C++ PATCH for c++/80840, ICE with constexpr and reference template parm

[Jason Merrill]

In convert_nontype_argument to reference type we were inappropriately
checking value_dependent_expression_p on an expression that might be a
VAR_DECL and might be a TEMPLATE_PARM_INDEX of reference type.  It's
inappropriate in the former case because we don't care about the value
of the object, only its address; we only want to test for the latter
case.

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

commit 388ec8de844dda6785d55daef0fffc90eb49118c
Author: Jason Merrill <ja...@redhat.com>
Date:   Tue May 30 21:38:17 2017 -0400

            PR c++/80840 - ICE with constexpr and reference
    
            * pt.c (convert_nontype_argument): Don't test whether a decl is
            value-dependent when binding to a reference.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index c1a8885..8239a3e 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -6749,7 +6749,11 @@ convert_nontype_argument (tree type, tree expr, 
tsubst_flags_t complain)
            }
        }
 
-      if (!value_dependent_expression_p (expr))
+      if (TREE_CODE (TREE_TYPE (expr)) == REFERENCE_TYPE
+         && value_dependent_expression_p (expr))
+       /* OK, dependent reference.  We don't want to ask whether a DECL is
+          itself value-dependent, since what we want here is its address.  */;
+      else
        {
          if (!DECL_P (expr))
            {
@@ -6771,8 +6775,11 @@ convert_nontype_argument (tree type, tree expr, 
tsubst_flags_t complain)
              return NULL_TREE;
            }
 
-         expr = build_nop (type, build_address (expr));
+         expr = build_address (expr);
        }
+
+      if (!same_type_p (type, TREE_TYPE (expr)))
+       expr = build_nop (type, expr);
     }
   /* [temp.arg.nontype]/5, bullet 4
 
diff --git a/gcc/testsuite/g++.dg/template/ref10.C 
b/gcc/testsuite/g++.dg/template/ref10.C
new file mode 100644
index 0000000..60e91d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/ref10.C
@@ -0,0 +1,13 @@
+// PR c++/80840
+// { dg-do compile { target c++11 } }
+
+template <class T, T X>
+struct Just;
+
+template <const double& X>
+struct Number {
+    static constexpr double value = X;
+    using result = Just<const double&, value>;
+};
+
+int main() {}