Do not throw an error for optional binding pattern params in function declaration (#12085)

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

@@ -2526,7 +2526,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     // Allow type annotations inside of a parameter list.
     parseAssignableListItemTypes(param: N.Pattern) {
       if (this.eat(tt.question)) {
-        if (param.type !== "Identifier") {
+        if (param.type !== "Identifier" && !this.state.isDeclareContext) {
           this.raise(param.start, TSErrors.PatternIsOptional);
         }
 

packages/babel-parser/test/fixtures/typescript/function/declare-pattern-parameters/input.ts

@@ -0,0 +1 @@
+declare function f([]?, {})

packages/babel-parser/test/fixtures/typescript/function/declare-pattern-parameters/output.json

@@ -0,0 +1,38 @@
+{
+  "type": "File",
+  "start":0,"end":27,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":27}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":27,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":27}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "TSDeclareFunction",
+        "start":0,"end":27,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":27}},
+        "declare": true,
+        "id": {
+          "type": "Identifier",
+          "start":17,"end":18,"loc":{"start":{"line":1,"column":17},"end":{"line":1,"column":18},"identifierName":"f"},
+          "name": "f"
+        },
+        "generator": false,
+        "async": false,
+        "params": [
+          {
+            "type": "ArrayPattern",
+            "start":19,"end":22,"loc":{"start":{"line":1,"column":19},"end":{"line":1,"column":22}},
+            "elements": [],
+            "optional": true
+          },
+          {
+            "type": "ObjectPattern",
+            "start":24,"end":26,"loc":{"start":{"line":1,"column":24},"end":{"line":1,"column":26}},
+            "properties": []
+          }
+        ]
+      }
+    ],
+    "directives": []
+  }
+}