[mypyc] always add implicit None return type to __init__ method (#9866)

Fixes mypyc/mypyc#792.

Always give __init__ an implicit None return type so that programs like

```
class C:
    def __init__(self):  # missing `-> None` annotation
        pass
```

compile.

mypyc/irbuild/mapper.py

@@ -132,7 +132,12 @@ def fdef_to_sig(self, fdef: FuncDef) -> FuncSignature:
         else:
             # Handle unannotated functions
             arg_types = [object_rprimitive for arg in fdef.arguments]
-            ret = object_rprimitive
+            # We at least know the return type for __init__ methods will be None.
+            is_init_method = fdef.name == '__init__' and bool(fdef.info)
+            if is_init_method:
+                ret = none_rprimitive
+            else:
+                ret = object_rprimitive
 
         args = [RuntimeArg(arg_name, arg_type, arg_kind)
                 for arg_name, arg_kind, arg_type in zip(fdef.arg_names, fdef.arg_kinds, arg_types)]

mypyc/test-data/run-classes.test

@@ -374,6 +374,16 @@ o = object()
 c1.d = o
 assert c1.d is o
 
+[case testInitMethodWithMissingNoneReturnAnnotation]
+class C:
+    def __init__(self):
+        self.x = 42
+[file driver.py]
+from native import C
+c = C()
+assert c is not None
+assert c.x == 42
+
 [case testConstructClassWithDefaultConstructor]
 class C:
     a: int

mypyc/test-data/run-functions.test

@@ -928,6 +928,15 @@ def f(x):
 from native import f
 assert f(3) == 6
 
+[case testUnannotatedModuleLevelInitFunction]
+# Ensure that adding an implicit `-> None` annotation only applies to `__init__`
+# _methods_ specifically (not module-level `__init__` functions).
+def __init__():
+    return 42
+[file driver.py]
+from native import __init__
+assert __init__() == 42
+
 [case testComplicatedArgs]
 from typing import Tuple, Dict