[parser] Fix scope handling of Flow declared functions (#12735)

packages/babel-parser/src/plugins/flow/index.js

@@ -5,26 +5,27 @@
 // Error messages are colocated with the plugin.
 /* eslint-disable @babel/development-internal/dry-error-messages */
 
-import type Parser from "../parser";
-import { types as tt, type TokenType } from "../tokenizer/types";
-import * as N from "../types";
-import type { Options } from "../options";
-import type { Pos, Position } from "../util/location";
-import type State from "../tokenizer/state";
-import { types as tc } from "../tokenizer/context";
+import type Parser from "../../parser";
+import { types as tt, type TokenType } from "../../tokenizer/types";
+import * as N from "../../types";
+import type { Pos, Position } from "../../util/location";
+import type State from "../../tokenizer/state";
+import { types as tc } from "../../tokenizer/context";
 import * as charCodes from "charcodes";
-import { isIteratorStart, isKeyword } from "../util/identifier";
+import { isIteratorStart, isKeyword } from "../../util/identifier";
+import FlowScopeHandler from "./scope";
 import {
   type BindingTypes,
   BIND_LEXICAL,
   BIND_VAR,
   BIND_FUNCTION,
+  BIND_FLOW_DECLARE_FN,
   SCOPE_ARROW,
   SCOPE_FUNCTION,
   SCOPE_OTHER,
-} from "../util/scopeflags";
-import type { ExpressionErrors } from "../parser/util";
-import { Errors } from "../parser/error";
+} from "../../util/scopeflags";
+import type { ExpressionErrors } from "../../parser/util";
+import { Errors } from "../../parser/error";
 
 const reservedTypes = new Set([
   "_",
@@ -185,11 +186,10 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     // The value of the @flow/@noflow pragma. Initially undefined, transitions
     // to "@flow" or "@noflow" if we see a pragma. Transitions to null if we are
     // past the initial comment.
-    flowPragma: void | null | "flow" | "noflow";
+    flowPragma: void | null | "flow" | "noflow" = undefined;
 
-    constructor(options: ?Options, input: string) {
-      super(options, input);
-      this.flowPragma = undefined;
+    getScopeHandler(): Class<FlowScopeHandler> {
+      return FlowScopeHandler;
     }
 
     shouldParseTypes(): boolean {
@@ -327,6 +327,8 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       this.resetEndLocation(id);
       this.semicolon();
 
+      this.scope.declareName(node.id.name, BIND_FLOW_DECLARE_FN, node.id.start);
+
       return this.finishNode(node, "DeclareFunction");
     }
 

packages/babel-parser/src/plugins/flow/scope.js

@@ -0,0 +1,56 @@
+// @flow
+
+import ScopeHandler, { Scope } from "../../util/scope";
+import {
+  BIND_FLAGS_FLOW_DECLARE_FN,
+  type ScopeFlags,
+  type BindingTypes,
+} from "../../util/scopeflags";
+import * as N from "../../types";
+
+// Reference implementation: https://github.com/facebook/flow/blob/23aeb2a2ef6eb4241ce178fde5d8f17c5f747fb5/src/typing/env.ml#L536-L584
+class FlowScope extends Scope {
+  // declare function foo(): type;
+  declareFunctions: string[] = [];
+}
+
+export default class FlowScopeHandler extends ScopeHandler<FlowScope> {
+  createScope(flags: ScopeFlags): FlowScope {
+    return new FlowScope(flags);
+  }
+
+  declareName(name: string, bindingType: BindingTypes, pos: number) {
+    const scope = this.currentScope();
+    if (bindingType & BIND_FLAGS_FLOW_DECLARE_FN) {
+      this.checkRedeclarationInScope(scope, name, bindingType, pos);
+      this.maybeExportDefined(scope, name);
+      scope.declareFunctions.push(name);
+      return;
+    }
+
+    super.declareName(...arguments);
+  }
+
+  isRedeclaredInScope(
+    scope: FlowScope,
+    name: string,
+    bindingType: BindingTypes,
+  ): boolean {
+    if (super.isRedeclaredInScope(...arguments)) return true;
+
+    if (bindingType & BIND_FLAGS_FLOW_DECLARE_FN) {
+      return (
+        !scope.declareFunctions.includes(name) &&
+        (scope.lexical.includes(name) || scope.functions.includes(name))
+      );
+    }
+
+    return false;
+  }
+
+  checkLocalExport(id: N.Identifier) {
+    if (this.scopeStack[0].declareFunctions.indexOf(id.name) === -1) {
+      super.checkLocalExport(id);
+    }
+  }
+}