c++: substituting CONST_DECL_USING_P enumerators [PR103081]

We implement class-scope using enum by injecting clones of the enum's
CONST_DECLs as fields of the class, for which CONST_DECL_USING_P is
true, so that qualified lookup naturally finds the enumerators.
Substitution into such a CONST_DECL currently ICEs however, because we
assume the DECL_CONTEXT is always the ENUMERAL_TYPE (which has
TYPE_VALUES) but in this case it's the RECORD_TYPE for the class scope
(which has TYPE_FIELDS).

Since these CONST_DECLs appear to always be non-dependent, this patch
fixes this by shortcutting substitution for CONST_DECLs that have
non-dependent DECL_CONTEXT.  This subsumes the existing (and seemingly
dead) DECL_NAMESPACE_SCOPE_P early exit test and also benefits
substitution into ordinary non-dependent CONST_DECLs.

	PR c++/103081

gcc/cp/ChangeLog:

	* pt.cc (tsubst_copy) <case CONST_DECL>: Generalize
	early exit test for namespace-scope decls to check dependence of
	the enclosing scope instead.  Remove dead early exit test.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/using-enum-10.C: New test.
	* g++.dg/cpp2a/using-enum-10a.C: New test.

gcc/cp/pt.cc

@@ -17066,13 +17066,8 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
 	if (DECL_TEMPLATE_PARM_P (t))
 	  return tsubst_copy (DECL_INITIAL (t), args, complain, in_decl);
-	/* There is no need to substitute into namespace-scope
-	   enumerators.  */
-	if (DECL_NAMESPACE_SCOPE_P (t))
+	if (!uses_template_parms (DECL_CONTEXT (t)))
 	  return t;
-	/* If ARGS is NULL, then T is known to be non-dependent.  */
-	if (args == NULL_TREE)
-	  return scalar_constant_value (t);
 
 	/* Unfortunately, we cannot just call lookup_name here.
 	   Consider:

gcc/testsuite/g++.dg/cpp2a/using-enum-10.C

@@ -0,0 +1,16 @@
+// PR c++/103081
+// { dg-do compile { target c++20 } }
+
+enum class Pig { OINK };
+
+struct Hog {
+  using enum Pig;
+  Hog(Pig) { }
+};
+
+template<int>
+void pen() {
+  Hog(Hog::OINK);
+}
+
+template void pen<0>();

gcc/testsuite/g++.dg/cpp2a/using-enum-10a.C

@@ -0,0 +1,19 @@
+// A version of using-enum-10.C where Hog is a template.
+// PR c++/103081
+// { dg-do compile { target c++20 } }
+
+enum class Pig { OINK };
+
+template<int>
+struct Hog {
+  using enum Pig;
+  Hog(Pig) { OINK; }
+};
+
+template<int N>
+void pen() {
+  Hog<1>(Hog<1>::OINK);
+  Hog<N>(Hog<N>::OINK);
+}
+
+template void pen<0>();