Fix parser to reject (|) as syntax error per ISO spec

Fixes #3140

The parser was incorrectly accepting (|) and converting it to the
atom '|', when it should produce a syntax_error according to ISO/IEC
13211-1 Technical Corrigendum 2, Section C2.

Changes:
- Modified reduce_brackets() in parser.rs to reject HeadTailSeparator
  inside parentheses, treating (|) as incomplete/forbidden syntax
- Updated builtins.pl to use quoted atom '|' instead of (|) in error term
- Added comprehensive tests in src/tests/iso_syntax_errors.pl covering:
  * (|) syntax error
  * ISO operator creation restrictions for {} and '|'
  * Valid operator declarations with proper priorities

Author: jjtolton

src/lib/builtins.pl

@@ -1466,7 +1466,7 @@
           lists:member(OpSpec, [xfx, xfy, yfx]),
           ( Priority >= 1001 ; Priority == 0 )
        -> '$op'(Priority, OpSpec, Op)
-       ;  throw(error(permission_error(create, operator, (|)), op/3))) % www.complang.tuwien.ac.at/ulrich/iso-prolog/conformity_testing#72
+       ;  throw(error(permission_error(create, operator, '|'), op/3))) % www.complang.tuwien.ac.at/ulrich/iso-prolog/conformity_testing#72
     ;  valid_op(Op), op_priority(Priority), op_specifier(OpSpec) ->
        '$op'(Priority, OpSpec, Op)
     ;  list_of_op_atoms(Op), op_priority(Priority), op_specifier(OpSpec) ->

src/parser/parser.rs

@@ -829,8 +829,17 @@ 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 and ISO-forbidden syntax
+                // See: https://www.complang.tuwien.ac.at/ulrich/iso-prolog/dtc2#C2
+                match self.stack[idx].tt {
+                    TokenType::Comma => {
+                        return false;
+                    }
+                    TokenType::HeadTailSeparator => {
+                        // (|) is forbidden by ISO spec
+                        return false;
+                    }
+                    _ => {}
                 }
 
                 if let Some(atom) = self.stack[idx].tt.sep_to_atom() {

src/tests/iso_syntax_errors.pl

@@ -0,0 +1,67 @@
+:- module(iso_syntax_errors_tests, []).
+:- use_module(test_framework).
+:- use_module(library(charsio)).
+
+% ISO/IEC 13211-1 Technical Corrigendum 2, Section C2
+% See https://www.complang.tuwien.ac.at/ulrich/iso-prolog/dtc2#C2
+
+test("single_bar_in_parens_should_error", (
+    % see https://github.com/mthom/scryer-prolog/issues/3140
+    % (|) should cause a syntax error when parsed
+    catch(
+        (read_from_chars("(|).", _), false),
+        error(syntax_error(_), _),
+        true
+    )
+)).
+
+test("op_create_empty_curly_should_error", (
+    catch(
+        op(500, xfy, {}),
+        error(permission_error(create, operator, {}), _),
+        true
+    )
+)).
+
+test("op_create_empty_curly_in_list_should_error", (
+    catch(
+        op(500, xfy, [{}]),
+        error(permission_error(create, operator, {}), _),
+        true
+    )
+)).
+
+test("op_create_bar_priority_1000_should_error", (
+    catch(
+        op(1000, xfy, '|'),
+        error(permission_error(create, operator, '|'), _),
+        true
+    )
+)).
+
+test("op_create_bar_in_list_priority_1000_should_error", (
+    catch(
+        op(1000, xfy, ['|']),
+        error(permission_error(create, operator, '|'), _),
+        true
+    )
+)).
+
+test("op_create_bar_prefix_should_error", (
+    catch(
+        op(1150, fx, '|'),
+        error(permission_error(create, operator, '|'), _),
+        true
+    )
+)).
+
+test("op_create_bar_priority_1105_should_succeed", (
+    op(1105, xfy, '|'),
+    % Clean up
+    op(0, xfy, '|')
+)).
+
+test("op_remove_bar_should_succeed", (
+    op(1105, xfy, '|'),
+    op(0, xfy, '|')
+)).

tests/scryer/cli/src_tests/iso_syntax_errors.toml

@@ -0,0 +1 @@
+args = ["-f", "--no-add-history", "src/tests/iso_syntax_errors.pl", "-f", "-g", "use_module(library(iso_syntax_errors_tests)), iso_syntax_errors_tests:main_quiet(iso_syntax_errors_tests)"]