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.

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;
 }

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);

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);

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>{}}),

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},