PR c++/83273 - constexpr if allows non-constant condition

	* semantics.c (finish_if_stmt_cond): Use require_constant_expression
	rather than is_constant_expression.
	* constexpr.c (potential_constant_expression_1) [LAMBDA_EXPR]: Allow
	in C++17.

From-SVN: r255390

gcc/cp/ChangeLog

+2017-12-04  Jason Merrill  <jason@redhat.com>
+
+	PR c++/83273 - constexpr if allows non-constant condition
+	* semantics.c (finish_if_stmt_cond): Use require_constant_expression
+	rather than is_constant_expression.
+	* constexpr.c (potential_constant_expression_1) [LAMBDA_EXPR]: Allow
+	in C++17.
+
 2017-12-01  Jason Merrill  <jason@redhat.com>
 
 	Give #include hints for <complex>.

gcc/cp/constexpr.c

@@ -5524,6 +5524,14 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
       return RECUR (STMT_EXPR_STMT (t), rval);
 
     case LAMBDA_EXPR:
+      if (cxx_dialect >= cxx17)
+	/* In C++17 lambdas can be constexpr, don't give up yet.  */
+	return true;
+      else if (flags & tf_error)
+	error_at (loc, "lambda-expression is not a constant expression "
+		  "before C++17");
+      return false;
+
     case DYNAMIC_CAST_EXPR:
     case PSEUDO_DTOR_EXPR:
     case NEW_EXPR:

gcc/cp/semantics.c

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

gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C

@@ -7,7 +7,7 @@ struct T {
 
 template <class MustBeTemplate>
 constexpr auto bf(T t) {
-    if constexpr(t.foo()) {
+    if constexpr(t.foo()) {	// { dg-error "constant expression" }
         return false;
     }
     return true;

gcc/testsuite/g++.dg/cpp1z/constexpr-if13.C

+// PR c++/83273
+// { dg-options -std=c++17 }
+
+int main()
+{
+  auto d = 42;
+  if constexpr (d > 0) {	// { dg-error "constant expression" }
+      return d;
+  }
+}