Fix assigning to exports from inside a function in scope hoisting

packages/core/integration-tests/test/integration/scope-hoisting/commonjs/export-assign-scope/a.js

@@ -0,0 +1,4 @@
+var b = require('./b');
+
+b.setValue(2);
+module.exports = b.value;

packages/core/integration-tests/test/integration/scope-hoisting/commonjs/export-assign-scope/b.js

@@ -0,0 +1,3 @@
+exports.setValue = function (value) {
+  exports.value = value;
+}

packages/core/integration-tests/test/scope-hoisting.js

@@ -1217,5 +1217,17 @@ describe('scope hoisting', function() {
       assert.equal(output.__esModule, true);
       assert.equal(output.default, 2);
     });
+
+    it('should support assigning to exports from inside a function', async function() {
+      let b = await bundle(
+        path.join(
+          __dirname,
+          '/integration/scope-hoisting/commonjs/export-assign-scope/a.js'
+        )
+      );
+
+      let output = await run(b);
+      assert.deepEqual(output, 2);
+    });
   });
 });

packages/core/parcel-bundler/src/scope-hoisting/hoist.js

@@ -268,13 +268,23 @@ module.exports = {
       let scope = path.scope.getProgramParent();
       if (!scope.hasBinding(identifier.name)) {
         asset.cacheData.exports[name] = identifier.name;
-        let [decl] = path.insertBefore(
-          t.variableDeclaration('var', [
-            t.variableDeclarator(t.clone(identifier), right)
-          ])
-        );
 
-        scope.registerDeclaration(decl);
+        // If in the program scope, create a variable declaration and initialize with the exported value.
+        // Otherwise, declare the variable in the program scope, and assign to it here.
+        if (path.scope === scope) {
+          let [decl] = path.insertBefore(
+            t.variableDeclaration('var', [
+              t.variableDeclarator(t.clone(identifier), right)
+            ])
+          );
+
+          scope.registerDeclaration(decl);
+        } else {
+          scope.push({id: t.clone(identifier)});
+          path.insertBefore(
+            t.assignmentExpression('=', t.clone(identifier), right)
+          );
+        }
       } else {
         path.insertBefore(
           t.assignmentExpression('=', t.clone(identifier), right)