Fix crash on destructured params in arrow function types (#278)

Fixes #277

There's a decision point on whether the upcoming type is a function type or not,
and it wasn't considering destructured params.

src/parser/plugins/typescript.ts

@@ -548,6 +548,21 @@ function tsSkipParameterStart(): boolean {
     next();
     return true;
   }
+  // If this is a possible array/object destructure, walk to the matching bracket/brace.
+  // The next token after will tell us definitively whether this is a function param.
+  if (match(tt.braceL) || match(tt.bracketL)) {
+    let depth = 1;
+    next();
+    while (depth > 0) {
+      if (match(tt.braceL) || match(tt.bracketL)) {
+        depth++;
+      } else if (match(tt.braceR) || match(tt.bracketR)) {
+        depth--;
+      }
+      next();
+    }
+    return true;
+  }
   return false;
 }
 

test/typescript-test.ts

@@ -1011,4 +1011,21 @@ describe("typescript transform", () => {
     `,
     );
   });
+
+  it("allows destructured params in function types", () => {
+    assertTypeScriptResult(
+      `
+      const f: ({a}: {a: number}) => void = () => {};
+      const g: ([a]: Array<number>) => void = () => {};
+      const h: ({a: {b: [c]}}: any) => void = () => {};
+      const o: ({a: {b: c}}) = {};
+    `,
+      `"use strict";
+      const f = () => {};
+      const g = () => {};
+      const h = () => {};
+      const o = {};
+    `,
+    );
+  });
 });