Fix parser for TypeVar bounds (#4602)

CHANGES.md

@@ -26,6 +26,9 @@
 
 <!-- Changes to the parser or to version autodetection -->
 
+- Fix bug where certain unusual expressions (e.g., lambdas) were not accepted
+  in type parameter bounds and defaults. (#4602)
+
 ### Performance
 
 <!-- Changes that improve Black's performance. -->

src/blib2to3/Grammar.txt

@@ -12,9 +12,9 @@ file_input: (NEWLINE | stmt)* ENDMARKER
 single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
 eval_input: testlist NEWLINE* ENDMARKER
 
-typevar: NAME [':' expr] ['=' expr]
-paramspec: '**' NAME ['=' expr]
-typevartuple: '*' NAME ['=' (expr|star_expr)]
+typevar: NAME [':' test] ['=' test]
+paramspec: '**' NAME ['=' test]
+typevartuple: '*' NAME ['=' (test|star_expr)]
 typeparam: typevar | paramspec | typevartuple
 typeparams: '[' typeparam (',' typeparam)* [','] ']'
 

tests/data/cases/type_param_defaults.py

@@ -20,6 +20,8 @@ def trailing_comma1[T=int,](a: str):
 def trailing_comma2[T=int](a: str,):
     pass
 
+def weird_syntax[T=lambda: 42, **P=lambda: 43, *Ts=lambda: 44](): pass
+
 # output
 
 type A[T = int] = float
@@ -61,3 +63,7 @@ def trailing_comma2[T = int](
     a: str,
 ):
     pass
+
+
+def weird_syntax[T = lambda: 42, **P = lambda: 43, *Ts = lambda: 44]():
+    pass

tests/data/cases/type_params.py

@@ -13,6 +13,8 @@ def it_gets_worse[WhatIsTheLongestTypeVarNameYouCanThinkOfEnoughToMakeBlackSplit
 
 def magic[Trailing, Comma,](): pass
 
+def weird_syntax[T: lambda: 42, U: a or b](): pass
+
 # output
 
 
@@ -56,3 +58,7 @@ def magic[
     Comma,
 ]():
     pass
+
+
+def weird_syntax[T: lambda: 42, U: a or b]():
+    pass