[gcc r12-1762] c++: excessive instantiation during CTAD [PR101174]

[Patrick Palka]

https://gcc.gnu.org/g:7da4eae3dcef6fd5d955eb2c80c453aa52368004

commit r12-1762-g7da4eae3dcef6fd5d955eb2c80c453aa52368004
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 23 17:23:39 2021 -0400

    c++: excessive instantiation during CTAD [PR101174]
    
    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 rather than setting their DECL_CONTEXT
    in build_deduction_guide.  We could alternatively make the class
    befriend the guide via DECL_BEFRIENDING_CLASSES, but that wouldn't
    be a complete fix and would break class-deduction-access3.C below
    since friendship isn't transitive.
    
            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 on the guide.
    
    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.

Diff:
---
 gcc/cp/pt.c                                          | 11 +++++++----
 gcc/testsuite/g++.dg/cpp1z/class-deduction-access3.C | 20 ++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp1z/class-deduction91.C       | 16 ++++++++++++++++
 .../23_containers/multiset/cons/deduction.cc         |  2 --
 .../testsuite/23_containers/set/cons/deduction.cc    |  2 --
 5 files changed, 43 insertions(+), 8 deletions(-)

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)));
   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..9df94808e86
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction-access3.C
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++17 } }
+
+template<class>
+struct Cont;
+
+template<class T>
+class Base
+{
+  using type = T;
+  friend Cont<T>;
+};
+
+template<class 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},