Issue #19611: handle implicit parameters in inspect.signature

inspect.signature now reports the implicit ``.0`` parameters generated by
the compiler for comprehension and generator expression scopes as if they
were positional-only parameters called ``implicit0``.

Patch by Jelle Zijlstra.

Lib/inspect.py

@@ -2396,6 +2396,20 @@ class Parameter:
         if not isinstance(name, str):
             raise TypeError("name must be a str, not a {!r}".format(name))
 
+        if name[0] == '.' and name[1:].isdigit():
+            # These are implicit arguments generated by comprehensions. In
+            # order to provide a friendlier interface to users, we recast
+            # their name as "implicitN" and treat them as positional-only.
+            # See issue 19611.
+            if kind != _POSITIONAL_OR_KEYWORD:
+                raise ValueError(
+                    'implicit arguments must be passed in as {}'.format(
+                        _POSITIONAL_OR_KEYWORD
+                    )
+                )
+            self._kind = _POSITIONAL_ONLY
+            name = 'implicit{}'.format(name[1:])
+
         if not name.isidentifier():
             raise ValueError('{!r} is not a valid parameter name'.format(name))