Fix parse tree for parentheses around !!

Part of r-lib#296

NEWS.md

@@ -113,6 +113,12 @@
   the first argument of an S3 method (which is necessarily evaluated
   in order to detect which class to dispatch to). (#295, #300).
 
+* Parentheses around `!!` are automatically removed. This is useful
+  for unquoting function names: `(!! sym("name"))(arg)`. If you need
+  the parenthesis wrapping, you can add a second layer since only one
+  layer is removed: `((!! x)) + 1` (#296).
+
+
 ## Breaking changes
 
 * `is_node()` now returns `TRUE` for calls as well and `is_pairlist()`

src/expr-interp.c

@@ -39,6 +39,19 @@ static void poke_bang_bang_operand(sexp* bang, struct expansion_info* info) {
 struct expansion_info which_bang_op(sexp* x) {
   struct expansion_info info = init_expansion_info();
 
+  if (r_is_call(x, "(")) {
+    sexp* paren = r_node_cadr(x);
+    if (r_is_call(paren, "(")) {
+      return info;
+    }
+    struct expansion_info inner_info = which_bang_op(paren);
+    if (inner_info.op == OP_EXPAND_UQ) {
+      return inner_info;
+    } else {
+      return info;
+    }
+  }
+
   if (!r_is_call(x, "!")) {
     return info;
   }

tests/testthat/test-tidy-unquote.R

@@ -225,6 +225,20 @@ test_that("`!!` works in prefixed calls", {
   expect_identical(expr_interp(~base::`!!`(~list)()), quo(base::list()))
 })
 
+test_that("one layer of parentheses around !! is removed", {
+  foo <- "foo"
+  expect_identical(expr((!! foo)), "foo")
+  expect_identical(expr(((!! foo))), quote(("foo")))
+
+  expect_identical(expr((!! foo) + 1), quote("foo" + 1))
+  expect_identical(expr(((!! foo)) + 1), quote(("foo") + 1))
+
+  expect_identical(expr((!! sym(foo))(bar)), quote(foo(bar)))
+  expect_identical(expr(((!! sym(foo)))(bar)), quote((foo)(bar)))
+
+  expect_identical(exprs((!! foo), ((!! foo))), named_list("foo", quote(("foo"))))
+})
+
 
 # quosures -----------------------------------------------------------