[Bug c++/52809] New: Template non-dependent static_assert diagnostics may confuse

["potswa at mac dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52809

             Bug #: 52809
           Summary: Template non-dependent static_assert diagnostics may
                    confuse
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: potswa@mac.com


A static_assert declaration that does not depend on template parameters will be
detected and reported while parsing the template, before any instantiation.
This is OK because such a template would have no valid instantiation, which
immediately renders the program ill-formed.

In terms of cosmetic QOI, however, it would be better to clarify the messages
for users who don't know all the rules. Intuitively, the assertion shouldn't be
evaluated until instantiation, regardless of dependence. The user might expect
an uninstantiated template to pass through. We should add something like
"error: template MyTemplate cannot have a valid instantiation" after the
static_assert diagnostic.

Example code and result, as currently implemented:

template< bool b >
struct s {
    static_assert( b, "uhoh" );
    static_assert( false, "yikes" );
};

s< false > q;

depassert.cpp:4:1: error: static assertion failed: yikes
depassert.cpp: In instantiation of ‘struct s<false>’:
depassert.cpp:7:12:   required from here
depassert.cpp:3:1: error: static assertion failed: uhoh