Improve error message for mixing return type declarations into def function syntax.

See #2529.

Cython/Compiler/Parsing.py

@@ -3370,7 +3370,11 @@ def p_def_statement(s, decorators=None, is_async_def=False):
         s.enter_async()
     s.next()
     name = _reject_cdef_modifier_in_py(s, p_ident(s))
-    s.expect('(')
+    s.expect(
+        '(',
+        "Expected '(', found '%s'. Did you use cdef syntax in a Python declaration? "
+        "Use decorators and Python type annotations instead." % (
+            s.systring if s.sy == 'IDENT' else s.sy))
     args, star_arg, starstar_arg = p_varargslist(s, terminator=')')
     s.expect(')')
     _reject_cdef_modifier_in_py(s, s.systring)

tests/errors/invalid_syntax_py.pyx

@@ -13,10 +13,15 @@ def nogil func() -> int:
 def func() nogil:
     pass
 
+def inline int* func():
+    pass
+
 
 _ERRORS = u"""
 4:11: Cannot use cdef modifier 'inline' in Python function signature. Use a decorator instead.
 7:8: Cannot use cdef modifier 'api' in Python function signature. Use a decorator instead.
 10:10: Cannot use cdef modifier 'nogil' in Python function signature. Use a decorator instead.
 13:11: Cannot use cdef modifier 'nogil' in Python function signature. Use a decorator instead.
+16:11: Cannot use cdef modifier 'inline' in Python function signature. Use a decorator instead.
+16:14: Expected '(', found '*'. Did you use cdef syntax in a Python declaration? Use decorators and Python type annotations instead.
 """