[mypyc] Fix vtable construction for deep trait inheritance (#20917)

Fixes mypyc/mypyc#1189

The core issue: `compute_vtable` iterates `cls.traits` (direct parents
only) instead of the full trait MRO, so when an intermediate trait is
empty, grandparent trait methods are never added to the vtable.

The difference between `cls.traits` and `all_traits` for the issue's
repro is the following:
 
```Python3
Class Child
-----------

 - cls.traits:
    •  Middle  --> {}

 - all_traits:
    •  Child   --> {'derived': <FuncIR Child.derived>}
    •  Middle  --> {}
    •  Base    --> {'value': <FuncIR Base.value>}
```

Author: VaggelisD

mypyc/irbuild/vtable.py

@@ -37,7 +37,7 @@ def compute_vtable(cls: ClassIR) -> None:
 
     all_traits = [t for t in cls.mro if t.is_trait]
 
-    for t in [cls] + cls.traits:
+    for t in [cls] + [t for t in all_traits if t is not cls]:
         for fn in itertools.chain(t.methods.values()):
             # TODO: don't generate a new entry when we overload without changing the type
             if fn == cls.get_method(fn.name, prefer_method=True):

mypyc/test-data/run-classes.test

@@ -5709,3 +5709,68 @@ def test_derived_acyclic() -> None:
     assert d.x == 3
     assert d.y == "hi"
     assert not gc.is_tracked(d)
+
+[case testTraitVtableGrandparentMethod]
+from typing import Any
+from mypy_extensions import trait
+
+@trait
+class Base:
+    def value(self) -> Any:
+        return 42
+
+@trait
+class Middle(Base):
+    pass
+
+@trait
+class Child(Middle):
+    def derived(self) -> Any:
+        return self.value()
+
+class Concrete(Child):
+    pass
+
+class Sub(Concrete):
+    def value(self) -> Any:
+        return 99
+
+[file driver.py]
+from native import Concrete, Sub
+c = Concrete()
+assert c.value() == 42
+assert c.derived() == 42
+s = Sub()
+assert s.value() == 99
+assert s.derived() == 99
+
+[case testTraitVtableGrandparentMethodAllowInterpretedSubclasses]
+from typing import Any
+from mypy_extensions import trait, mypyc_attr
+
+@mypyc_attr(allow_interpreted_subclasses=True)
+@trait
+class Base:
+    def value(self) -> Any:
+        return 42
+
+@mypyc_attr(allow_interpreted_subclasses=True)
+@trait
+class Middle(Base):
+    pass
+
+@mypyc_attr(allow_interpreted_subclasses=True)
+@trait
+class Child(Middle):
+    def derived(self) -> Any:
+        return self.value()
+
+@mypyc_attr(allow_interpreted_subclasses=True)
+class Concrete(Child):
+    pass
+
+[file driver.py]
+from native import Concrete
+c = Concrete()
+assert c.value() == 42
+assert c.derived() == 42