[Fix] export: false positive for typescript namespace merging

CHANGELOG.md

@@ -10,9 +10,9 @@ This change log adheres to standards from [Keep a CHANGELOG](https://keepachange
 - [`no-named-default`, `no-default-export`, `prefer-default-export`, `no-named-export`, `export`, `named`, `namespace`, `no-unused-modules`]: support arbitrary module namespace names ([#2358], thanks [@sosukesuzuki])
 - [`no-dynamic-require`]: support dynamic import with espree ([#2371], thanks [@sosukesuzuki])
 
-
 ### Fixed
 - [`default`]: `typescript-eslint-parser`: avoid a crash on exporting as namespace (thanks [@ljharb])
+- [`export`]/TypeScript: false positive for typescript namespace merging ([#1964], thanks [@magarcia])
 
 ### Changed
 - [Tests] `no-nodejs-modules`: add tests for node protocol URL ([#2367], thanks [@sosukesuzuki])
@@ -1575,6 +1575,7 @@ for info on changes for earlier releases.
 [@ludofischer]: https://github.com/ludofischer
 [@lukeapage]: https://github.com/lukeapage
 [@lydell]: https://github.com/lydell
+[@magarcia]: https://github.com/magarcia
 [@Mairu]: https://github.com/Mairu
 [@malykhinvi]: https://github.com/malykhinvi
 [@manovotny]: https://github.com/manovotny
@@ -1665,4 +1666,4 @@ for info on changes for earlier releases.
 [@wtgtybhertgeghgtwtg]: https://github.com/wtgtybhertgeghgtwtg
 [@xpl]: https://github.com/xpl
 [@yordis]: https://github.com/yordis
-[@zloirock]: https://github.com/zloirock
+[@zloirock]: https://github.com/zloirock

src/rules/export.js

@@ -36,13 +36,59 @@ const tsTypePrefix = 'type:';
  */
 function isTypescriptFunctionOverloads(nodes) {
   const types = new Set(Array.from(nodes, node => node.parent.type));
-  return (
-    types.has('TSDeclareFunction') &&
-    (
-      types.size === 1 ||
-      (types.size === 2 && types.has('FunctionDeclaration'))
-    )
-  );
+  return types.has('TSDeclareFunction')
+    && (
+      types.size === 1
+      || (types.size === 2 && types.has('FunctionDeclaration'))
+    );
+}
+
+/**
+ * Detect merging Namespaces with Classes, Functions, or Enums like:
+ * ```ts
+ * export class Foo { }
+ * export namespace Foo { }
+ * ```
+ * @param {Set<Object>} nodes
+ * @returns {boolean}
+ */
+function isTypescriptNamespaceMerging(nodes) {
+  const types = new Set(Array.from(nodes, node => node.parent.type));
+  const noNamespaceNodes = Array.from(nodes).filter((node) => node.parent.type !== 'TSModuleDeclaration');
+
+  return types.has('TSModuleDeclaration')
+    && (
+      types.size === 1
+      // Merging with functions
+      || (types.size === 2 && (types.has('FunctionDeclaration') || types.has('TSDeclareFunction')))
+      || (types.size === 3 && types.has('FunctionDeclaration') && types.has('TSDeclareFunction'))
+      // Merging with classes or enums
+      || (types.size === 2 && (types.has('ClassDeclaration') || types.has('TSEnumDeclaration')) && noNamespaceNodes.length === 1)
+    );
+}
+
+/**
+ * Detect if a typescript namespace node should be reported as multiple export:
+ * ```ts
+ * export class Foo { }
+ * export function Foo();
+ * export namespace Foo { }
+ * ```
+ * @param {Object} node
+ * @param {Set<Object>} nodes
+ * @returns {boolean}
+ */
+function shouldSkipTypescriptNamespace(node, nodes) {
+  const types = new Set(Array.from(nodes, node => node.parent.type));
+
+  return !isTypescriptNamespaceMerging(nodes)
+    && node.parent.type === 'TSModuleDeclaration'
+    && (
+      types.has('TSEnumDeclaration')
+      || types.has('ClassDeclaration')
+      || types.has('FunctionDeclaration')
+      || types.has('TSDeclareFunction')
+    );
 }
 
 module.exports = {
@@ -85,15 +131,19 @@ module.exports = {
     }
 
     return {
-      'ExportDefaultDeclaration': (node) => addNamed('default', node, getParent(node)),
+      ExportDefaultDeclaration(node) {
+        addNamed('default', node, getParent(node));
+      },
 
-      'ExportSpecifier': (node) => addNamed(
-        node.exported.name || node.exported.value,
-        node.exported,
-        getParent(node.parent),
-      ),
+      ExportSpecifier(node) {
+        addNamed(
+          node.exported.name || node.exported.value,
+          node.exported,
+          getParent(node.parent),
+        );
+      },
 
-      'ExportNamedDeclaration': function (node) {
+      ExportNamedDeclaration(node) {
         if (node.declaration == null) return;
 
         const parent = getParent(node);
@@ -119,7 +169,7 @@ module.exports = {
         }
       },
 
-      'ExportAllDeclaration': function (node) {
+      ExportAllDeclaration(node) {
         if (node.source == null) return; // not sure if this is ever true
 
         // `export * as X from 'path'` does not conflict
@@ -156,9 +206,11 @@ module.exports = {
           for (const [name, nodes] of named) {
             if (nodes.size <= 1) continue;
 
-            if (isTypescriptFunctionOverloads(nodes)) continue;
+            if (isTypescriptFunctionOverloads(nodes) || isTypescriptNamespaceMerging(nodes)) continue;
 
             for (const node of nodes) {
+              if (shouldSkipTypescriptNamespace(node, nodes)) continue;
+
               if (name === 'default') {
                 context.report(node, 'Multiple default exports.');
               } else {