5 files changed, 75 insertions, 5 deletions

diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 40e1714..c75b889 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -5415,10 +5415,11 @@ type_has_user_provided_or_explicit_constructor (tree t)
 
 /* Returns true iff class T has a non-user-provided (i.e. implicitly
    declared or explicitly defaulted in the class body) default
-   constructor.  */
+   constructor.  If SYNTH, only return true if it hasn't been
+   implicitly defined yet.  */
 
-bool
-type_has_non_user_provided_default_constructor (tree t)
+static bool
+type_has_non_user_provided_default_constructor_1 (tree t, bool synth)
 {
   if (!TYPE_HAS_DEFAULT_CONSTRUCTOR (t))
     return false;
@@ -5431,12 +5432,28 @@ type_has_non_user_provided_default_constructor (tree t)
       if (TREE_CODE (fn) == FUNCTION_DECL
 	  && default_ctor_p (fn)
 	  && !user_provided_p (fn))
-	return true;
+	{
+	  if (synth)
+	    return !DECL_INITIAL (fn);
+	  return true;
+	}
     }
 
   return false;
 }
 
+bool
+type_has_non_user_provided_default_constructor (tree t)
+{
+  return type_has_non_user_provided_default_constructor_1 (t, false);
+}
+
+bool
+type_has_default_ctor_to_be_synthesized (tree t)
+{
+  return type_has_non_user_provided_default_constructor_1 (t, true);
+}
+
 /* TYPE is being used as a virtual base, and has a non-trivial move
    assignment.  Return true if this is due to there being a user-provided
    move assignment in TYPE or one of its subobjects; if there isn't, then
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 2f71885..02734a4 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6720,6 +6720,7 @@ extern tree in_class_defaulted_default_constructor (tree);
 extern bool user_provided_p			(tree);
 extern bool type_has_user_provided_constructor  (tree);
 extern bool type_has_non_user_provided_default_constructor (tree);
+extern bool type_has_default_ctor_to_be_synthesized (tree);
 extern bool vbase_has_user_provided_move_assign (tree);
 extern tree default_init_uninitialized_part (tree);
 extern bool trivial_default_constructor_is_constexpr (tree);
diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 903ee66..e0fe217 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -2056,6 +2056,28 @@ assignable_expr (tree to, tree from)
   return r;
 }
 
+/* An unparsed default member initializer prevents calling a defaulted default
+   constructor; make checking std::is_constructible ill-formed until the DMI
+   has been parsed, to avoid caching the wrong value.  */
+
+static bool
+complain_about_unparsed_dmi (tree t)
+{
+  if (type_has_default_ctor_to_be_synthesized (t)
+      && TYPE_HAS_COMPLEX_DFLT (t))
+    for (tree f = TYPE_FIELDS (t); f; f = DECL_CHAIN (f))
+      if (TREE_CODE (f) == FIELD_DECL
+	  && DECL_INITIAL (f)
+	  && TREE_CODE (DECL_INITIAL (f)) == DEFERRED_PARSE)
+	{
+	  error ("default member initializer for %qD required by %qs before "
+		 "the end of its enclosing class", f, "std::is_constructible");
+	  inform (location_of (f), "defined here");
+	  return true;
+	}
+  return false;
+}
+
 /* The predicate condition for a template specialization
    is_constructible<T, Args...> shall be satisfied if and only if the
    following variable definition would be well-formed for some invented
@@ -2070,6 +2092,8 @@ constructible_expr (tree to, tree from)
   cp_unevaluated cp_uneval_guard;
   if (CLASS_TYPE_P (to))
     {
+      if (!from && complain_about_unparsed_dmi (to))
+	return error_mark_node;
       tree ctype = to;
       vec<tree, va_gc> *args = NULL;
       if (!TYPE_REF_P (to))
diff --git a/gcc/testsuite/g++.dg/ext/is_constructible3.C b/gcc/testsuite/g++.dg/ext/is_constructible3.C
index c7c5874..305751d 100644
--- a/gcc/testsuite/g++.dg/ext/is_constructible3.C
+++ b/gcc/testsuite/g++.dg/ext/is_constructible3.C
@@ -8,7 +8,7 @@ struct A {
     B() = default;
   };
 
-  static constexpr bool v = __is_constructible (B);
+  static constexpr bool v = __is_constructible (B); // { dg-error "member initializer" }
 
 };
 
diff --git a/gcc/testsuite/g++.dg/ext/is_constructible7.C b/gcc/testsuite/g++.dg/ext/is_constructible7.C
new file mode 100644
index 0000000..76a63bb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_constructible7.C
@@ -0,0 +1,28 @@
+// PR c++/96645
+// { dg-do compile { target c++11 } }
+
+template<bool B>
+struct bool_constant
+{
+  static constexpr bool value = B;
+  using type = bool_constant;
+};
+
+using true_type = bool_constant<true>;
+
+template<typename T>
+struct is_default_constructible
+  : bool_constant<__is_constructible(T)> // { dg-error "default member init" }
+{ };
+
+void testVarStruct()
+{
+  struct DataWithStruct {
+    struct A {
+      int number = 5; // compiles, if remove initialization
+    };
+
+    // { dg-prune-output "could not convert" }
+    is_default_constructible<A>::type t = true_type{};
+  };
+}