Don't error when exception_check is set to True and return type is PyObject. (GH-4433)

With Cython syntax, we ignore the exception_check when a Python object is returned.
In pure Python mode, with a Python object type as return type, we reject it.
Instead of raising an error, we now just reset exception_check to False.

Found in https://github.com/cython/cython/issues/2529

Cython/Compiler/Nodes.py

@@ -712,6 +712,12 @@ class CFuncDeclaratorNode(CDeclaratorNode):
             env.add_include_file('new')         # for std::bad_alloc
             env.add_include_file('stdexcept')
             env.add_include_file('typeinfo')    # for std::bad_cast
+        elif return_type.is_pyobject and self.exception_check:
+            # Functions in pure Python mode default to always check return values for exceptions
+            # (equivalent to the "except*" declaration). In this case, the exception clause
+            # is silently ignored for functions returning a Python object.
+            self.exception_check = False
+
         if (return_type.is_pyobject
                 and (self.exception_value or self.exception_check)
                 and self.exception_check != '+'):

tests/errors/pure_errors.py

@@ -50,8 +50,15 @@ def pyfunc(x):  # invalid
     return x + 1
 
 
+@cython.exceptval(-1)
+@cython.cfunc
+def test_cdef_return_object_broken(x: object) -> object:
+    return x
+
+
 _ERRORS = """
 44:22: Calling gil-requiring function not allowed without gil
 45:24: Calling gil-requiring function not allowed without gil
 48:0: Python functions cannot be declared 'nogil'
+53:0: Exception clause not allowed for function returning Python object
 """

tests/run/pure_py3.py

@@ -85,3 +85,19 @@ def call_cdef_inline(x):
     """
     ret = cdef_inline(x)
     return ret, cython.typeof(ret)
+
+@cython.cfunc
+def test_cdef_return_object(x: object) -> object:
+    """
+    Test support of python object in annotations
+    >>> test_cdef_return_object(3)
+    3
+    >>> test_cdef_return_object(None)
+    Traceback (most recent call last):
+        ...
+    RuntimeError
+    """
+    if x:
+        return x
+    else:
+        raise RuntimeError()