[@babel/parser] Add "allowUndeclaredExports" option (#9864)

Ref: https://github.com/babel/notes/blob/master/2019/04/09.md#export-changes-in-scope-pr-break-meteor-issue

packages/babel-parser/src/options.js

@@ -15,6 +15,7 @@ export type Options = {
   allowReturnOutsideFunction: boolean,
   allowImportExportEverywhere: boolean,
   allowSuperOutsideMethod: boolean,
+  allowUndeclaredExports: boolean,
   plugins: PluginList,
   strictMode: ?boolean,
   ranges: boolean,
@@ -41,6 +42,8 @@ export const defaultOptions: Options = {
   allowImportExportEverywhere: false,
   // TODO
   allowSuperOutsideMethod: false,
+  // When enabled, export statements can reference undeclared variables.
+  allowUndeclaredExports: false,
   // An array of plugins to enable
   plugins: [],
   // TODO

packages/babel-parser/src/parser/statement.js

@@ -45,7 +45,11 @@ export default class StatementParser extends ExpressionParser {
 
     this.parseBlockBody(program, true, true, tt.eof);
 
-    if (this.inModule && this.scope.undefinedExports.size > 0) {
+    if (
+      this.inModule &&
+      !this.options.allowUndeclaredExports &&
+      this.scope.undefinedExports.size > 0
+    ) {
       for (const [name] of Array.from(this.scope.undefinedExports)) {
         const pos = this.scope.undefinedExports.get(name);
         // $FlowIssue

packages/babel-parser/test/fixtures/core/opts/allowUndeclaredExports/input.js

@@ -0,0 +1 @@
+export { foo };

packages/babel-parser/test/fixtures/core/opts/allowUndeclaredExports/options.json

@@ -0,0 +1,4 @@
+{
+  "sourceType": "module",
+  "allowUndeclaredExports": true
+}

packages/babel-parser/test/fixtures/core/opts/allowUndeclaredExports/output.json

@@ -0,0 +1,103 @@
+{
+  "type": "File",
+  "start": 0,
+  "end": 15,
+  "loc": {
+    "start": {
+      "line": 1,
+      "column": 0
+    },
+    "end": {
+      "line": 1,
+      "column": 15
+    }
+  },
+  "program": {
+    "type": "Program",
+    "start": 0,
+    "end": 15,
+    "loc": {
+      "start": {
+        "line": 1,
+        "column": 0
+      },
+      "end": {
+        "line": 1,
+        "column": 15
+      }
+    },
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExportNamedDeclaration",
+        "start": 0,
+        "end": 15,
+        "loc": {
+          "start": {
+            "line": 1,
+            "column": 0
+          },
+          "end": {
+            "line": 1,
+            "column": 15
+          }
+        },
+        "specifiers": [
+          {
+            "type": "ExportSpecifier",
+            "start": 9,
+            "end": 12,
+            "loc": {
+              "start": {
+                "line": 1,
+                "column": 9
+              },
+              "end": {
+                "line": 1,
+                "column": 12
+              }
+            },
+            "local": {
+              "type": "Identifier",
+              "start": 9,
+              "end": 12,
+              "loc": {
+                "start": {
+                  "line": 1,
+                  "column": 9
+                },
+                "end": {
+                  "line": 1,
+                  "column": 12
+                },
+                "identifierName": "foo"
+              },
+              "name": "foo"
+            },
+            "exported": {
+              "type": "Identifier",
+              "start": 9,
+              "end": 12,
+              "loc": {
+                "start": {
+                  "line": 1,
+                  "column": 9
+                },
+                "end": {
+                  "line": 1,
+                  "column": 12
+                },
+                "identifierName": "foo"
+              },
+              "name": "foo"
+            }
+          }
+        ],
+        "source": null,
+        "declaration": null
+      }
+    ],
+    "directives": []
+  }
+}