JuliaSyntax: fix anonymous function parsing (JuliaLang#60221)

Author: IanButterworth

JuliaSyntax/src/julia/parser.jl

@@ -2218,10 +2218,23 @@ function parse_function_signature(ps::ParseState, is_function::Bool)
             opts = parse_brackets(ps, K")") do had_commas, had_splat, num_semis, num_subexprs
                 _parsed_call = was_eventually_call(ps)
                 _maybe_grouping_parens = !had_commas && !had_splat && num_semis == 0 && num_subexprs == 1
-                # Skip intervening newlines only when the parentheses hold a single
-                # expression, which is the ambiguous case between a name like (::T)
-                # and an anonymous function parameter list.
-                next_kind = peek(ps, 2, skip_newlines=_maybe_grouping_parens)
+                # Check if there's a newline between `)` and the next `(` or `.`.
+                # We need to find where `)` is and check what immediately follows it.
+                # If peek(1, skip_newlines=false) is `)`, we're directly before it.
+                # Otherwise there's whitespace/newline before `)`.
+                next_token_pos = if peek(ps, 1, skip_newlines=false) == K")"
+                    # Directly before ), token after ) is at 2
+                    2
+                else
+                    # There's whitespace before ), so ) is at 2
+                    # and what follows ) is at 3
+                    3
+                end
+                token_after_paren = peek(ps, next_token_pos, skip_newlines=false)
+                # If token_after_paren is a newline, this is an anonymous function
+                has_newline_after_paren = _maybe_grouping_parens && token_after_paren == K"NewlineWs"
+                # Get the next significant token to determine if we need to parse a call
+                next_kind = peek(ps, 2, skip_newlines=_maybe_grouping_parens && !has_newline_after_paren)
                 _needs_parse_call = next_kind ∈ KSet"( ."
                 _is_anon_func = (!_needs_parse_call && !_parsed_call) || had_commas
                 return (needs_parameters      = _is_anon_func,

JuliaSyntax/test/parser.jl

@@ -611,6 +611,8 @@ tests = [
         "macro \$f()    end"   =>  "(macro (call (\$ f)) (block))"
         "macro (\$f)()  end"   =>  "(macro (call (parens (\$ f))) (block))"
         "function (x) body end"=>  "(function (tuple-p x) (block body))"
+        "function (x)\n    body\nend"=>  "(function (tuple-p x) (block body))"
+        "function (x)\n() end" =>  "(function (tuple-p x) (block (tuple-p)))"
         "function (x,y) end"   =>  "(function (tuple-p x y) (block))"
         "function (x,y,) end"  =>  "(function (tuple-p-, x y) (block))"
         "function (x=1) end"   =>  "(function (tuple-p (= x 1)) (block))"