fix(eslint-plugin): [no-unnecessary-condition] allow nullish coalescing for naked type parameter (typescript-eslint#6910)

dora1998 · web-flow · commit 3e5f8583f1e4 · 2023-04-17T09:52:51.000-04:00
* fix(eslint-plugin): [no-unnecessary-condition] allow nullish coalescing for naked type parameter

* add tests
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts b/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts
@@ -263,8 +263,14 @@ export default createRule<Options, MessageId>({
 
     function checkNodeForNullish(node: TSESTree.Expression): void {
       const type = getNodeType(node);
-      // Conditional is always necessary if it involves `any` or `unknown`
-      if (isTypeAnyType(type) || isTypeUnknownType(type)) {
+
+      // Conditional is always necessary if it involves `any`, `unknown` or a naked type parameter
+      if (
+        isTypeFlagSet(
+          type,
+          ts.TypeFlags.Any | ts.TypeFlags.Unknown | ts.TypeFlags.TypeParameter,
+        )
+      ) {
         return;
       }
 
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts
@@ -280,6 +280,16 @@ function test(a: string | null | undefined) {
     `
 function test(a: unknown) {
   return a ?? 'default';
+}
+    `,
+    `
+function test<T>(a: T) {
+  return a ?? 'default';
+}
+    `,
+    `
+function test<T extends string | null>(a: T) {
+  return a ?? 'default';
 }
     `,
     // Indexing cases
@@ -827,6 +837,14 @@ function test(a: string) {
       code: `
 function test(a: string | false) {
   return a ?? 'default';
+}
+      `,
+      errors: [ruleError(3, 10, 'neverNullish')],
+    },
+    {
+      code: `
+function test<T extends string>(a: T) {
+  return a ?? 'default';
 }
       `,
       errors: [ruleError(3, 10, 'neverNullish')],
@@ -858,6 +876,14 @@ function test(a: null[]) {
     },
     {
       code: `
+function test<T extends null>(a: T) {
+  return a ?? 'default';
+}
+      `,
+      errors: [ruleError(3, 10, 'alwaysNullish')],
+    },
+    {
+      code: `
 function test(a: never) {
   return a ?? 'default';
 }
