c++: delayed parse DMI [PR96645]

With the changes for PR81359 and PR88368 to make get_nsdmi errors be treated
as substitution failure, we have the problem that if we check
std::is_default_constructible for a complete class that still has unparsed
default member initializers, we get an answer (false) that will be wrong
once the DMIs have been parsed.  The traits avoid this problem for regular
incomplete classes by giving an error if the operand is incomplete; we
should do the same if get_nsdmi is going to fail due to unparsed DMI.

	PR c++/96645

gcc/cp/ChangeLog:

	* cp-tree.h (type_has_default_ctor_to_be_synthesized): Declare.
	* class.cc (type_has_default_ctor_to_be_synthesized): New.
	(type_has_non_user_provided_default_constructor_1): Support it.
	(type_has_non_user_provided_default_constructor): Now a wrapper.
	* method.cc (complain_about_unparsed_dmi): New.
	(constructible_expr): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_constructible3.C: Expect error.
	* g++.dg/ext/is_constructible7.C: New test.

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{};
+  };
+}