[gcc r13-4477] c++: substituting CONST_DECL_USING_P enumerators [PR103081]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:b3912122c9dfaba6c8229e8f095885f69782ceda

commit r13-4477-gb3912122c9dfaba6c8229e8f095885f69782ceda
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Dec 3 10:28:25 2022 -0500

    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.

Diff:
---
 gcc/cp/pt.cc                                |  7 +------
 gcc/testsuite/g++.dg/cpp2a/using-enum-10.C  | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/using-enum-10a.C | 19 +++++++++++++++++++
 3 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 31691618d1b..bc8ea06ceae 100644
--- a/gcc/cp/pt.cc
+++ b/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:
diff --git a/gcc/testsuite/g++.dg/cpp2a/using-enum-10.C b/gcc/testsuite/g++.dg/cpp2a/using-enum-10.C
new file mode 100644
index 00000000000..98fe0644729
--- /dev/null
+++ b/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>();
diff --git a/gcc/testsuite/g++.dg/cpp2a/using-enum-10a.C b/gcc/testsuite/g++.dg/cpp2a/using-enum-10a.C
new file mode 100644
index 00000000000..daa3221b77a
--- /dev/null
+++ b/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>();