PR c++/84421 - type-dependent if constexpr

	* semantics.c (finish_if_stmt_cond): Check
	type_dependent_expression_p.

From-SVN: r257744

gcc/cp/ChangeLog

+2018-02-16  Jason Merrill  <jason@redhat.com>
+
+	PR c++/84421 - type-dependent if constexpr
+	* semantics.c (finish_if_stmt_cond): Check type_dependent_expression_p.
+
 2018-02-16  Nathan Sidwell  <nathan@acm.org>
 
 	Deprecate -ffriend-injection.

gcc/cp/semantics.c

@@ -731,6 +731,7 @@ finish_if_stmt_cond (tree cond, tree if_stmt)
 {
   cond = maybe_convert_cond (cond);
   if (IF_STMT_CONSTEXPR_P (if_stmt)
+      && !type_dependent_expression_p (cond)
       && require_constant_expression (cond)
       && !value_dependent_expression_p (cond))
     {

gcc/testsuite/g++.dg/cpp1z/constexpr-if14.C

+// PR c++/84421
+// { dg-options -std=c++17 }
+
+struct A{
+  constexpr operator bool() const { return true; }
+};
+
+int main(){
+  auto f = [](auto v){
+    if constexpr(v){}
+  };
+  A a;
+  f(a);
+}