[PATCH][gdb] Fix build breaker with gcc 4.8

[Tom de Vries <tdevries@suse.de>]

Hi,

When compiling with gcc 4.8, we run into:
...
/usr/include/c++/4.8/bits/unordered_map.h:100:18: required from \
  ‘class std::unordered_map<sect_offset, std::vector<sect_offset> >’
src/gdb/dwarf2read.h:260:5:   required from here
/usr/include/c++/4.8/bits/hashtable_policy.h:1070:12: error: invalid use of \
  incomplete type ‘struct std::hash<sect_offset>’
...

Fix this by adding std::hash<sect_offset>.

Build and reg-tested on x86_64-linux with gcc 4.8.

OK for trunk?

Thanks,
- Tom

[gdb] Fix build breaker with gcc 4.8

gdb/ChangeLog:

2019-06-19  Tom de Vries  <tdevries@suse.de>

	* dwarf2read.h (std::hash<sect_offset>): New specialization.

---
 gdb/dwarf2read.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/gdb/dwarf2read.h b/gdb/dwarf2read.h
index 776860e454..494dcb32c1 100644
--- a/gdb/dwarf2read.h
+++ b/gdb/dwarf2read.h
@@ -99,6 +99,26 @@ struct signatured_type;
 struct die_info;
 typedef struct die_info *die_info_ptr;
 
+/* An std::hash specialization, required for use of sect_offset in
+   std::unordered_map with gcc 4.8.  */
+
+namespace std
+{
+  template<> struct hash<sect_offset>
+  {
+    typedef sect_offset argument_type;
+    typedef std::size_t result_type;
+    using underlying_type
+    = typename std::underlying_type<argument_type>::type;
+
+    result_type operator() (const argument_type &o) const noexcept
+    {
+      underlying_type u = static_cast<underlying_type> (o);
+      return std::hash<underlying_type> {} (u);
+    }
+  };
+};
+
 /* Collection of data recorded per objfile.
    This hangs off of dwarf2_objfile_data_key.  */