C++ PATCH for c++/85258, ICE with invalid range-based for-loop

[Marek Polacek]

This is a crash on invalid which started when we changed 
decl_maybe_constant_var_p
to say true for references.  Then in tsubst_copy we take this branch:
                  if (decl_maybe_constant_var_p (r))
                    {
                      /* We can't call cp_finish_decl, so handle the
                         initializer by hand.  */
                      tree init = tsubst_init (DECL_INITIAL (t), r, args,
                                               complain, in_decl);
but tsubst_init can return NULL_TREE, which potential_constant_expression
knows how to handle, but reduced_constant_expression_p didn't.  So the
following patch will fix the ICE.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-04-12  Marek Polacek  <pola...@redhat.com>

        PR c++/85258
        * constexpr.c (reduced_constant_expression_p): Return false for null
        trees.

        * g++.dg/parse/error61.C: New test.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 75f56df4465..82f14baaefd 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -1773,6 +1773,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree 
t,
 bool
 reduced_constant_expression_p (tree t)
 {
+  if (t == NULL_TREE)
+    return false;
+
   switch (TREE_CODE (t))
     {
     case PTRMEM_CST:
@@ -1794,9 +1797,8 @@ reduced_constant_expression_p (tree t)
        field = NULL_TREE;
       FOR_EACH_CONSTRUCTOR_ELT (CONSTRUCTOR_ELTS (t), i, idx, val)
        {
-         if (!val)
-           /* We're in the middle of initializing this element.  */
-           return false;
+         /* If VAL is null, we're in the middle of initializing this
+            element.  */
          if (!reduced_constant_expression_p (val))
            return false;
          if (field)
diff --git gcc/testsuite/g++.dg/parse/error61.C 
gcc/testsuite/g++.dg/parse/error61.C
index e69de29bb2d..199e1aa721c 100644
--- gcc/testsuite/g++.dg/parse/error61.C
+++ gcc/testsuite/g++.dg/parse/error61.C
@@ -0,0 +1,14 @@
+// PR c++/85258
+// { dg-do compile { target c++11 } }
+
+template<int> void foo()
+{
+  int x[8];
+  for (int& i, j : x) // { dg-error "multiple" }
+    i = 0; // { dg-error "local variable" }
+}
+
+void bar()
+{
+  foo<0>();
+}

        Marek