c++: explicit inst of template method not generated [PR110323]

c++: explicit inst of template method not generated [PR110323]

Consider

  constexpr int VAL = 1;
  struct foo {
      template <int B>
      void bar(typename std::conditional<B==VAL, int, float>::type arg) { }
  };
  template void foo::bar<1>(int arg);

where we since r11-291 fail to emit the code for the explicit
instantiation.  That's because cp_walk_subtrees/TYPENAME_TYPE now
walks TYPE_CONTEXT ('conditional' here) as well, and in a template
finds the B==VAL template argument.  VAL is constexpr, which implies const,
which in the global scope implies static.  constrain_visibility_for_template
then makes "struct conditional<(B == VAL), int, float>" non-TREE_PUBLIC.
Then symtab_node::needed_p checks TREE_PUBLIC, sees it's 0, and we don't
emit any code.

I thought the fix would be some ODR-esque check to not consider
constexpr variables/fns that are used just for their value.  But
it turned out to be tricky.  For instance, we can't skip
determine_visibility in a template; we can't even skip it for value-dep
expressions.  For example, no-linkage-expr1.C has

  using P = struct {}*;
  template <int N>
  void f(int(*)[((P)0, N)]) {}

where ((P)0, N) is value-dep, but N is not relevant here: we have to
ferret out the anonymous type.  When instantiating, it's already gone.

This patch uses decl_constant_var_p.  This is to implement (an
approximation) [basic.def.odr]#14.5.1 and [basic.def.odr]#5.2.

	PR c++/110323

gcc/cp/ChangeLog:

	* decl2.cc (min_vis_expr_r) <case VAR_DECL>: Do nothing for
	decl_constant_var_p VAR_DECLs.

gcc/testsuite/ChangeLog:

	* g++.dg/template/explicit-instantiation6.C: New test.
	* g++.dg/template/explicit-instantiation7.C: New test.