Reenable lazy compilation in formal parameters

Summary:
This works properly now.

Partial revert of D58147864 (didn't want to revert the test).

Reviewed By: tmikov

Differential Revision: D58532227

fbshipit-source-id: 5608761366506c9eb51d6a182249432c00bb1808

lib/Parser/JSParserImpl.cpp

@@ -586,9 +586,6 @@ bool JSParserImpl::parseFormalParameters(
     Param param,
     ESTree::NodeList &paramList) {
   assert(check(TokenKind::l_paren) && "FormalParameters must start with '('");
-
-  llvh::SaveAndRestore oldFormalParams{isFormalParams_, true};
-
   // (
   SMLoc lparenLoc = advance().Start;
 
@@ -718,11 +715,7 @@ Optional<ESTree::BlockStatementNode *> JSParserImpl::parseFunctionBody(
     bool paramAwait,
     JSLexer::GrammarContext grammarContext,
     bool parseDirectives) {
-  // Disable lazy compilation in formal parameters,
-  // because initializer expressions capture a different environment than the
-  // environment the function body names are added to, and it would add some
-  // complexity to optimize a use case that's never actually encountered.
-  if (pass_ == LazyParse && !eagerly && !isFormalParams_) {
+  if (pass_ == LazyParse && !eagerly) {
     auto startLoc = tok_->getStartLoc();
     assert(
         preParsed_->functionInfo.count(startLoc) == 1 &&

lib/Parser/JSParserImpl.h

@@ -215,11 +215,6 @@ class JSParserImpl {
   /// so we can recover directive nodes back in the lazyParse pass.
   llvh::SmallVector<UniqueString *, 1> seenDirectives_{};
 
-  /// Whether we're currently parsing formal parameters of a function.
-  /// Saved/restored by parseFormalParameters.
-  /// Used to control lazy compilation, which is disabled in parameters.
-  bool isFormalParams_ = false;
-
   /// Whether the current function is an arrow function.
   /// Only set/restored by SaveFunctionState when entering/exiting a new
   /// function.

test/hermes/lazy-capture-in-params.js

@@ -25,3 +25,37 @@
   print(paramsFunc());
   // CHECK: outside
 })();
+
+(function arrowCapture() {
+  var x = 'outside';
+  var paramsFunc;
+
+  function foo(
+    _ = (paramsFunc = () => {
+      // Capture the outer 'x'
+      return x;
+    })
+  ) {
+    let x = 'inside';
+  }
+  foo();
+
+  print(paramsFunc());
+  // CHECK: outside
+})();
+
+(function setCapture() {
+  var x = 'outside';
+  var probeParams;
+
+  var obj = {
+    set a(_ = probeParams = function() { return x; }) {
+      var x = 'inside';
+    }
+  };
+  // Force default params on setter to trigger.
+  obj.a = undefined;
+
+  print(probeParams());
+  // CHECK: outside
+})();