[red-knot] Add failing tests for iterating over maybe-iterable unions (#14016)

crates/red_knot_python_semantic/resources/mdtest/loops/for_loop.md

@@ -223,3 +223,61 @@ flag = bool_instance()
 for x in Test() if flag else Test2():
     reveal_type(x)  # revealed: int | Exception | str | tuple[int, int] | bytes | memoryview
 ```
+
+## Union type as iterable where one union element has no `__iter__` method
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter:
+        return TestIter()
+
+def coinflip() -> bool:
+    return True
+
+# TODO: we should emit a diagnostic here (it might not be iterable)
+for x in Test() if coinflip() else 42:
+    reveal_type(x)  # revealed: int | Unknown
+```
+
+## Union type as iterable where one union element has invalid `__iter__` method
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter:
+        return TestIter()
+
+class Test2:
+    def __iter__(self) -> int:
+        return 42
+
+def coinflip() -> bool:
+    return True
+
+# TODO: we should emit a diagnostic here (it might not be iterable)
+for x in Test() if coinflip() else Test2():
+    reveal_type(x)  # revealed: int | Unknown
+```
+
+## Union type as iterator where one union element has no `__next__` method
+
+```py
+class TestIter:
+    def __next__(self) -> int:
+        return 42
+
+class Test:
+    def __iter__(self) -> TestIter | int:
+        return TestIter()
+
+# TODO: we should emit a diagnostic here (it might not be iterable)
+for x in Test():
+    reveal_type(x)  # revealed: int | Unknown
+```

crates/red_knot_python_semantic/src/types.rs

@@ -1090,7 +1090,7 @@ impl<'db> Type<'db> {
             };
         }
 
-        if let Type::Unknown | Type::Any = self {
+        if matches!(self, Type::Unknown | Type::Any | Type::Todo) {
             // Explicit handling of `Unknown` and `Any` necessary until `type[Unknown]` and
             // `type[Any]` are not defined as `Todo` anymore.
             return IterationOutcome::Iterable { element_ty: self };
@@ -1185,9 +1185,9 @@ impl<'db> Type<'db> {
             // TODO can we do better here? `type[LiteralString]`?
             Type::StringLiteral(_) | Type::LiteralString => KnownClass::Str.to_class(db),
             // TODO: `type[Any]`?
-            Type::Any => Type::Todo,
+            Type::Any => Type::Any,
             // TODO: `type[Unknown]`?
-            Type::Unknown => Type::Todo,
+            Type::Unknown => Type::Unknown,
             // TODO intersections
             Type::Intersection(_) => Type::Todo,
             Type::Todo => Type::Todo,