c++: access checking in aggregate initialization [PR96673]

We were deferring access checks while parsing B<int>{}, didn't adjust that
when we went to instantiate the default member initializer for B::c,
deferred access checking for C::C, and then checked it after parsing
B<int>{}, back in the main() context which has no access.  We need to do the
access checks in the class context of the DMI.

I tried fixing this in push_to/pop_from_top_level, but that caused several
regressions.

gcc/cp/ChangeLog:

	PR c++/96673
	* init.c (get_nsdmi): Don't defer access checking.

gcc/testsuite/ChangeLog:

	PR c++/96673
	* g++.dg/cpp1y/nsdmi-aggr13.C: New test.

gcc/cp/init.c

@@ -591,6 +591,7 @@ get_nsdmi (tree member, bool in_ctor, tsubst_flags_t complain)
 	    {
 	      push_to_top_level ();
 	      push_nested_class (ctx);
+	      push_deferring_access_checks (dk_no_deferred);
 	      pushed = true;
 	    }
 
@@ -616,6 +617,7 @@ get_nsdmi (tree member, bool in_ctor, tsubst_flags_t complain)
 
 	  if (pushed)
 	    {
+	      pop_deferring_access_checks ();
 	      pop_nested_class ();
 	      pop_from_top_level ();
 	    }

gcc/testsuite/g++.dg/cpp1y/nsdmi-aggr13.C

@@ -0,0 +1,33 @@
+// PR c++/96673
+// { dg-do compile { target c++11 } }
+
+template <class T>
+class A {};
+
+template <class T>
+class B;
+
+template <class T>
+class C {
+    private:
+
+    friend class B<T>;
+
+    explicit C(A<T>&) {};
+};
+
+
+template <class T>
+class B {
+    public:
+    B() = default;
+    //B() {};       // << This implementation of the constructor makes it work
+
+    A<T> a = {};
+    C<T> c = C<T>{a};
+};
+
+int main() {
+    auto b = B<int>{};
+    auto &c = b.c;
+}