,7 @@ namespace ts {
                     // In a 'switch' statement, each 'case' expression must be of a type that is comparable
                     // to or from the type of the 'switch' expression.
                     const caseType = checkExpression(caseClause.expression);
-                    if (!isTypeComparableTo(expressionType, caseType)) {
+                    if (!isTypeEqualityComparableTo(expressionType, caseType)) {
                         // expressionType is not comparable to caseType, try the reversed check and report errors if it fails
                         checkTypeComparableTo(caseType, expressionType, caseClause.expression, /*headMessage*/ undefined);
                     }

tests/baselines/reference/equalityStrictNulls.errors.txt

@@ -81,4 +81,14 @@ tests/cases/conformance/types/typeRelationships/comparable/equalityStrictNulls.t
 !!! error TS2365: Operator '<=' cannot be applied to types 'number' and 'undefined'.
         }
     }
+    function f5(x: string) {
+        switch(x) {
+            case null:
+                break;
+            case undefined:
+                break;
+            default:
+                return;
+        }
+    }
 

tests/baselines/reference/equalityStrictNulls.js

@@ -67,6 +67,16 @@ function f4(x: number) {
     if (x <= undefined) {
     }
 }
+function f5(x: string) {
+    switch(x) {
+        case null:
+            break;
+        case undefined:
+            break;
+        default:
+            return;
+    }
+}
 
 
 //// [equalityStrictNulls.js]
@@ -134,3 +144,13 @@ function f4(x) {
     if (x <= undefined) {
     }
 }
+function f5(x) {
+    switch (x) {
+        case null:
+            break;
+        case undefined:
+            break;
+        default:
+            return;
+    }
+}

tests/cases/conformance/types/typeRelationships/comparable/equalityStrictNulls.ts

@@ -67,3 +67,13 @@ function f4(x: number) {
     if (x <= undefined) {
     }
 }
+function f5(x: string) {
+    switch(x) {
+        case null:
+            break;
+        case undefined:
+            break;
+        default:
+            return;
+    }
+}