Proposed resolution (November, 2014):

Change 13.2 [temp.param] paragraph 10 as follows:

The set of default template-arguments available for use with a template declaration or definition is obtained by merging the default arguments from the definition (if in scope) and all prior declarations in scope of the template in the same way default function arguments are (9.3.4.7 [dcl.fct.default]). [Example:...