PR c++/45923

	* class.c (explain_non_literal_class): New.
	(finalize_literal_type_property): Call it.
	* cp-tree.h: Declare it.
	* semantics.c (ensure_literal_type_for_constexpr_object): Call it.
	(is_valid_constexpr_fn): Likewise.
	(massage_constexpr_body): Split out from...
	(register_constexpr_fundef): ...here.
	(is_instantiation_of_constexpr): New.
	(expand_or_defer_fn_1): Leave DECL_SAVED_TREE alone in that case.
	(explain_invalid_constexpr_fn): New.
	(cxx_eval_call_expression): Call it.
	(potential_constant_expression_1): Likewise.  Avoid redundant errors.
	* method.c (process_subob_fn): Diagnose non-constexpr.
	(walk_field_subobs): Likewise.
	(synthesized_method_walk): Don't shortcut if we want diagnostics.
	(explain_implicit_non_constexpr): New.
	(defaulted_late_check): Use it.
	* call.c (build_cxx_call): Remember location.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@175646 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 39 insertions, 5 deletions

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index ec1c5025278..f10e846d0ea 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -958,7 +958,15 @@ process_subob_fn (tree fn, bool move_p, tree *spec_p, bool *trivial_p,
 	  && !DECL_TEMPLATE_INSTANTIATED (fn))
 	instantiate_decl (fn, /*defer_ok*/false, /*expl_class*/false);
       if (!DECL_DECLARED_CONSTEXPR_P (fn))
-	*constexpr_p = false;
+	{
+	  *constexpr_p = false;
+	  if (msg)
+	    {
+	      inform (0, "defaulted constructor calls non-constexpr "
+		      "%q+D", fn);
+	      explain_invalid_constexpr_fn (fn);
+	    }
+	}
     }
 
   return;
@@ -1037,7 +1045,12 @@ walk_field_subobs (tree fields, tree fnname, special_function_kind sfk,
 	  /* FIXME will need adjustment for non-static data member
 	     initializers.  */
 	  if (constexpr_p && !CLASS_TYPE_P (mem_type))
-	    *constexpr_p = false;
+	    {
+	      *constexpr_p = false;
+	      if (msg)
+		inform (0, "defaulted default constructor does not "
+			"initialize %q+#D", field);
+	    }
 	}
 
       if (!CLASS_TYPE_P (mem_type))
@@ -1071,8 +1084,9 @@ walk_field_subobs (tree fields, tree fnname, special_function_kind sfk,
 /* The caller wants to generate an implicit declaration of SFK for CTYPE
    which is const if relevant and CONST_P is set.  If spec_p, trivial_p and
    deleted_p are non-null, set their referent appropriately.  If diag is
-   true, we're being called from maybe_explain_implicit_delete to give
-   errors.  */
+   true, we're either being called from maybe_explain_implicit_delete to
+   give errors, or if constexpr_p is non-null, from
+   explain_invalid_constexpr_fn.  */
 
 static void
 synthesized_method_walk (tree ctype, special_function_kind sfk, bool const_p,
@@ -1175,6 +1189,7 @@ synthesized_method_walk (tree ctype, special_function_kind sfk, bool const_p,
      resolution, so a constructor can be trivial even if it would otherwise
      call a non-trivial constructor.  */
   if (expected_trivial
+      && !diag
       && (!copy_arg_p || cxx_dialect < cxx0x))
     {
       if (constexpr_p && sfk == sfk_constructor)
@@ -1366,6 +1381,20 @@ maybe_explain_implicit_delete (tree decl)
   return false;
 }
 
+/* DECL is a defaulted function which was declared constexpr.  Explain why
+   it can't be constexpr.  */
+
+void
+explain_implicit_non_constexpr (tree decl)
+{
+  tree parm_type = TREE_VALUE (FUNCTION_FIRST_USER_PARMTYPE (decl));
+  bool const_p = CP_TYPE_CONST_P (non_reference (parm_type));
+  bool dummy;
+  synthesized_method_walk (DECL_CLASS_CONTEXT (decl),
+			   special_function_p (decl), const_p,
+			   NULL, NULL, NULL, &dummy, true);
+}
+
 /* Implicitly declare the special function indicated by KIND, as a
    member of TYPE.  For copy constructors and assignment operators,
    CONST_P indicates whether these functions should take a const
@@ -1581,7 +1610,12 @@ defaulted_late_check (tree fn)
       && DECL_DECLARED_CONSTEXPR_P (fn))
     {
       if (!CLASSTYPE_TEMPLATE_INSTANTIATION (ctx))
-	error ("%qD cannot be declared as constexpr", fn);
+	{
+	  error ("explicitly defaulted function %q+D cannot be declared "
+		 "as constexpr because the implicit declaration is not "
+		 "constexpr:", fn);
+	  explain_implicit_non_constexpr (fn);
+	}
       DECL_DECLARED_CONSTEXPR_P (fn) = false;
     }