fix co_firstlineno for functions that have decorators

Cython/Compiler/Parsing.py

@@ -3367,7 +3367,7 @@ def _reject_cdef_modifier_in_py(s, name):
 
 def p_def_statement(s, decorators=None, is_async_def=False):
     # s.sy == 'def'
-    pos = s.position()
+    pos = decorators[0].pos if decorators else s.position()
     # PEP 492 switches the async/await keywords on in "async def" functions
     if is_async_def:
         s.enter_async()

tests/run/cyfunction.pyx

@@ -391,3 +391,22 @@ cdef class TestOptimisedBuiltinMethod:
 
     def call(self, arg, obj=None):
         (obj or self).append(arg+1)  # optimistically optimised => uses fast fallback method call
+
+
+def do_nothing(f):
+    """Dummy decorator for `test_firstlineno_decorated_function`"""
+    return f
+
+
+@do_nothing
+@do_nothing
+def test_firstlineno_decorated_function():
+    """
+    check that `test_firstlineno_decorated_function` starts 5 lines below `do_nothing`
+
+    >>> test_firstlineno_decorated_function()
+    5
+    """
+    l1 = do_nothing.__code__.co_firstlineno
+    l2 = test_firstlineno_decorated_function.__code__.co_firstlineno
+    return l2 - l1