Improve syntax error for not after a unary operator

Author: lysnikolaou

Grammar/python.gram

@@ -795,6 +795,7 @@ term[expr_ty]:
     | a=term '//' b=factor { _PyAST_BinOp(a, FloorDiv, b, EXTRA) }
     | a=term '%' b=factor { _PyAST_BinOp(a, Mod, b, EXTRA) }
     | a=term '@' b=factor { CHECK_VERSION(expr_ty, 5, "The '@' operator is", _PyAST_BinOp(a, MatMult, b, EXTRA)) }
+    | invalid_factor
     | factor
 
 factor[expr_ty] (memo):
@@ -1419,3 +1420,5 @@ invalid_conversion_character:
 
 invalid_arithmetic:
     | sum ('+'|'-'|'*'|'/'|'%'|'//'|'@') a='not' b=inversion { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "'not' after an operator must be parenthesized") }
+invalid_factor:
+    | ('+' | '-' | '~') a='not' b=factor { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "'not' after an operator must be parenthesized") }

Lib/test/test_syntax.py

@@ -1722,7 +1722,15 @@
 Traceback (most recent call last):
 SyntaxError: 'not' after an operator must be parenthesized
 
->>> 3 ~ not 3
+>>> + not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> - not 3
+Traceback (most recent call last):
+SyntaxError: 'not' after an operator must be parenthesized
+
+>>> ~ not 3
 Traceback (most recent call last):
 SyntaxError: 'not' after an operator must be parenthesized
 
@@ -1735,6 +1743,10 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
+>>> not + 1 +
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
 Corner-cases that used to fail to raise the correct error:
 
     >>> def f(*, x=lambda __debug__:0): pass