[PATCH] Fix building gdb with gcc-4.x

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

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Hi,

with Luis' commit of today the trunk is no longer able to
be compiled with gcc-4.x.

The problem is std::is_trivially_default_constructible is
not defined before gcc-5 although the compiler supports C++11 

I am not sure about what's the best approach for conditionally
enabling the code, especially for compilers other than g++.


This fixes the build for me.
Is it OK for trunk?


Thanks
Bernd.

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From 0879e2cff0ea732769aa80b89fa6fbba70c84260 Mon Sep 17 00:00:00 2001
From: Bernd Edlinger <bernd.edlinger@hotmail.de>
Date: Mon, 4 Jan 2021 21:40:41 +0100
Subject: [PATCH] Fix building gdb with gcc-4.x
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Since is_trivially_default_constructible was not implemented before gcc-5
it cannot be used with gcc-4.x.

../../binutils-gdb/gdb/trad-frame.c: In function ‘trad_frame_saved_reg* trad_frame_alloc_saved_regs(gdbarch*)’:
../../binutils-gdb/gdb/trad-frame.c:64:22: error: ‘is_trivially_default_constructible’ is not a member of ‘std’
   gdb_static_assert (std::is_trivially_default_constructible<trad_frame_saved_reg>::value);

Fix build by using conditional compilation around that line.

Fixes: 098caef485a ("Refactor struct trad_frame_saved_regs")

gdb:
2021-01-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>

        * trad-frame.c (trad_frame_alloc_saved_regs): Avoid compile-error
	because is_trivially_default_constructible was first implemented with
	gcc-5.
---
 gdb/trad-frame.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/gdb/trad-frame.c b/gdb/trad-frame.c
index 17375e8..de1acc1 100644
--- a/gdb/trad-frame.c
+++ b/gdb/trad-frame.c
@@ -60,7 +60,9 @@ struct trad_frame_cache *
 trad_frame_saved_reg *
 trad_frame_alloc_saved_regs (struct gdbarch *gdbarch)
 {
+#if defined(__GNUC__) && __GNUC__ >= 5
   gdb_static_assert (std::is_trivially_default_constructible<trad_frame_saved_reg>::value);
+#endif
 
   int numregs = gdbarch_num_cooked_regs (gdbarch);
   trad_frame_saved_reg *this_saved_regs
-- 
1.9.1