From 58db655133aaae006efe3e3ceee971cf88dc348f Mon Sep 17 00:00:00 2001
From: Brad Zacher <brad.zacher@gmail.com>
Date: Wed, 3 Jun 2020 11:00:27 -0700
Subject: [PATCH] fix(eslint-plugin): [explicit-module-boundary-types] handle
 bodyless arrow functions with explicit return types that return functions
 (#2169)

---
 .../rules/explicit-module-boundary-types.ts   |  7 ++----
 .../src/util/explicitReturnTypeUtils.ts       | 25 ++++++-------------
 .../explicit-module-boundary-types.test.ts    |  6 +++++
 3 files changed, 16 insertions(+), 22 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/explicit-module-boundary-types.ts b/packages/eslint-plugin/src/rules/explicit-module-boundary-types.ts
index 2c065eb6212..fe4dbf28662 100644
--- a/packages/eslint-plugin/src/rules/explicit-module-boundary-types.ts
+++ b/packages/eslint-plugin/src/rules/explicit-module-boundary-types.ts
@@ -402,7 +402,7 @@ export default util.createRule<Options, MessageIds>({
       if (
         isAllowedName(node.parent) ||
         isTypedFunctionExpression(node, options) ||
-        ancestorHasReturnType(node.parent, options)
+        ancestorHasReturnType(node.parent)
       ) {
         return;
       }
@@ -424,10 +424,7 @@ export default util.createRule<Options, MessageIds>({
       }
       checkedFunctions.add(node);
 
-      if (
-        isAllowedName(node.parent) ||
-        ancestorHasReturnType(node.parent, options)
-      ) {
+      if (isAllowedName(node.parent) || ancestorHasReturnType(node.parent)) {
         return;
       }
 
diff --git a/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts b/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts
index 473a9d1549f..6c9c76645ac 100644
--- a/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts
+++ b/packages/eslint-plugin/src/util/explicitReturnTypeUtils.ts
@@ -300,10 +300,8 @@ function isValidFunctionExpressionReturnType(
 function isValidFunctionReturnType(
   node: FunctionNode,
   options: Options,
-  isParentCheck = false,
 ): boolean {
   if (
-    !isParentCheck &&
     options.allowHigherOrderFunctions &&
     doesImmediatelyReturnFunctionExpression(node)
   ) {
@@ -353,29 +351,22 @@ function checkFunctionExpressionReturnType(
  * Check whether any ancestor of the provided node has a valid return type, with
  * the given options.
  */
-function ancestorHasReturnType(
-  ancestor: TSESTree.Node | undefined,
-  options: Options,
-): boolean {
-  // Exit early if this ancestor is not a ReturnStatement.
-  if (ancestor?.type !== AST_NODE_TYPES.ReturnStatement) {
+function ancestorHasReturnType(ancestor: TSESTree.Node | undefined): boolean {
+  // if the ancestor is not a return, then this function was not returned at all, so we can exit early
+  const isReturnStatne = ancestor?.type === AST_NODE_TYPES.ReturnStatement;
+  const isBodylessArrow =
+    ancestor?.type === AST_NODE_TYPES.ArrowFunctionExpression &&
+    ancestor.body.type !== AST_NODE_TYPES.BlockStatement;
+  if (!isReturnStatne && !isBodylessArrow) {
     return false;
   }
 
-  // This boolean tells the `isValidFunctionReturnType` that it is being called
-  // by an ancestor check.
-  const isParentCheck = true;
-
   while (ancestor) {
     switch (ancestor.type) {
       case AST_NODE_TYPES.ArrowFunctionExpression:
       case AST_NODE_TYPES.FunctionExpression:
-        return (
-          isValidFunctionExpressionReturnType(ancestor, options) ||
-          isValidFunctionReturnType(ancestor, options, isParentCheck)
-        );
       case AST_NODE_TYPES.FunctionDeclaration:
-        return isValidFunctionReturnType(ancestor, options, isParentCheck);
+        return ancestor.returnType != null;
     }
 
     ancestor = ancestor.parent;
diff --git a/packages/eslint-plugin/tests/rules/explicit-module-boundary-types.test.ts b/packages/eslint-plugin/tests/rules/explicit-module-boundary-types.test.ts
index 74efa619d90..44aa3c3d066 100644
--- a/packages/eslint-plugin/tests/rules/explicit-module-boundary-types.test.ts
+++ b/packages/eslint-plugin/tests/rules/explicit-module-boundary-types.test.ts
@@ -618,6 +618,12 @@ export function foo(...[a]: any): void {}
     `
 export function foo(arg = 1): void {}
     `,
+    // https://github.com/typescript-eslint/typescript-eslint/issues/2161
+    {
+      code: `
+export const foo = (): ((n: number) => string) => n => String(n);
+      `,
+    },
   ],
   invalid: [
     {