Ensure signature extensions reflected in the pxd if needed.

This fixes #1732.

Cython/Compiler/Symtab.py

@@ -2088,6 +2088,11 @@ 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:
+                        error(pos,
+                            "Compatible but non-identical C method '%s' not redeclared "
+                            "in definition part of extension type" % name)
+                        error(entry.pos, "Previous declaration is here")
                     entry = self.add_cfunction(name, type, pos, cname, visibility='ignore', modifiers=modifiers)
                 else:
                     error(pos, "Signature not compatible with previous declaration")

tests/errors/cmethbasematch.pyx

@@ -8,7 +8,32 @@ cdef class D(C):
     cdef void f(self, int x):
         pass
 
+# These are declared in the pxd.
+cdef class Base(object):
+  cdef f(self):
+    pass
+
+cdef class MissingRedeclaration(Base):
+  # Not declared (so assumed cdef) in the pxd.
+  cpdef f(self):
+    pass
+
+cdef class BadRedeclaration(Base):
+  # Declared as cdef in the pxd.
+  cpdef f(self):
+    pass
+
+cdef class UnneededRedeclaration(Base):
+  # This is OK, as it's not declared in the pxd.
+  cpdef f(self):
+    pass
+
+
 _ERRORS = u"""
 8: 9: Signature not compatible with previous declaration
 4: 9: Previous declaration is here
+18:8: Compatible but non-identical C method 'f' not redeclared in definition part of extension type
+2:9: Previous declaration is here
+23:8: Compatible but non-identical C method 'f' not redeclared in definition part of extension type
+2:9: Previous declaration is here
 """