[PATCH] Do not define basic_string_view::to_string

[Tom Tromey <tromey@adacore.com>]

gdb's copy of basic_string_view includes a to_string method.  However,
according to cppreference, this is not a method on the real
std::basic_string_view:

https://en.cppreference.com/w/cpp/string/basic_string_view

This difference matters because gdb_string_view.h will use the
standard implementation when built with a C++17 or later.  This caused
PR build/26183.

This patch fixes the problem by changing the method to be a standalone
helper function, and then rewriting the uses.  Tested by rebuilding
with a version of GCC that defaults to C++17.

(Note that the build still is not clean; and also I noticed that the
libstdc++ string_view forbids the use of nullptr ... I wonder if gdb
violates that.)

gdb/ChangeLog
2020-06-29  Tom Tromey  <tromey@adacore.com>

	PR build/26183:
	* ada-lang.c (ada_lookup_name_info::ada_lookup_name_info): Use
	gdb::to_string.

gdbsupport/ChangeLog
2020-06-29  Tom Tromey  <tromey@adacore.com>

	PR build/26183:
	* gdb_string_view.h (basic_string_view::to_string): Remove.
	(gdb::to_string): New function.
---
 gdb/ChangeLog                |  6 ++++++
 gdb/ada-lang.c               |  8 ++++----
 gdbsupport/ChangeLog         |  6 ++++++
 gdbsupport/gdb_string_view.h | 17 ++++++++++-------
 4 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index 9b0c2efbfe2..98508c168bc 100644
--- a/gdb/ada-lang.c
+++ b/gdb/ada-lang.c
@@ -13553,10 +13553,10 @@ ada_lookup_name_info::ada_lookup_name_info (const lookup_name_info &lookup_name)
     {
       if (user_name.back () == '>')
 	m_encoded_name
-	  = user_name.substr (1, user_name.size () - 2).to_string ();
+	  = gdb::to_string (user_name.substr (1, user_name.size () - 2));
       else
 	m_encoded_name
-	  = user_name.substr (1, user_name.size () - 1).to_string ();
+	  = gdb::to_string (user_name.substr (1, user_name.size () - 1));
       m_encoded_p = true;
       m_verbatim_p = true;
       m_wild_match_p = false;
@@ -13575,10 +13575,10 @@ ada_lookup_name_info::ada_lookup_name_info (const lookup_name_info &lookup_name)
 	  if (encoded != NULL)
 	    m_encoded_name = encoded;
 	  else
-	    m_encoded_name = user_name.to_string ();
+	    m_encoded_name = gdb::to_string (user_name);
 	}
       else
-	m_encoded_name = user_name.to_string ();
+	m_encoded_name = gdb::to_string (user_name);
 
       /* Handle the 'package Standard' special case.  See description
 	 of m_standard_p.  */
diff --git a/gdbsupport/gdb_string_view.h b/gdbsupport/gdb_string_view.h
index c0ae7a8a2d9..65124e67e54 100644
--- a/gdbsupport/gdb_string_view.h
+++ b/gdbsupport/gdb_string_view.h
@@ -245,13 +245,6 @@ namespace gdb {
 	  return { this->_M_str, this->_M_len };
 	}
 
-      template<typename _Allocator = std::allocator<_CharT>>
-	std::basic_string<_CharT, _Traits, _Allocator>
-	to_string(const _Allocator& __alloc = _Allocator()) const
-	{
-	  return { this->_M_str, this->_M_len, __alloc };
-	}
-
       size_type
       copy(_CharT* __str, size_type __n, size_type __pos = 0) const
       {
@@ -560,4 +553,14 @@ namespace gdb {
 
 #endif // __cplusplus < 201703L
 
+namespace gdb {
+
+static inline std::string
+to_string(const gdb::string_view &view)
+{
+  return { view.data (), view.size () };
+}
+
+}
+
 #endif /* COMMON_GDB_STRING_VIEW_H */
-- 
2.21.3