c++: constexpr init of union sub-aggr w/ base [PR105491]

Here ever since r10-7313-gb599bf9d6d1e18, reduced_constant_expression_p
in C++11/14 is rejecting the marked sub-aggregate initializer (of type S)

  W w = {.D.2445={.s={.D.2387={.m=0}, .b=0}}};
                     ^
ultimately because said initializer has CONSTRUCTOR_NO_CLEARING set,
hence the function must verify that all fields of S are initialized.
And before C++17 it doesn't expect to see base class fields (since
next_initializable_field skips over them), so the presence thereof
causes r_c_e_p to return false.

The reason r10-7313-gb599bf9d6d1e18 causes this is because in that
commit we began using CONSTRUCTOR_NO_CLEARING to precisely track whether
we're in middle of activating a union member.  This ends up affecting
clear_no_implicit_zero, which recurses into sub-aggregate initializers
only if the outer initializer has CONSTRUCTOR_NO_CLEARING set.  After
that commit, the outer union initializer above no longer has the flag
set at this point and so clear_no_implicit_zero no longer recurses into
the marked inner initializer.

But arguably r_c_e_p should be able to accept the marked initializer
regardless of whether CONSTRUCTOR_NO_CLEARING is set.  The primary bug
therefore seems to be that r_c_e_p relies on next_initializable_field
which skips over base class fields in C++11/14.  To fix this, this patch
introduces a new helper function next_subobject_field which is like
next_initializable_field except that it never skips base class fields,
and makes r_c_e_p use it.  This patch then renames next_initializable_field
to next_aggregate_field (and makes it skip over vptr fields again).

NB: This minimal backport of r13-211-g0c7bce0ac184c0 for 12.2 just adds
next_subobject_field and makes reduced_constant_expression_p use it.

	PR c++/105491

gcc/cp/ChangeLog:

	* constexpr.cc (reduced_constant_expression_p): Use
	next_subobject_field instead.
	* cp-tree.h (next_subobject_field): Declare.
	* decl.cc (next_subobject_field): Define.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-union7.C: New test.
	* g++.dg/cpp0x/constexpr-union7a.C: New test.
	* g++.dg/cpp2a/constinit17.C: New test.

(cherry picked from commit 0c7bce0ac184c057bacad9c8e615ce82923835fd)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 47d5113ace26791031dd55619c546589bc75bb4d..1e3342adbb9971d2ac2d7bc416cf806fa78e787d 100644 (file)
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3053,7 +3053,7 @@ reduced_constant_expression_p (tree t)
              field = NULL_TREE;
            }
          else
-           field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
+           field = next_subobject_field (TYPE_FIELDS (TREE_TYPE (t)));
        }
       else
        field = NULL_TREE;
@@ -3065,15 +3065,15 @@ reduced_constant_expression_p (tree t)
            return false;
          /* Empty class field may or may not have an initializer.  */
          for (; field && e.index != field;
-              field = next_initializable_field (DECL_CHAIN (field)))
+              field = next_subobject_field (DECL_CHAIN (field)))
            if (!is_really_empty_class (TREE_TYPE (field),
                                        /*ignore_vptr*/false))
              return false;
          if (field)
-           field = next_initializable_field (DECL_CHAIN (field));
+           field = next_subobject_field (DECL_CHAIN (field));
        }
       /* There could be a non-empty field at the end.  */
-      for (; field; field = next_initializable_field (DECL_CHAIN (field)))
+      for (; field; field = next_subobject_field (DECL_CHAIN (field)))
        if (!is_really_empty_class (TREE_TYPE (field), /*ignore_vptr*/false))
          return false;
 ok:

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 72177999968ccd1475414cb555740e900e2db4cb..b540c2342c07325465eb76702421fe0eb4218a6c 100644 (file)
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6885,6 +6885,7 @@ extern void initialize_artificial_var             (tree, vec<constructor_elt, va_gc> *);
 extern tree check_var_type                     (tree, tree, location_t);
 extern tree reshape_init                        (tree, tree, tsubst_flags_t);
 extern tree next_initializable_field (tree);
+extern tree next_subobject_field               (tree);
 extern tree first_field                                (const_tree);
 extern tree fndecl_declared_return_type                (tree);
 extern bool undeduced_auto_decl                        (tree);

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index d1fc7e10a523b5495409bd740eb9f27be5045a8f..269c6679ca26c58a38b2ff5fecdb667b421d36a1 100644 (file)
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -6409,6 +6409,25 @@ next_initializable_field (tree field)
   return field;
 }
 
+/* FIELD is an element of TYPE_FIELDS or NULL.  In the former case, the value
+   returned is the next FIELD_DECL (possibly FIELD itself) that corresponds
+   to a subobject.  If there are no more such fields, the return value will be
+   NULL.  */
+
+tree
+next_subobject_field (tree field)
+{
+  while (field
+        && (TREE_CODE (field) != FIELD_DECL
+            || DECL_UNNAMED_BIT_FIELD (field)
+            || (DECL_ARTIFICIAL (field)
+                && !DECL_FIELD_IS_BASE (field)
+                && !DECL_VIRTUAL_P (field))))
+     field = DECL_CHAIN (field);
+
+  return field;
+}
+
 /* Return true for [dcl.init.list] direct-list-initialization from
    single element of enumeration with a fixed underlying type.  */
 

diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C
new file mode 100644 (file)
index 0000000..b3147d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C
@@ -0,0 +1,17 @@
+// PR c++/105491
+// { dg-do compile { target c++11 } }
+
+struct V {
+  int m = 0;
+};
+struct S : V {
+  constexpr S(int) : b() { }
+  bool b;
+};
+struct W {
+  constexpr W() : s(0) { }
+  union {
+    S s;
+  };
+};
+constexpr W w;

diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C
new file mode 100644 (file)
index 0000000..b676e7d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C
@@ -0,0 +1,15 @@
+// PR c++/105491
+// { dg-do compile { target c++11 } }
+
+struct V {
+  int m = 0;
+};
+struct S : V {
+  constexpr S(int) : b() { }
+  bool b;
+};
+union W {
+  constexpr W() : s(0) { }
+  S s;
+};
+constexpr W w;

diff --git a/gcc/testsuite/g++.dg/cpp2a/constinit17.C b/gcc/testsuite/g++.dg/cpp2a/constinit17.C
new file mode 100644 (file)
index 0000000..6431654
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constinit17.C
@@ -0,0 +1,24 @@
+// PR c++/105491
+// { dg-do compile { target c++11 } }
+
+class Message {
+  virtual int GetMetadata();
+};
+class ProtobufCFileOptions : Message {
+public:
+  constexpr ProtobufCFileOptions(int);
+  bool no_generate_;
+  bool const_strings_;
+  bool use_oneof_field_name_;
+  bool gen_pack_helpers_;
+  bool gen_init_helpers_;
+};
+constexpr ProtobufCFileOptions::ProtobufCFileOptions(int)
+    : no_generate_(), const_strings_(), use_oneof_field_name_(),
+      gen_pack_helpers_(), gen_init_helpers_() {}
+struct ProtobufCFileOptionsDefaultTypeInternal {
+  constexpr ProtobufCFileOptionsDefaultTypeInternal() : _instance({}) {}
+  union {
+    ProtobufCFileOptions _instance;
+  };
+} __constinit _ProtobufCFileOptions_default_instance_;