Ban templating C++ classes with reference counted types (GH-4337)

Specifically this disallows memoryviews, but it extends to any future type that Cython has to generate manual reference counting code for.

Closes https://github.com/cython/cython/issues/3085

Cython/Compiler/PyrexTypes.py

@@ -3897,10 +3897,12 @@ class CppClassType(CType):
             return error_type
         has_object_template_param = False
         for value in template_values:
-            if value.is_pyobject:
+            if value.is_pyobject or value.needs_refcounting:
                 has_object_template_param = True
+                type_description = "Python object" if value.is_pyobject else "Reference-counted"
                 error(pos,
-                      "Python object type '%s' cannot be used as a template argument" % value)
+                      "%s type '%s' cannot be used as a template argument" % (
+                          type_description, value))
         if has_object_template_param:
             return error_type
         return self.specialize(dict(zip(self.templates, template_values)))

Cython/Compiler/Symtab.py

@@ -2021,7 +2021,7 @@ class StructOrUnionScope(Scope):
     def declare_var(self, name, type, pos,
                     cname = None, visibility = 'private',
                     api = 0, in_pxd = 0, is_cdef = 0,
-                    allow_pyobject=False, allow_memoryview=False):
+                    allow_pyobject=False, allow_memoryview=False, allow_refcounted=False):
         # Add an entry for an attribute.
         if not cname:
             cname = name
@@ -2032,11 +2032,16 @@ class StructOrUnionScope(Scope):
         entry = self.declare(name, cname, type, pos, visibility)
         entry.is_variable = 1
         self.var_entries.append(entry)
-        if type.is_pyobject and not allow_pyobject:
-            error(pos, "C struct/union member cannot be a Python object")
-        elif type.is_memoryviewslice and not allow_memoryview:
-            # Memory views wrap their buffer owner as a Python object.
-            error(pos, "C struct/union member cannot be a memory view")
+        if type.is_pyobject:
+            if not allow_pyobject:
+                error(pos, "C struct/union member cannot be a Python object")
+        elif type.is_memoryviewslice:
+            if not allow_memoryview:
+                # Memory views wrap their buffer owner as a Python object.
+                error(pos, "C struct/union member cannot be a memory view")
+        elif type.needs_refcounting:
+            if not allow_refcounted:
+                error(pos, "C struct/union member cannot be reference-counted type '%s'" % type)
         if visibility != 'private':
             error(pos, "C struct/union member cannot be declared %s" % visibility)
         return entry

tests/errors/cpp_object_template.pyx

@@ -12,7 +12,13 @@ def main():
     cdef vector[A] va
     va.push_back(A())
 
+def memview():
+    import array
+    cdef vector[int[:]] vmv
+    vmv.push_back(array.array("i", [1,2,3]))
+
 _ERRORS = u"""
 10:16: Python object type 'Python object' cannot be used as a template argument
 12:16: Python object type 'A' cannot be used as a template argument
+17:16: Reference-counted type 'int[:]' cannot be used as a template argument
 """