Bail on singleline cells with sunken brackets

Sunken bracket is a closing bracket character that is put before its
opening counterpart.

In IPython, this leads to unnecessary annoyances, where a multiline
prompt is opened for seemingly no reason at all, e.g. if a line contains
a ']]' mistyped brackets.

IPython/core/inputtransformer2.py

@@ -508,6 +508,20 b' def make_tokens_by_line(lines:List[str]):'
 
     return tokens_by_line
 
+
+def has_sunken_brackets(tokens: List[tokenize.TokenInfo]):
+    """Check if the depth of brackets in the list of tokens drops below 0"""
+    parenlev = 0
+    for token in tokens:
+        if token.string in {"(", "[", "{"}:
+            parenlev += 1
+        elif token.string in {")", "]", "}"}:
+            parenlev -= 1
+            if parenlev < 0:
+                return True
+    return False
+
+
 def show_linewise_tokens(s: str):
     """For investigation and debugging"""
     if not s.endswith('\n'):
@@ -662,6 +676,11 b' class TransformerManager:'
 
         tokens_by_line = make_tokens_by_line(lines)
 
+        # Bail if we got one line and there are more closing parentheses than
+        # the opening ones
+        if len(lines) == 1 and has_sunken_brackets(tokens_by_line[0]):
+            return "invalid", None
+
         if not tokens_by_line:
             return 'incomplete', find_last_indent(lines)
 

IPython/core/tests/test_inputtransformer2.py

@@ -297,6 +297,24 b' def test_check_complete_II():'
     nt.assert_equal(cc('''def foo():\n    """'''), ('incomplete', 4))
 
 
+def test_check_complete_invalidates_sunken_brackets():
+    """
+    Test that a single line with more closing brackets than the opening ones is
+    interpretted as invalid
+    """
+    cc = ipt2.TransformerManager().check_complete
+    nt.assert_equal(cc(")"), ("invalid", None))
+    nt.assert_equal(cc("]"), ("invalid", None))
+    nt.assert_equal(cc("}"), ("invalid", None))
+    nt.assert_equal(cc(")("), ("invalid", None))
+    nt.assert_equal(cc("]["), ("invalid", None))
+    nt.assert_equal(cc("}{"), ("invalid", None))
+    nt.assert_equal(cc("[()("), ("invalid", None))
+    nt.assert_equal(cc("())("), ("invalid", None))
+    nt.assert_equal(cc(")[]("), ("invalid", None))
+    nt.assert_equal(cc("()]("), ("invalid", None))
+
+
 def test_null_cleanup_transformer():
     manager = ipt2.TransformerManager()
     manager.cleanup_transforms.insert(0, null_cleanup_transformer)