[mypyc] Support ellipsis (...) expressions in class bodies (#17923)

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

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: hauntsaninja <[email protected]>

Author: 3 people

mypyc/irbuild/classdef.py

@@ -12,6 +12,7 @@
     CallExpr,
     ClassDef,
     Decorator,
+    EllipsisExpr,
     ExpressionStmt,
     FuncDef,
     Lvalue,
@@ -145,7 +146,9 @@ 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

@@ -3,20 +3,36 @@ class Empty: pass
 
 def f(e: Empty) -> Empty:
     return e
+
+class EmptyEllipsis: ...
+
+def g(e: EmptyEllipsis) -> EmptyEllipsis:
+    return e
 [file driver.py]
-from native import Empty, f
+from native import Empty, EmptyEllipsis, f, g
 
 print(isinstance(Empty, type))
 print(Empty)
 print(str(Empty())[:20])
 
 e = Empty()
 print(f(e) is e)
+
+print(isinstance(EmptyEllipsis, type))
+print(EmptyEllipsis)
+print(str(EmptyEllipsis())[:28])
+
+e2 = EmptyEllipsis()
+print(g(e2) is e2)
 [out]
 True
 <class 'native.Empty'>
 <native.Empty object
 True
+True
+<class 'native.EmptyEllipsis'>
+<native.EmptyEllipsis object
+True
 
 [case testClassWithFields]
 class C: