6d27ee7fcc
For a (complex) alias template-id, dependent_alias_template_spec_p returns true if any template argument of the template-id is dependent. This predicate indicates that substitution into the template-id may behave differently with respect to SFINAE than substitution into the expanded alias, and so the alias is in a way non-transparent. For example, 'first_t<T, T&>' in template<class T, class...> using first_t = T; template<class T> first_t<T, T&> f(); is such an alias template-id since first_t doesn't use its second template parameter and so the substitution into the expanded alias would discard the SFINAE effects of the corresponding (dependent) argument 'T&'. But this predicate is overly conservative since what really matters for sake of SFINAE equivalence is whether a template argument corresponding to an _unused_ template parameter is dependent. So the predicate should return false for e.g. 'first_t<T&, int>'. This patch refines the predicate appropriately. We need to be able to efficiently determine which template parameters of a complex alias template are unused, so to that end we add a new out parameter to complex_alias_template_p and cache its result in an on-the-side hash_map that replaces the existing TEMPLATE_DECL_COMPLEX_ALIAS_P flag. PR c++/90679 gcc/cp/ChangeLog: * cp-tree.h (TEMPLATE_DECL_COMPLEX_ALIAS_P): Remove. (most_general_template): Constify parameter. * pt.cc (push_template_decl): Adjust after removing TEMPLATE_DECL_COMPLEX_ALIAS_P. (complex_alias_tmpl_info): New hash_map. (uses_all_template_parms_data::seen): Change type to tree* from bool*. (complex_alias_template_r): Adjust accordingly. (complex_alias_template_p): Add 'seen_out' out parameter. Call most_general_template and check PRIMARY_TEMPLATE_P. Use complex_alias_tmpl_info to cache the result and set '*seen_out' accordigly. (dependent_alias_template_spec_p): Add !processing_template_decl early exit test. Consider dependence of only template arguments corresponding to seen template parameters as per gcc/testsuite/ChangeLog: * g++.dg/cpp0x/alias-decl-76.C: New test. |
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