bpo-44838: Refine the custom syntax errors for invalid 'if' expressio…

…ns (pythonGH-27615)

Grammar/python.gram

@@ -1083,7 +1083,7 @@ invalid_expression:
     # Soft keywords need to also be ignored because they can be parsed as NAME NAME
    | !(NAME STRING | SOFT_KEYWORD) a=disjunction b=expression_without_invalid {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
-   | a=disjunction 'if' b=disjunction !'else' { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
+   | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
 
 invalid_named_expression:
     | a=expression ':=' expression {

Lib/test/test_syntax.py

@@ -152,6 +152,14 @@
 Traceback (most recent call last):
 SyntaxError: expected 'else' after 'if' expression
 
+>>> if True:
+...     print("Hello"
+...
+... if 2:
+...    print(123))
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
 >>> True = True = 3
 Traceback (most recent call last):
 SyntaxError: cannot assign to True

Misc/NEWS.d/next/Core and Builtins/2021-08-05-17-42-03.bpo-44838.r_Lkj_.rst

@@ -0,0 +1,2 @@
+Fixed a bug that was causing the parser to raise an incorrect custom
+:exc:`SyntaxError` for invalid 'if' expressions. Patch by Pablo Galindo.