PR c++/87594 - constexpr rejects-valid with range-based for.

	* constexpr.c (potential_constant_expression_1): If the condition
	can't be evaluated, return true.

	* g++.dg/cpp1y/constexpr-loop8.C: New test.

From-SVN: r265600

gcc/cp/ChangeLog

+2018-10-29  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/87594 - constexpr rejects-valid with range-based for.
+	* constexpr.c (potential_constant_expression_1): If the condition
+	can't be evaluated, return true.
+
 2018-10-29  Joseph Myers  <joseph@codesourcery.com>
 	    Julian Brown  <julian@codesourcery.com>
 

gcc/cp/constexpr.c

@@ -5825,7 +5825,9 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	{
 	  if (!processing_template_decl)
 	    tmp = cxx_eval_outermost_constant_expr (tmp, true);
-	  if (integer_zerop (tmp))
+	  /* If we couldn't evaluate the condition, it might not ever be
+	     true.  */
+	  if (!integer_onep (tmp))
 	    return true;
 	}
       if (!RECUR (FOR_EXPR (t), any))
@@ -5853,7 +5855,8 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	return false;
       if (!processing_template_decl)
 	tmp = cxx_eval_outermost_constant_expr (tmp, true);
-      if (integer_zerop (tmp))
+      /* If we couldn't evaluate the condition, it might not ever be true.  */
+      if (!integer_onep (tmp))
 	return true;
       if (!RECUR (WHILE_BODY (t), any))
 	return false;

gcc/testsuite/ChangeLog

+2018-10-29  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/87594 - constexpr rejects-valid with range-based for.
+	* g++.dg/cpp1y/constexpr-loop8.C: New test.
+
 2018-10-29  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/87785

gcc/testsuite/g++.dg/cpp1y/constexpr-loop8.C

+// PR c++/87594
+// { dg-do compile { target c++14 } }
+
+constexpr bool always_false() { return false; }
+int f() { return 1; }
+
+constexpr int
+fn1()
+{
+  struct empty_range {
+    constexpr int* begin() { return 0; }
+    constexpr int* end() { return 0; }
+  } e;
+  for (auto x : e)
+    f();
+  return 0;
+}
+
+constexpr int
+fn2 ()
+{
+  int a[] = { 1, 2, 3 };
+  for (auto x : a)
+    f(); // { dg-error "call to non-.constexpr. function" }
+  return 0;
+}
+
+constexpr int
+fn3 ()
+{
+  __extension__ int a[] = { };
+  for (auto x : a)
+    f();
+  return 0;
+}
+
+
+void
+bar ()
+{
+  constexpr int i1 = fn1 ();
+  constexpr int i2 = fn2 (); // { dg-message "in .constexpr. expansion of " }
+  constexpr int i3 = fn3 ();
+}