fix(eslint-plugin): [nuta] correctly handle null/undefined separation (#1201)

Author: bradzacher

packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts

@@ -216,10 +216,17 @@ export default util.createRule<Options, MessageIds>({
               contextualType,
               ts.TypeFlags.Null,
             );
-            if (
-              (typeIncludesUndefined && contextualTypeIncludesUndefined) ||
-              (typeIncludesNull && contextualTypeIncludesNull)
-            ) {
+
+            // make sure that the parent accepts the same types
+            // i.e. assigning `string | null | undefined` to `string | undefined` is invalid
+            const isValidUndefined = typeIncludesUndefined
+              ? contextualTypeIncludesUndefined
+              : true;
+            const isValidNull = typeIncludesNull
+              ? contextualTypeIncludesNull
+              : true;
+
+            if (isValidUndefined && isValidNull) {
               context.report({
                 node,
                 messageId: 'contextuallyUnnecessary',

packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts

@@ -104,6 +104,17 @@ class Mx {
   private prop = 1;
 }
     `,
+    // https://github.com/typescript-eslint/typescript-eslint/issues/1199
+    `
+function testFunction(_param: string | undefined): void { /* noop */ }
+const value = 'test' as string | null | undefined
+testFunction(value!)
+    `,
+    `
+function testFunction(_param: string | null): void { /* noop */ }
+const value = 'test' as string | null | undefined
+testFunction(value!)
+    `,
   ],
 
   invalid: [