[C++ Patch] PR 79790 ("[C++17] ICE class template argument deduction")

[Paolo Carlini]

Hi,

in this C++17 ICE on invalid we crash when, in do_class_deduction, 
build_new_function_call is called with a null first argument. What is 
happening is pretty simple to analyze: for the broken snippet, there are 
no cands and elided is false, because the issue isn't that candidates 
are elided because explicit, instead that there are no viable candidates 
whatsoever, because args->length () == 3 and we don't even have the 
implicit deduction guides. Alternately to the straightforward fix I'm 
proposing below, a literal reading of the standard [16.3.1.8] suggests 
that we could also consider generating the implicit deduction guides 
even when args->length () >= 2 and the copy deduction guide even when 
args->length () != 1, that is unconditionally.

While working on the bug I also noticed that we can simplify a bit the 
code generating the implicit deduction guides: if I'm not mistaken, when 
we pass types as first argument of build_deduction_guide - for implicit 
guides, that is - the deduction guide is never explicit. thus 
DECL_NONCONVERTING_P is never true. It's an unrelated tweak, anyway, 
which we can consider applying by itself if we don't change the code 
generating the implicit deduction guides.

Thanks! Paolo.

///////////////////////

/cp
2017-07-14  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/79790
        * pt.c (do_class_deduction): Handle the case of no viable implicit
        deduction guides; simplify the code generating implicit deduction
        guides.

/testsuite
2017-07-14  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/79790
        * g++.dg/cpp1z/class-deduction42.C: New.
Index: cp/pt.c
===================================================================
--- cp/pt.c     (revision 250186)
+++ cp/pt.c     (working copy)
@@ -25439,11 +25439,7 @@ do_class_deduction (tree ptype, tree tmpl, tree in
       if (gtype)
        {
          tree guide = build_deduction_guide (gtype, outer_args, complain);
-         if ((flags & LOOKUP_ONLYCONVERTING)
-             && DECL_NONCONVERTING_P (STRIP_TEMPLATE (guide)))
-           elided = true;
-         else
-           cands = lookup_add (guide, cands);
+         cands = lookup_add (guide, cands);
        }
     }
 
@@ -25454,6 +25450,12 @@ do_class_deduction (tree ptype, tree tmpl, tree in
             "user-declared constructors", type);
       return error_mark_node;
     }
+  else if (!cands && call == error_mark_node)
+    {
+      error ("cannot deduce template arguments for copy-initialization"
+            " of %qT, as it has no viable implicit deduction guides", type);
+      return error_mark_node;
+    }
 
   if (call == error_mark_node)
     {
Index: testsuite/g++.dg/cpp1z/class-deduction42.C
===================================================================
--- testsuite/g++.dg/cpp1z/class-deduction42.C  (revision 0)
+++ testsuite/g++.dg/cpp1z/class-deduction42.C  (working copy)
@@ -0,0 +1,10 @@
+// PR c++/79790
+// { dg-options -std=c++1z }
+
+template <int N>
+struct array
+{
+  int a [N];
+};
+
+array a = { 1, 2, 3 };  // { dg-error "cannot deduce" }