PR c++/87567 - constexpr rejects call to non-constexpr function.

	* constexpr.c (potential_constant_expression_1) <case FOR_STMT>: Return
	true if the condition is always false.
	<case WHILE_STMT>: Likewise.

	* g++.dg/cpp1y/constexpr-loop7.C: New test.

From-SVN: r265027

gcc/cp/ChangeLog

+2018-10-10  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/87567 - constexpr rejects call to non-constexpr function.
+	* constexpr.c (potential_constant_expression_1) <case FOR_STMT>: Return
+	true if the condition is always false.
+	<case WHILE_STMT>: Likewise.
+
 2018-10-09  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/84423

gcc/cp/constexpr.c

@@ -5818,8 +5818,16 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
     case FOR_STMT:
       if (!RECUR (FOR_INIT_STMT (t), any))
 	return false;
-      if (!RECUR (FOR_COND (t), rval))
+      tmp = FOR_COND (t);
+      if (!RECUR (tmp, rval))
 	return false;
+      if (tmp)
+	{
+	  if (!processing_template_decl)
+	    tmp = cxx_eval_outermost_constant_expr (tmp, true);
+	  if (integer_zerop (tmp))
+	    return true;
+	}
       if (!RECUR (FOR_EXPR (t), any))
 	return false;
       if (!RECUR (FOR_BODY (t), any))
@@ -5840,8 +5848,13 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
       return true;
 
     case WHILE_STMT:
-      if (!RECUR (WHILE_COND (t), rval))
+      tmp = WHILE_COND (t);
+      if (!RECUR (tmp, rval))
 	return false;
+      if (!processing_template_decl)
+	tmp = cxx_eval_outermost_constant_expr (tmp, true);
+      if (integer_zerop (tmp))
+	return true;
       if (!RECUR (WHILE_BODY (t), any))
 	return false;
       if (breaks (jump_target) || continues (jump_target))

gcc/testsuite/ChangeLog

+2018-10-10  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/87567 - constexpr rejects call to non-constexpr function.
+	* g++.dg/cpp1y/constexpr-loop7.C: New test.
+
 2018-10-10  Paul A. Clarke  <pc@us.ibm.com>
 
 	PR target/87579

gcc/testsuite/g++.dg/cpp1y/constexpr-loop7.C

+// PR c++/87567
+// { dg-do compile { target c++14 } }
+
+constexpr bool always_false() { return false; }
+int f() { return 1; }
+
+constexpr int
+fn1 ()
+{
+  while (always_false ())
+    return f();
+  return 0;
+}
+
+constexpr int
+fn2 ()
+{
+  for (;always_false();)
+    return f();
+  return 0;
+}