stubtest: emit error if a stub defines a public type alias that doesn't exist at runtime

mypy/stubtest.py

@@ -956,10 +956,13 @@ def verify_decorator(
 def verify_typealias(
     stub: nodes.TypeAlias, runtime: MaybeMissing[Any], object_path: List[str]
 ) -> Iterator[Error]:
+    stub_target = mypy.types.get_proper_type(stub.target)
     if isinstance(runtime, Missing):
-        # ignore type aliases that don't have a runtime counterpart
+        yield Error(
+            object_path, "is not present at runtime", stub, runtime,
+            stub_desc=f"Type alias for {stub_target}"
+        )
         return
-    stub_target = mypy.types.get_proper_type(stub.target)
     if isinstance(stub_target, mypy.types.Instance):
         yield from verify(stub_target.type, runtime, object_path)
         return

mypy/test/teststubtest.py

@@ -712,6 +712,18 @@ class Y: ...
             runtime="A = (int, str)",
             error="A",
         )
+        # Error if an alias isn't present at runtime...
+        yield Case(
+            stub="B = str",
+            runtime="",
+            error="B"
+        )
+        # ... but only if the alias isn't private
+        yield Case(
+            stub="_C = int",
+            runtime="",
+            error=None
+        )
 
     @collect_cases
     def test_enum(self) -> Iterator[Case]: