[mypyc] Support ellipsis (...) expressions in class bodies

This can be used to declare concise custom exceptions, e.g.

    class UnknownReleaseError(ValueError): ...

which otherwise probably would be written

    class UnknownReleaseError(ValueError):
        pass

and is supported by CPython.

Closes mypyc/mypyc#1069

Author: Newbytee

mypyc/irbuild/classdef.py

@@ -12,6 +12,7 @@
     CallExpr,
     ClassDef,
     Decorator,
+    EllipsisExpr,
     ExpressionStmt,
     FuncDef,
     Lvalue,
@@ -145,7 +146,11 @@ def transform_class_def(builder: IRBuilder, cdef: ClassDef) -> None:
                 continue
             with builder.catch_errors(stmt.line):
                 cls_builder.add_method(get_func_def(stmt))
-        elif isinstance(stmt, PassStmt):
+        elif (
+            isinstance(stmt, PassStmt)
+            or (isinstance(stmt, ExpressionStmt)
+            and isinstance(stmt.expr, EllipsisExpr))
+        ):
             continue
         elif isinstance(stmt, AssignmentStmt):
             if len(stmt.lvalues) != 1:

mypyc/test-data/run-classes.test

@@ -18,6 +18,26 @@ True
 <native.Empty object
 True
 
+[case testEmptyClassEllipsis]
+class EmptyEllipsis: ...
+
+def f(e: EmptyEllipsis) -> EmptyEllipsis:
+    return e
+[file driver.py]
+from native import EmptyEllipsis, f
+
+print(isinstance(EmptyEllipsis, type))
+print(EmptyEllipsis)
+print(str(EmptyEllipsis())[:20])
+
+e = EmptyEllipsis()
+print(f(e) is e)
+[out]
+True
+<class 'native.EmptyEllipsis'>
+<native.EmptyEllipsis object
+True
+
 [case testClassWithFields]
 class C:
     x: int