Fix parser to reject incomplete reductions like ([, ({, ((

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:
- Another opening parenthesis (
- Opening bracket [
- Opening curly brace {
- Comma ,

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

Added comprehensive tests in src/tests/syntax_errors.pl to verify
the fix and prevent regressions.

Author: jjtolton

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: (,  ([  ({  ((
+                match self.stack[idx].tt {
+                    TokenType::Comma
+                    | TokenType::OpenList
+                    | TokenType::OpenCurly
+                    | TokenType::Open
+                    | TokenType::OpenCT => {
+                        return false;
+                    }
+                    _ => {}
                 }
 
                 if let Some(atom) = self.stack[idx].tt.sep_to_atom() {

src/tests/syntax_errors.pl

@@ -0,0 +1,48 @@
+:- 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 {[ produces syntax error (already tested in ISO conformity)
+test("{[ should produce syntax_error", (
+    catch(
+        (read_from_chars("{[}.", _), false),
+        error(syntax_error(_), _),
+        true
+    )
+)).
+
+% Test that {( produces syntax error (already tested in ISO conformity)
+test("{( should produce syntax_error", (
+    catch(
+        (read_from_chars("{(}.", _), false),
+        error(syntax_error(_), _),
+        true
+    )
+)).
+
+% Test that (( produces syntax error
+test("(( should produce syntax_error", (
+    catch(
+        (read_from_chars("((}.", _), false),
+        error(syntax_error(_), _),
+        true
+    )
+)).

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)"]