From d9bf40a181b32e4d5e581ec10d48803ea4a43f9d Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Tue, 20 Mar 2018 08:44:49 -0400
Subject: [PATCH] PR c++/84937 - ICE with class deduction and auto.

	* pt.c (rewrite_template_parm): Fix auto handling.

From-SVN: r258680
---
 gcc/cp/ChangeLog                               |  7 ++++++-
 gcc/cp/pt.c                                    | 17 +++++++++++++++--
 gcc/testsuite/g++.dg/cpp1z/class-deduction51.C | 11 +++++++++++
 3 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction51.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 236ea215452..fd7329f0622 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+2018-03-19  Jason Merrill  <jason@redhat.com>
+
+	PR c++/84937 - ICE with class deduction and auto.
+	* pt.c (rewrite_template_parm): Fix auto handling.
+
 2018-03-19  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/84925
@@ -9,7 +14,7 @@
 	(cxx_eval_constant_expression): Verify constructor's flags
 	unconditionally.
 
-2018-03-16  Jason Merrill  <jason@redhat.com>
+2018-03-19  Jason Merrill  <jason@redhat.com>
 
 	PR c++/71834 - template-id with too few arguments.
 	* pt.c (coerce_template_parms): Check fixed_parameter_pack_p.
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 21a4de57745..d7c0c7bec81 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -25781,8 +25781,21 @@ rewrite_template_parm (tree olddecl, unsigned index, unsigned level,
 	  = TEMPLATE_TYPE_PARM_FOR_CLASS (oldtype);
     }
   else
-    newtype = tsubst (TREE_TYPE (olddecl), tsubst_args,
-		      complain, NULL_TREE);
+    {
+      newtype = TREE_TYPE (olddecl);
+      if (type_uses_auto (newtype))
+	{
+	  // Substitute once to fix references to other template parameters.
+	  newtype = tsubst (newtype, tsubst_args,
+			    complain|tf_partial, NULL_TREE);
+	  // Now substitute again to reduce the level of the auto.
+	  newtype = tsubst (newtype, current_template_args (),
+			    complain, NULL_TREE);
+	}
+      else
+	newtype = tsubst (newtype, tsubst_args,
+			  complain, NULL_TREE);
+    }
 
   tree newdecl
     = build_decl (DECL_SOURCE_LOCATION (olddecl), TREE_CODE (olddecl),
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction51.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction51.C
new file mode 100644
index 00000000000..eba7972c3c6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction51.C
@@ -0,0 +1,11 @@
+// PR c++/84937
+// { dg-additional-options -std=c++17 }
+
+template<int, int> struct A {};
+
+template<int I> struct B
+{
+  template<auto J> B(A<I,J>);
+};
+
+B b(A<0,0>{});
-- 
2.30.2