Allow undeclared narrower return for cdef methods.

Cython/Compiler/PyrexTypes.py

@@ -2553,11 +2553,11 @@ class CFuncType(CType):
     def as_argument_type(self):
         return c_ptr_type(self)
 
-    def same_c_signature_as(self, other_type, as_cmethod = 0):
+    def same_c_signature_as(self, other_type, as_cmethod = 0, as_pxd_definition = 0):
         return self.same_c_signature_as_resolved_type(
             other_type.resolve(), as_cmethod)
 
-    def same_c_signature_as_resolved_type(self, other_type, as_cmethod = 0):
+    def same_c_signature_as_resolved_type(self, other_type, as_cmethod = 0, as_pxd_definition = 0):
         #print "CFuncType.same_c_signature_as_resolved_type:", \
         #    self, other_type, "as_cmethod =", as_cmethod ###
         if other_type is error_type:
@@ -2579,6 +2579,11 @@ class CFuncType(CType):
             return 0
         if self.optional_arg_count != other_type.optional_arg_count:
             return 0
+        if as_pxd_definition:
+            # A narrowing of the return type declared in the pxd is allowed.
+            if not self.return_type.subtype_of_resolved_type(other_type.return_type):
+                return 0
+        else:
             if not self.return_type.same_as(other_type.return_type):
                 return 0
         if not self.same_calling_convention_as(other_type):

Cython/Compiler/Symtab.py

@@ -2094,7 +2094,8 @@ class CClassScope(ClassScope):
                     # Fix with_gil vs nogil.
                     entry.type = entry.type.with_with_gil(type.with_gil)
                 elif type.compatible_signature_with(entry.type, as_cmethod = 1) and type.nogil == entry.type.nogil:
-                    if self.defined and not in_pxd:
+                    if (self.defined and not in_pxd
+                        and not type.same_c_signature_as_resolved_type(entry.type, as_cmethod = 1, as_pxd_definition = 1)):
                         error(pos,
                             "Compatible but non-identical C method '%s' not redeclared "
                             "in definition part of extension type '%s'" % (name, self.class_name))

tests/errors/cmethbasematch.pxd

@@ -6,3 +6,6 @@ cdef class MissingRedeclaration(Base):
 
 cdef class BadRedeclaration(Base):
   cdef f(self)
+
+cdef class NarrowerReturn(Base):
+  pass

tests/errors/cmethbasematch.pyx

@@ -28,6 +28,11 @@ cdef class UnneededRedeclaration(Base):
   cpdef f(self):
     pass
 
+cdef class NarrowerReturn(Base):
+  # This does not require a new vtable entry.
+  cdef Base f(self):
+    pass
+
 
 _ERRORS = u"""
 8: 9: Signature not compatible with previous declaration