C++ PATCH for c++/79503 (inherited ctor taking base class)

C++ PATCH for c++/79503 (inherited ctor taking base class)

[Jason Merrill]

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In the old inheriting constructor scheme, copy constructors were not
inherited.  In the new scheme we needed a rule to get a similar
effect; I proposed that an inherited constructor should not be viable
if it has a single parameter of a type reference-related to the
derived type.  Richard Smith proposed a further refinement that this
would apply only if the base type is also reference-related to the
parameter type.  This testcase demonstrates why this refinement is
needed.

Tested x86_64-pc-linux-gnu, applying to trunk.

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commit 5e1645da2601d0c31a2fd0a5687224d6e3087824
Author: Jason Merrill <jason@redhat.com>
Date:   Sun Feb 19 16:22:43 2017 -0500

            PR c++/79503 - inherited ctor taking base class
    
            * call.c (add_function_candidate): Also check that
            DECL_INHERITED_CTOR_BASE is reference-related to the parameter type.

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 4ef444b..d6d3a8f 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -2057,7 +2057,9 @@ add_function_candidate (struct z_candidate **candidates,
     {
       tree ptype = non_reference (TREE_VALUE (parmlist));
       tree dtype = DECL_CONTEXT (fn);
-      if (reference_related_p (ptype, dtype))
+      tree btype = DECL_INHERITED_CTOR_BASE (fn);
+      if (reference_related_p (ptype, dtype)
+	  && reference_related_p (btype, ptype))
 	{
 	  viable = false;
 	  reason = inherited_ctor_rejection ();
diff --git a/gcc/testsuite/g++.dg/cpp0x/inh-ctor26.C b/gcc/testsuite/g++.dg/cpp0x/inh-ctor26.C
new file mode 100644
index 0000000..e1e6b9e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/inh-ctor26.C
@@ -0,0 +1,21 @@
+// PR c++/79503
+// { dg-do compile { target c++11 } }
+
+struct payload {};
+
+struct base: private payload {
+    base(payload) {}
+};
+
+struct derived: base {
+    using base::base;
+};
+
+int main()
+{
+    payload data;
+    // error: no matching function for call to 'derived::derived(payload&)'
+    // note: candidate: base::base(payload)
+    // note:   an inherited constructor is not a candidate for initialization from an expression of the same or derived type
+    derived demo(data);
+}