Hi!

The removal of remove_zero_width_bit_fields, in addition to triggering
some ABI issues that need solving anyway (ABI incompatibility between
C and C++) also resulted in UB inside of gcc, we now call build_zero_init
which calls build_int_cst on an integral type with TYPE_PRECISION of 0.

Fixed by ignoring the zero width bitfields.  I understand
build_value_init_noctor wants to initialize to 0 even unnamed bitfields
(of non-zero width), at least until we have some CONSTRUCTOR flag that says
that even all the padding bits should be cleared.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2021-08-23  Jakub Jelinek  <ja...@redhat.com>

        PR c++/102019
        * init.c (build_value_init_noctor): Ignore unnamed zero-width
        bitfields.

--- gcc/cp/init.c.jj    2021-07-29 13:24:42.698012879 +0200
+++ gcc/cp/init.c       2021-08-23 14:43:08.791947395 +0200
@@ -427,6 +427,11 @@ build_value_init_noctor (tree type, tsub
                      == NULL_TREE))
                continue;
 
+             /* Ignore unnamed zero-width bitfields.  */
+             if (DECL_UNNAMED_BIT_FIELD (field)
+                 && integer_zerop (DECL_SIZE (field)))
+               continue;
+
              /* We could skip vfields and fields of types with
                 user-defined constructors, but I think that won't improve
                 performance at all; it should be simpler in general just

        Jakub

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