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.

gcc/cp/decl2.cc

@@ -2718,7 +2718,12 @@ min_vis_expr_r (tree *tp, int */*walk_subtrees*/, void *data)
       /* Fall through.  */
     case VAR_DECL:
     case FUNCTION_DECL:
-      if (! TREE_PUBLIC (t))
+      if (decl_constant_var_p (t))
+	/* The ODR allows definitions in different TUs to refer to distinct
+	   constant variables with internal or no linkage, so such a reference
+	   shouldn't affect visibility (PR110323).  FIXME but only if the
+	   lvalue-rvalue conversion is applied.  */;
+      else if (! TREE_PUBLIC (t))
 	tpvis = VISIBILITY_ANON;
       else
 	tpvis = DECL_VISIBILITY (t);

gcc/testsuite/g++.dg/template/explicit-instantiation6.C

@@ -0,0 +1,43 @@
+// PR c++/110323
+// { dg-do compile { target c++14 } }
+
+template<bool B, class T, class F>
+struct conditional { using type = T; };
+
+template<class T, class F>
+struct conditional<false, T, F> { using type = F; };
+
+constexpr int VAL = 1;
+
+static constexpr int getval () { return 1; }
+
+template<typename>
+constexpr int TVAL = 1;
+
+static struct S {
+  constexpr operator bool() { return true; }
+} s;
+
+struct foo {
+    template <int B>
+    void bar(typename conditional<B == VAL, int, float>::type arg) { }
+
+    template <int B>
+    void qux(typename conditional<B == TVAL<int>, int, float>::type arg) { }
+
+    template <int B>
+    void sox(typename conditional<B == noexcept (VAL), int, float>::type arg) { }
+
+    template <int B>
+    void nim(typename conditional<B != sizeof (VAL), int, float>::type arg) { }
+};
+
+template void foo::bar<1>(int arg);
+template void foo::qux<1>(int arg);
+template void foo::sox<1>(int arg);
+template void foo::nim<1>(int arg);
+
+// { dg-final { scan-assembler "_ZN3foo3barILi1EEEvN11conditionalIXeqT_L_ZL3VALEEifE4typeE" } }
+// { dg-final { scan-assembler "_ZN3foo3quxILi1EEEvN11conditionalIXeqT_L_Z4TVALIiEEEifE4typeE" } }
+// { dg-final { scan-assembler "_ZN3foo3soxILi1EEEvN11conditionalIXeqT_nxL_ZL3VALEEifE4typeE" } }
+// { dg-final { scan-assembler "_ZN3foo3nimILi1EEEvN11conditionalIXneT_szL_ZL3VALEEifE4typeE" } }

gcc/testsuite/g++.dg/template/explicit-instantiation7.C

@@ -0,0 +1,22 @@
+// PR c++/110323
+// { dg-do compile { target c++11 } }
+
+using P = struct { }*;
+using N = struct A { }*;
+
+template<bool B, class T, class F>
+struct conditional { using type = T; };
+
+struct foo {
+    template <int B>
+    void bar(typename conditional<((P) 0, B), int, float>::type arg) { }
+
+    template <int B>
+    void baz(typename conditional<((N) 0, B), int, float>::type arg) { }
+};
+
+template void foo::bar<1>(int arg);
+template void foo::baz<1>(int arg);
+
+// { dg-final { scan-assembler-not "_ZN3foo3barILi1EEEvN11conditionalIXcmcvP1XLi0EneT_Li0EEifE4typeE" } }
+// { dg-final { scan-assembler "_ZN3foo3bazILi1EEEvN11conditionalIXcmcvP1ALi0EneT_Li0EEifE4typeE" } }