[fix] export: false positive for typescript overloads

Fixes #1357.

src/rules/export.js

@@ -24,6 +24,21 @@ ambient namespaces:
 const rootProgram = 'root'
 const tsTypePrefix = 'type:'
 
+/**
+ * Detect function overloads like:
+ * ```ts
+ * export function foo(a: number);
+ * export function foo(a: string);
+ * export function foo(a: number|string) { return a; }
+ * ```
+ * @param {Set<Object>} nodes
+ * @returns {boolean}
+ */
+function isTypescriptFunctionOverloads(nodes) {
+  const types = new Set(Array.from(nodes, node => node.parent.type))
+  return types.size === 2 && types.has('TSDeclareFunction') && types.has('FunctionDeclaration')
+}
+
 module.exports = {
   meta: {
     type: 'problem',
@@ -123,6 +138,8 @@ module.exports = {
           for (let [name, nodes] of named) {
             if (nodes.size <= 1) continue
 
+            if (isTypescriptFunctionOverloads(nodes)) continue
+
             for (let node of nodes) {
               if (name === 'default') {
                 context.report(node, 'Multiple default exports.')

tests/src/rules/export.js

@@ -118,7 +118,7 @@ context('Typescript', function () {
     parsers.push(require.resolve('@typescript-eslint/parser'))
   }
 
-  if (semver.satisfies(eslintPkg.version, '<6.0.0')) {
+  if (semver.satisfies(eslintPkg.version, '>=4.0.0 <6.0.0')) {
     parsers.push(require.resolve('typescript-eslint-parser'))
   }
 
@@ -147,6 +147,14 @@ context('Typescript', function () {
           `,
         }, parserConfig)),
 
+        test(Object.assign({
+          code: `
+            export function fff(a: string);
+            export function fff(a: number);
+            export function fff(a: string|number) {};
+          `,
+        }, parserConfig)),
+
         // namespace
         test(Object.assign({
           code: `