Check for exceptions also when @returns() is used, not only for "->" return...

Check for exceptions also when @returns() is used, not only for "->" return type annotations. (GH-3664)

When you use Python type annotations, it would be weird if you lost Python exception propagation semantics along the way, just by compiling the code. So the default behaviour is "except? -1" here for C integer types.

Arguably, this would also be a better default for the decorator case.

See https://github.com/cython/cython/issues/3625#issuecomment-631931675

Cython/Compiler/ParseTreeTransforms.py

@@ -2445,8 +2445,8 @@ class AdjustDefByDirectives(CythonTransform, SkipDeclarations):
             if return_type_node is not None and except_val is None:
                 except_val = (None, True)  # except *
         elif except_val is None:
-            # backward compatible default: no exception check
-            except_val = (None, False)
+            # backward compatible default: no exception check, unless there's also a "@returns" declaration
+            except_val = (None, True if return_type_node else False)
         if 'ccall' in self.directives:
             node = node.as_cfunction(
                 overridable=True, modifiers=modifiers, nogil=nogil,