names that end with __ are not positional-only (#5158)

Also removed a duplicate test case (apparently a holdover
from before fastparse).

Author: JelleZijlstra

mypy/sharedparse.py

@@ -104,4 +104,4 @@ def special_function_elide_names(name: str) -> bool:
 
 
 def argument_elide_name(name: Optional[str]) -> bool:
-    return name is not None and name.startswith("__")
+    return name is not None and name.startswith("__") and not name.endswith("__")

test-data/unit/check-functions.test

@@ -2073,25 +2073,18 @@ h(7) # E: Cannot call function of unknown type
 
 [case testPositionalOnlyArg]
 def f(__a: int) -> None: pass
+def g(__a__: int) -> None: pass
 
 f(1)
 f(__a=1) # E: Unexpected keyword argument "__a" for "f"
 
-[builtins fixtures/bool.pyi]
-[out]
-main:1: note: "f" defined here
-
-[case testPositionalOnlyArgFastparse]
-
-
-def f(__a: int) -> None: pass
-
-f(1)
-f(__a=1) # E: Unexpected keyword argument "__a" for "f"
+g(1)
+# Argument names that also end with __ are not positional-only.
+g(__a__=1)
 
 [builtins fixtures/bool.pyi]
 [out]
-main:3: note: "f" defined here
+main:1: note: "f" defined here
 
 [case testMagicMethodPositionalOnlyArg]
 class A(object):