fix: fix span for (x)=>{} expression; fix crash

If an arrow function expression has parentheses, we are calculating the
begin span for the expression incorrectly. This leads to an assertion
failure with the following code:

    // expression.h:438: internal check failed in arrow_function: !this->children_.empty()
    ((x)) => { }

Fix the assertion failure by keeping track of where the first '(' is.

Author: strager

docs/CHANGELOG.md

@@ -31,6 +31,7 @@ Semantic Versioning.
   no longer falsely interpreted as an async function. It is instead interpreted
   as the use of a variable called `async` followed by a non-async function, per
   the language standard.
+* `((x)) => {}` no longer crashes the parser with an assertion failure.
 
 ## 2.5.0 (2022-05-23)
 

src/parse-expression.cpp

@@ -1573,14 +1573,15 @@ void parser::parse_arrow_function_expression_remainder(
   expression* lhs = binary_builder.last_expression();
   function_attributes attributes = function_attributes::normal;
 
+  const char8* left_paren_begin = nullptr;
   if (lhs->kind() == expression_kind::paren) {
+    left_paren_begin = lhs->span().begin();
     lhs = static_cast<expression::paren*>(lhs)->child_;
   }
 
   expression_arena::vector<expression*> parameters(
       "parse_arrow_function_expression_remainder",
       this->expressions_.allocator());
-  const char8* left_paren_begin = nullptr;
   switch (lhs->kind()) {
   case expression_kind::binary_operator:
   case expression_kind::trailing_comma:

test/test-parse-expression.cpp

@@ -2928,7 +2928,7 @@ TEST_F(test_parse_expression, arrow_function) {
     EXPECT_EQ(ast->attributes(), function_attributes::normal);
     EXPECT_EQ(ast->child_count(), 1);
     EXPECT_EQ(summarize(ast->child(0)), "var a");
-    // TODO(strager): Implement begin_offset.
+    EXPECT_EQ(p.range(ast).begin_offset(), 0);
     EXPECT_EQ(p.range(ast).end_offset(), 8);
     EXPECT_THAT(p.errors(), IsEmpty());
   }