C++ PATCH for c++/87567, constexpr rejects call to non-constexpr function

[Marek Polacek]

In this testcase, the call to f() can never be a constant expression, but
that's not a problem because it is never reached.  We handle a similar scenario
for IF_STMT, so we can just do the same.  The RANGE_FOR_STMT case seems to
never be reached in the whole testsuite, so I didn't change that.

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

2018-10-10  Marek Polacek  <pola...@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.

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

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 403edda8c47..4fa8c965a9d 100644
--- gcc/cp/constexpr.c
+++ 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))
diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-loop7.C 
gcc/testsuite/g++.dg/cpp1y/constexpr-loop7.C
index e69de29bb2d..6733820fee2 100644
--- gcc/testsuite/g++.dg/cpp1y/constexpr-loop7.C
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-loop7.C
@@ -0,0 +1,21 @@
+// 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;
+}