Fix crash in Self type on forward reference in upper bound (#14206)

Fixes #14199 

This is straightforward, just use the same logic as for regular (user
defined) type variables.

Author: ilevkivskyi

mypy/semanal.py

@@ -1047,7 +1047,11 @@ def setup_self_type(self) -> None:
         assert self.type is not None
         info = self.type
         if info.self_type is not None:
-            return
+            if has_placeholder(info.self_type.upper_bound):
+                # Similar to regular (user defined) type variables.
+                self.defer(force_progress=True)
+            else:
+                return
         info.self_type = TypeVarType("Self", f"{info.fullname}.Self", 0, [], fill_typevars(info))
 
     def visit_overloaded_func_def(self, defn: OverloadedFuncDef) -> None:

test-data/unit/check-classes.test

@@ -7679,3 +7679,18 @@ def g(b: Type[Any]) -> None:
 
 def h(b: type) -> None:
     class D(b): ...
+
+[case testNoCrashOnSelfWithForwardRefGenericClass]
+from typing import Generic, Sequence, TypeVar, Self
+
+_T = TypeVar('_T', bound="Foo")
+
+class Foo:
+    foo: int
+
+class Element(Generic[_T]):
+    elements: Sequence[Self]
+
+class Bar(Foo): ...
+e: Element[Bar]
+reveal_type(e.elements)  # N: Revealed type is "typing.Sequence[__main__.Element[__main__.Bar]]"

test-data/unit/check-dataclasses.test

@@ -1981,3 +1981,23 @@ def const_two(x: T) -> str:
 c = Cont(Box(const_two))
 reveal_type(c)  # N: Revealed type is "__main__.Cont[builtins.str]"
 [builtins fixtures/dataclasses.pyi]
+
+[case testNoCrashOnSelfWithForwardRefGenericDataclass]
+from typing import Generic, Sequence, TypeVar, Self
+from dataclasses import dataclass
+
+_T = TypeVar('_T', bound="Foo")
+
+@dataclass
+class Foo:
+    foo: int
+
+@dataclass
+class Element(Generic[_T]):
+    elements: Sequence[Self]
+
+@dataclass
+class Bar(Foo): ...
+e: Element[Bar]
+reveal_type(e.elements)  # N: Revealed type is "typing.Sequence[__main__.Element[__main__.Bar]]"
+[builtins fixtures/dataclasses.pyi]