[stubgen] Include simple decorators in stub files (#18489)

Stubgen historically only included a selected number of decorators in
the generated stubs. I couldn't find the actual reason for it, however
it's likely fair to assume that decorator typing only started being
possible with PEP 612 thus most had been untyped previously.

As it's fairly simple to annotate decorators with `ParamSpec` now, it's
probably fair to include them in the stub file now.

mypy/stubgen.py

@@ -755,6 +755,9 @@ def process_decorator(self, o: Decorator) -> None:
             elif fullname in DATACLASS_TRANSFORM_NAMES:
                 p = AliasPrinter(self)
                 self._decorators.append(f"@{decorator.accept(p)}")
+            elif isinstance(decorator, (NameExpr, MemberExpr)):
+                p = AliasPrinter(self)
+                self._decorators.append(f"@{decorator.accept(p)}")
 
     def get_fullname(self, expr: Expression) -> str:
         """Return the expression's full name."""

test-data/unit/stubgen.test

@@ -338,10 +338,24 @@ class A: ...
 class B(A): ...
 
 [case testDecoratedFunction]
+import x
+
 @decorator
 def foo(x): ...
+
+@x.decorator
+def bar(x): ...
+
+@decorator(x=1, y={"a": 1})
+def foo_bar(x): ...
 [out]
+import x
+
+@decorator
 def foo(x) -> None: ...
+@x.decorator
+def bar(x) -> None: ...
+def foo_bar(x) -> None: ...
 
 [case testMultipleAssignment]
 x, y = 1, 2