Re: [PATCH] c++: substituting CONST_DECL_USING_P enumerator [PR103081]

[Jason Merrill via Gcc-patches]

On 12/2/22 14:01, Patrick Palka wrote:

We implement using enum at class scope 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 of 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 which have a
non-dependent scope.  This subsumes the existing (and seemingly dead)
DECL_NAMESPACE_SCOPE_P early exit test and also benefits substitution
into ordinary non-dependent CONST_DECLs.

Bootstrapped and regtested on x86_64-pc-linu-xgnu, does this look OK for
trunk/12?

OK.

        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 args 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                                |  7 +------
  gcc/testsuite/g++.dg/cpp2a/using-enum-10.C  | 16 ++++++++++++++++
  gcc/testsuite/g++.dg/cpp2a/using-enum-10a.C | 18 ++++++++++++++++++
  3 files changed, 35 insertions(+), 6 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/using-enum-10.C
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/using-enum-10a.C

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..43688e69b19
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/using-enum-10a.C
@@ -0,0 +1,18 @@
+// 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<N>(Hog<N>::OINK);
+}
+
+template void pen<0>();