Re: [PATCH] c++: excessive instantiation during CTAD [PR101174]

[Jason Merrill via Gcc-patches]

On 6/23/21 4:49 PM, Patrick Palka wrote:

On Wed, 23 Jun 2021, Jason Merrill wrote:

On 6/23/21 2:18 PM, Patrick Palka wrote:

We set DECL_CONTEXT on implicitly generated deduction guides so that
their access is consistent with that of the constructor.  But this
apparently leads to excessive instantiation in some cases, ultimately
because instantiation of a deduction guide should be independent of
instantiation of the resulting class specialization, but setting the
DECL_CONTEXT of the former to the latter breaks this independence.

To fix this, this patch makes push_access_scope handle artificial
deduction guides specifically instead of setting their DECL_CONTEXT
in build_deduction_guide.  We could alternatively make the class
befriend the guide via DECL_BEFRIENDING_CLASSES, but that'd break
class-deduction-access3.C below since friendship isn't transitive.

Bootstrapped and regtested on x86_64-pc-linxu-gnu, does this look OK for
trunk?

        PR c++/101174

gcc/cp/ChangeLog:

        * pt.c (push_access_scope): For artificial deduction guides,
        set the access scope to that of the constructor.
        (pop_access_scope): Likewise.
        (build_deduction_guide): Don't set DECL_CONTEXT.

libstdc++-v3/ChangeLog:

        * testsuite/23_containers/multiset/cons/deduction.cc:
        Uncomment CTAD example that was rejected by this bug.
        * testsuite/23_containers/set/cons/deduction.cc: Likewise.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1z/class-deduction-access3.C: New test.
        * g++.dg/cpp1z/class-deduction91.C: New test.
---
   gcc/cp/pt.c                                   | 11 ++++++----
   .../g++.dg/cpp1z/class-deduction-access3.C    | 22 +++++++++++++++++++
   .../g++.dg/cpp1z/class-deduction91.C          | 16 ++++++++++++++
   .../23_containers/multiset/cons/deduction.cc  |  2 --
   .../23_containers/set/cons/deduction.cc       |  2 --
   5 files changed, 45 insertions(+), 8 deletions(-)
   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction-access3.C
   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction91.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 732fb405adf..5c55507203a 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -236,6 +236,10 @@ push_access_scope (tree t)
       push_nested_class (DECL_FRIEND_CONTEXT (t));
     else if (DECL_CLASS_SCOPE_P (t))
       push_nested_class (DECL_CONTEXT (t));
+  else if (deduction_guide_p (t) && DECL_ARTIFICIAL (t))
+    /* An artificial deduction guide should have the same access as
+       the constructor.  */
+    push_nested_class (TREE_TYPE (TREE_TYPE (t)));

Let's be more specific and actually push into the access scope of the
constructor (DECL_ABSTRACT_ORIGIN) if set.

Hmm, IIUC this doesn't seem to work because DECL_ABSTRACT_ORIGIN is
not set on specializations of a deduction guide, only on the
TEMPLATE_DECL.

Ah, true.  Your patch is OK, then.

     else
       push_to_top_level ();
   @@ -255,7 +259,9 @@ pop_access_scope (tree t)
     if (TREE_CODE (t) == FUNCTION_DECL)
       current_function_decl = saved_access_scope->pop();
   -  if (DECL_FRIEND_CONTEXT (t) || DECL_CLASS_SCOPE_P (t))
+  if (DECL_FRIEND_CONTEXT (t)
+      || DECL_CLASS_SCOPE_P (t)
+      || (deduction_guide_p (t) && DECL_ARTIFICIAL (t)))
       pop_nested_class ();
     else
       pop_from_top_level ();
@@ -28804,9 +28810,6 @@ build_deduction_guide (tree type, tree ctor, tree
outer_args, tsubst_flags_t com
       DECL_ABSTRACT_ORIGIN (ded_tmpl) = fn_tmpl;
     if (ci)
       set_constraints (ded_tmpl, ci);
-  /* The artificial deduction guide should have same access as the
-     constructor.  */
-  DECL_CONTEXT (ded_fn) = type;
       return ded_tmpl;
   }
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction-access3.C
b/gcc/testsuite/g++.dg/cpp1z/class-deduction-access3.C
new file mode 100644
index 00000000000..44b4b5c455f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction-access3.C
@@ -0,0 +1,22 @@
+// { dg-do compile { target c++17 } }
+
+template<typename T>
+struct Cont;
+
+template<typename T>
+struct Base
+{
+private:
+  using type = T;
+  friend Cont<T>;
+};
+
+template<typename T>
+struct Cont
+{
+  using argument_type = typename Base<T>::type;
+
+  Cont(T, argument_type) { }
+};
+
+Cont c(1, 1);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction91.C
b/gcc/testsuite/g++.dg/cpp1z/class-deduction91.C
new file mode 100644
index 00000000000..f474c8e35ec
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction91.C
@@ -0,0 +1,16 @@
+// PR c++/101174
+// { dg-do compile { target c++17 } }
+
+struct S { using type = int; };
+
+template<class T = int, class U = S>
+struct multiset {
+  using type = typename U::type;
+  multiset(T);
+  multiset(U);
+};
+
+template<class T>
+multiset(T) -> multiset<T>;
+
+multiset c(42);
diff --git a/libstdc++-v3/testsuite/23_containers/multiset/cons/deduction.cc
b/libstdc++-v3/testsuite/23_containers/multiset/cons/deduction.cc
index a4ccc6fa467..8b7a16042a4 100644
--- a/libstdc++-v3/testsuite/23_containers/multiset/cons/deduction.cc
+++ b/libstdc++-v3/testsuite/23_containers/multiset/cons/deduction.cc
@@ -19,11 +19,9 @@ static_assert(std::is_same_v<
              decltype(std::multiset{{1, 2, 3}, std::less<int>{}, {}}),
              std::multiset<int>>);
   -/* FIXME: GCC 12 rejects this due to PR c++/101174
   static_assert(std::is_same_v<
              decltype(std::multiset{{1, 2, 3}, std::less<int>{}}),
              std::multiset<int>>);
-*/
     static_assert(std::is_same_v<
              decltype(std::multiset{{1, 2, 3}, SimpleAllocator<int>{}}),
diff --git a/libstdc++-v3/testsuite/23_containers/set/cons/deduction.cc
b/libstdc++-v3/testsuite/23_containers/set/cons/deduction.cc
index 0ae4c2a5c5f..14f36b7c05d 100644
--- a/libstdc++-v3/testsuite/23_containers/set/cons/deduction.cc
+++ b/libstdc++-v3/testsuite/23_containers/set/cons/deduction.cc
@@ -20,12 +20,10 @@ static_assert(std::is_same_v<
                    std::less<int>{}, {}}),
              std::set<int>>);
   -/* FIXME: GCC 12 rejects this due to PR c++/101174
   static_assert(std::is_same_v<
              decltype(std::set{{1, 2, 3},
                    std::less<int>{}}),
              std::set<int>>);
-*/
     static_assert(std::is_same_v<
              decltype(std::set{{1, 2, 3},