C++ PATCH for c++/85695, rejects-valid with constexpr if

[Marek Polacek]

Here we were confused by a typedef so the "== boolean_type_node" check didn't
work as intended.  We can use TYPE_MAIN_VARIANT to see the real type.

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

2018-05-08  Marek Polacek  <pola...@redhat.com>

        PR c++/85695
        * semantics.c (finish_if_stmt_cond): See through typedefs.

        * g++.dg/cpp1z/constexpr-if22.C: New test.

diff --git gcc/cp/semantics.c gcc/cp/semantics.c
index 2b2b51b2a7e..195286ca751 100644
--- gcc/cp/semantics.c
+++ gcc/cp/semantics.c
@@ -736,7 +736,7 @@ finish_if_stmt_cond (tree cond, tree if_stmt)
       && !instantiation_dependent_expression_p (cond)
       /* Wait until instantiation time, since only then COND has been
         converted to bool.  */
-      && TREE_TYPE (cond) == boolean_type_node)
+      && TYPE_MAIN_VARIANT (TREE_TYPE (cond)) == boolean_type_node)
     {
       cond = instantiate_non_dependent_expr (cond);
       cond = cxx_constant_value (cond, NULL_TREE);
diff --git gcc/testsuite/g++.dg/cpp1z/constexpr-if22.C 
gcc/testsuite/g++.dg/cpp1z/constexpr-if22.C
index e69de29bb2d..76f0c73476b 100644
--- gcc/testsuite/g++.dg/cpp1z/constexpr-if22.C
+++ gcc/testsuite/g++.dg/cpp1z/constexpr-if22.C
@@ -0,0 +1,21 @@
+// PR c++/85695
+// { dg-options -std=c++17 }
+
+template <typename T, T v>
+struct integral_constant {
+    using value_type = T;
+    static constexpr const value_type value = v;
+    constexpr operator value_type (void) const { return value; }
+};
+template <typename T> struct is_trivial
+    : public integral_constant<bool, __is_trivial(T)> {};
+
+template <typename T>
+T clone_object (const T& p)
+{
+    if constexpr (is_trivial<T>::value)
+        return p;
+    else
+        return p.clone();
+}
+int main (void) { return clone_object(0); }