From fd81f5d4f316eb0fd275b0743c80e213c29ced52 Mon Sep 17 00:00:00 2001
From: Kirk Waiblinger <kirk.waiblinger@gmail.com>
Date: Sat, 4 May 2024 00:52:34 -0600
Subject: [PATCH] [explicit-function-return-types] default parameters where a
 type annotation is present should count as typed function expressions

fix https://github.com/typescript-eslint/typescript-eslint/issues/8950
---
 .../src/util/explicitReturnTypeUtils.ts       | 10 +++
 .../explicit-function-return-type.test.ts     | 74 +++++++++++++++++++
 2 files changed, 84 insertions(+)

diff --git a/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts b/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts
index 2604e6185cc..f65273a502e 100644
--- a/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts
+++ b/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts
@@ -87,12 +87,22 @@ function isTypedParent(
   return (
     isTypeAssertion(parent) ||
     isVariableDeclaratorWithTypeAnnotation(parent) ||
+    isDefaultFunctionParameterWithTypeAnnotation(parent) ||
     isPropertyDefinitionWithTypeAnnotation(parent) ||
     isFunctionArgument(parent, callee) ||
     isTypedJSX(parent)
   );
 }
 
+function isDefaultFunctionParameterWithTypeAnnotation(
+  node: TSESTree.Node,
+): boolean {
+  return (
+    node.type === AST_NODE_TYPES.AssignmentPattern &&
+    node.left.typeAnnotation != null
+  );
+}
+
 /**
  * Checks if a node belongs to:
  * ```
diff --git a/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts b/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts
index 7556cee3ce7..0e8c5182b7d 100644
--- a/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts
+++ b/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts
@@ -770,7 +770,32 @@ class Bar {
 }
       `,
     },
+    {
+      code: `
+type CallBack = () => void;
+
+function f(gotcha: CallBack = () => {}): void {}
+      `,
+      options: [{ allowTypedFunctionExpressions: true }],
+    },
+    {
+      code: `
+type CallBack = () => void;
+
+const f = (gotcha: CallBack = () => {}): void => {};
+      `,
+      options: [{ allowTypedFunctionExpressions: true }],
+    },
+    {
+      code: `
+type ObjectWithCallback = { callback: () => void };
+
+const f = (gotcha: ObjectWithCallback = { callback: () => {} }): void => {};
+      `,
+      options: [{ allowTypedFunctionExpressions: true }],
+    },
   ],
+
   invalid: [
     {
       code: `
@@ -1940,5 +1965,54 @@ let foo = (() => () => {})()();
         },
       ],
     },
+    {
+      code: `
+type CallBack = () => void;
+
+function f(gotcha: CallBack = () => {}): void {}
+      `,
+      options: [{ allowTypedFunctionExpressions: false }],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 4,
+          column: 34,
+          endLine: 4,
+          endColumn: 36,
+        },
+      ],
+    },
+    {
+      code: `
+type CallBack = () => void;
+
+const f = (gotcha: CallBack = () => {}): void => {};
+      `,
+      options: [{ allowTypedFunctionExpressions: false }],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 4,
+          column: 34,
+          endLine: 4,
+          endColumn: 36,
+        },
+      ],
+    },
+    {
+      code: `
+type ObjectWithCallback = { callback: () => void };
+
+const f = (gotcha: ObjectWithCallback = { callback: () => {} }): void => {};
+      `,
+      options: [{ allowTypedFunctionExpressions: false }],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 4,
+          column: 43,
+        },
+      ],
+    },
   ],
 });