c++: Allow indeterminate unsigned char or std::byte in bit_cast - P1272R4

P1272R4 has added to the std::byteswap new stuff to me quite unrelated
clarification for std::bit_cast.
The patch treats it as DR, applying to all languages.
We no longer diagnose if padding bits are stored into unsigned char
or std::byte result, fields or bitfields, instead arrange for that result,
those fields or bitfields to get indeterminate value (empty
CONSTRUCTOR with CONSTRUCTOR_NO_ZEROING or just leaving the member's
initializer out and setting CONSTRUCTOR_NO_ZEROING on parent).

We still have a bug that we don't diagnose in lots of places lvalue-to-rvalue
conversions of indeterminate values or class objects with some indeterminate
members.

2021-12-04  Jakub Jelinek <jakub@redhat.com>

	* cp-tree.h (is_byte_access_type_not_plain_char): Declare.
	* tree.c (is_byte_access_type_not_plain_char): New function.
	* constexpr.c (clear_uchar_or_std_byte_in_mask): New function.
	(cxx_eval_bit_cast): Don't error about padding bits if target
	type is unsigned char or std::byte, instead return no clearing
	ctor.  Use clear_uchar_or_std_byte_in_mask.

	* g++.dg/cpp2a/bit-cast11.C: New test.
	* g++.dg/cpp2a/bit-cast12.C: New test.
	* g++.dg/cpp2a/bit-cast13.C: New test.
	* g++.dg/cpp2a/bit-cast14.C: New test.

1 files changed, 12 insertions, 0 deletions

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 1471ed8..52c5683 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -4311,6 +4311,18 @@ is_byte_access_type (tree type)
 	  && !strcmp ("byte", TYPE_NAME_STRING (type)));
 }
 
+/* Returns true if TYPE is unsigned char or std::byte.  */
+
+bool
+is_byte_access_type_not_plain_char (tree type)
+{
+  type = TYPE_MAIN_VARIANT (type);
+  if (type == char_type_node)
+    return false;
+
+  return is_byte_access_type (type);
+}
+
 /* Returns 1 iff type T is something we want to treat as a scalar type for
    the purpose of deciding whether it is trivial/POD/standard-layout.  */