From 25264267a6cab3dcba815955aae7e7bf302862e1 Mon Sep 17 00:00:00 2001
From: "J.J. Tolton" <jjtolton@gmail.com>
Date: Sun, 26 Oct 2025 11:40:48 -0400
Subject: [PATCH] Fix parser to reject incomplete reductions like ([ and ({

Fixes #3138

The parser was incorrectly accepting expressions like ([ and ({
as valid syntax, parsing them as atoms instead of rejecting them as
incomplete reductions.

This fix adds checks in reduce_brackets() to reject when an opening
parenthesis is followed by:
- Opening bracket [
- Opening curly brace {

Note: (, and (( were already correctly rejected by existing checks.

This brings the behavior in line with how curly braces work, where
{[ and {( correctly produce syntax errors.

Added tests in src/tests/syntax_errors.pl to verify the fix and
prevent regressions.
---
 src/parser/parser.rs                          | 12 ++++++--
 src/tests/syntax_errors.pl                    | 30 +++++++++++++++++++
 tests/scryer/cli/src_tests/syntax_errors.toml |  1 +
 3 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 src/tests/syntax_errors.pl
 create mode 100644 tests/scryer/cli/src_tests/syntax_errors.toml

diff --git a/src/parser/parser.rs b/src/parser/parser.rs
index 61e85c7ec..ed578f815 100644
--- a/src/parser/parser.rs
+++ b/src/parser/parser.rs
@@ -829,8 +829,16 @@ impl<'a, R: CharRead> Parser<'a, R> {
 
         match td.tt {
             TokenType::Open | TokenType::OpenCT => {
-                if self.stack[idx].tt == TokenType::Comma {
-                    return false;
+                // Reject incomplete reductions like ([  and  ({
+                // Note: (,  is already rejected by checking Comma
+                // Note: ((  is already rejected by the check at line 823
+                match self.stack[idx].tt {
+                    TokenType::Comma
+                    | TokenType::OpenList
+                    | TokenType::OpenCurly => {
+                        return false;
+                    }
+                    _ => {}
                 }
 
                 if let Some(atom) = self.stack[idx].tt.sep_to_atom() {
diff --git a/src/tests/syntax_errors.pl b/src/tests/syntax_errors.pl
new file mode 100644
index 000000000..1a963269e
--- /dev/null
+++ b/src/tests/syntax_errors.pl
@@ -0,0 +1,30 @@
+:- module(syntax_errors_tests, []).
+:- use_module(test_framework).
+:- use_module(library(charsio)).
+
+% Test for issue #3138 - ([) should be a syntax error when reading
+test("read of ([). should produce syntax_error", (
+    catch(
+        (read_from_chars("([).", _), false),
+        error(syntax_error(_), _),
+        true
+    )
+)).
+
+% Test that writeq(([)) produces syntax error when reading
+test("read of writeq(([)). should produce syntax_error", (
+    catch(
+        (read_from_chars("writeq(([)).", _), false),
+        error(syntax_error(_), _),
+        true
+    )
+)).
+
+% Test that ({) should be a syntax error (similar to ([)
+test("read of ({). should produce syntax_error", (
+    catch(
+        (read_from_chars("({).", _), false),
+        error(syntax_error(_), _),
+        true
+    )
+)).
diff --git a/tests/scryer/cli/src_tests/syntax_errors.toml b/tests/scryer/cli/src_tests/syntax_errors.toml
new file mode 100644
index 000000000..f0d0ccafc
--- /dev/null
+++ b/tests/scryer/cli/src_tests/syntax_errors.toml
@@ -0,0 +1 @@
+args = ["-f", "--no-add-history", "src/tests/syntax_errors.pl", "-f", "-g", "use_module(library(syntax_errors_tests)), syntax_errors_tests:main_quiet(syntax_errors_tests)"]