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Re: [c++-concepts] Class template constraints


On 09/03/2013 11:01 AM, Andrew Sutton wrote:
Attached is a patch for constrained class templates. It's the 3rd time
I've sent it.

Please feel free to ping me if you're waiting for a patch review; once a week is not too much.

1. Type constraints are checked on lookup rather than instantiation.

How is this different from function template constraints? Is this just a difference in internal function name (instantiate_template vs lookup_template_class)?

+// Returns the type of a template specialization only if that
+// specializaiton needs to defined. Otherwise (e.g., if the type has

specialization

+  // Do the constraints match the most general template? Note that
+  // the absence of constraints will also match.
+  if (equivalent_constraints (cur_constr, DECL_CONSTRAINTS (tmpl)))

If absence matches, I think the name "equivalent" is misleading. But the implementation seems to require subsumes in both directions. What's up here?

+  // Find the template parameter list at the a depth appropriate to
+  // the scope we're trying to enter.
+  tree parms = current_template_parms;
+  int depth = template_class_depth (type);
+  for (int n = processing_template_decl; n > depth && parms; --n)
+    parms = TREE_CHAIN (parms);

If you're going to use this function from lookup_template_class_1, it can't use current_template_*, since those are parser state which might be something completely unrelated when we get here during instantiation.

+        return resolve_template_scope(gen_tmpl, template_type);

Space before (.

+          // Diaagnose constraints here since they are not diagnosed

Extra 'a'.

+  // If not, it is equivalent to have failed to compute the binding.

"to having failed"

Jason


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