Register switch's discriminant in the outer scope (#15167)

* Add failing test

* Register `switch`'s `discriminant` in the outer scope

packages/babel-plugin-transform-block-scoping/test/fixtures/general/switch-shadow/input.js

@@ -0,0 +1,3 @@
+switch (x) {
+  default: let x;
+}

packages/babel-plugin-transform-block-scoping/test/fixtures/general/switch-shadow/output.js

@@ -0,0 +1,4 @@
+switch (x) {
+  default:
+    var _x;
+}

packages/babel-plugin-transform-block-scoping/test/fixtures/tdz/switch-shadow/input.js

@@ -0,0 +1,4 @@
+let x;
+switch (x) {
+  default: let x;
+}

packages/babel-plugin-transform-block-scoping/test/fixtures/tdz/switch-shadow/output.js

@@ -0,0 +1,5 @@
+var x;
+switch (x) {
+  default:
+    var _x;
+}

packages/babel-traverse/src/path/context.ts

@@ -133,10 +133,12 @@ export function setScope(this: NodePath) {
 
   let path = this.parentPath;
 
-  // Skip method scope if is computed method key or decorator expression
   if (
-    (this.key === "key" || this.listKey === "decorators") &&
-    path.isMethod()
+    // Skip method scope if is computed method key or decorator expression
+    ((this.key === "key" || this.listKey === "decorators") &&
+      path.isMethod()) ||
+    // Skip switch scope if for discriminant (`x` in `switch (x) {}`).
+    (this.key === "discriminant" && path.isSwitchStatement())
   ) {
     path = path.parentPath;
   }

packages/babel-traverse/test/scope.js

@@ -282,6 +282,14 @@ describe("scope", () => {
       });
     });
 
+    it("switch discriminant scope", () => {
+      expect(
+        getPath(`let a = "outside"; switch (a) { default: let a = "inside" }`)
+          .get("body.1.discriminant")
+          .scope.getBinding("a").path.node.init.value,
+      ).toBe("outside");
+    });
+
     it("variable declaration", function () {
       expect(getPath("var foo = null;").scope.getBinding("foo").path.type).toBe(
         "VariableDeclarator",