Re: PR libstdc++/78420 Make std::less etc. yield total order for pointers

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On 14 March 2018 at 23:01, Jonathan Wakely wrote:
> Here's a very different patch. This gets rid of the __ptr_rel_ops and
> just puts the special handling for pointers directly in the
> std::less<_Tp*> etc. specializations. Then std::less<void> uses
> std::less<P*> for some pointer type P*. I've also added a ton of ugly
> metaprogramming to detect the "if the call operator calls a built-in
> operator comparing pointers" condition. The idea is borrowed from
> Casey Carter and Eric Niebler's Ranges work, but basically it checks
> if operator<(T,U) or T.operator<(U) can be called, and if not the
> comparison must be using a built-in operator. If both T and U are
> convertible to pointers (specifically, to const volatile void* which
> is the most accepting of all pointers) then we assume we're using a
> built-in operator comparing pointers, and delegate to std::less<const
> volatile void*>, which ensures a total order.
>
> Tested powerpc64le-linux, committed to trunk. This fixes a regression,
> but I'm not sure about backporting it yet, I haven't even tried
> testing it on the branches.

This adjusts the new test to avoid using 'auto' so it works as C++98 too.

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commit be039c0c67a6d18ddeb86bddc50cd9704a120f75
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Mar 17 13:35:50 2018 +0000

    Fix new test that fails in C++98 mode
    
            * testsuite/20_util/function_objects/comparisons_pointer.cc: Adjust
            to compile as C++98.

diff --git a/libstdc++-v3/testsuite/20_util/function_objects/comparisons_pointer.cc b/libstdc++-v3/testsuite/20_util/function_objects/comparisons_pointer.cc
index 6b2b8be0f51..aad0bc3dd2e 100644
--- a/libstdc++-v3/testsuite/20_util/function_objects/comparisons_pointer.cc
+++ b/libstdc++-v3/testsuite/20_util/function_objects/comparisons_pointer.cc
@@ -27,7 +27,7 @@ int a[8];
 void
 test01()
 {
-  auto p = a + 8;
+  int* p = a + 8;
   std::greater<int*> gt;
 
   std::stringstream ss;
@@ -44,7 +44,7 @@ test01()
   ss.str("");
   ss.clear();
   sum = 0;
-  auto p2 = a + 8;
+  int* p2 = a + 8;
   std::greater<> gt2;
   ss << gt2(p2, b) << ' ' << gt2(b, p2) << ' ' << (!gt2(p2, b) && !gt2(b, p2));
   while (ss >> n)
@@ -59,7 +59,7 @@ test01()
 void
 test02()
 {
-  auto p = a + 8;
+  int* p = a + 8;
   std::less<int*> lt;
 
   std::stringstream ss;
@@ -76,7 +76,7 @@ test02()
   ss.str("");
   ss.clear();
   sum = 0;
-  auto p2 = a + 8;
+  int* p2 = a + 8;
   std::less<> lt2;
   ss << lt2(p2, b) << ' ' << lt2(b, p2) << ' ' << (!lt2(p2, b) && !lt2(b, p2));
   while (ss >> n)
@@ -91,7 +91,7 @@ test02()
 void
 test03()
 {
-  auto p = a + 8;
+  int* p = a + 8;
   std::greater_equal<int*> ge;
 
   std::stringstream ss;
@@ -109,7 +109,7 @@ test03()
   ss.str("");
   ss.clear();
   sum = 0;
-  auto p2 = a + 8;
+  int* p2 = a + 8;
   std::greater_equal<> ge2;
   ss << !ge2(p2, b) << ' ' << !ge2(b, p2) << ' ' << (ge2(p2, b) && ge2(b, p2));
   while (ss >> n)
@@ -125,7 +125,7 @@ test03()
 void
 test04()
 {
-  auto p = a + 8;
+  int* p = a + 8;
   std::less_equal<int*> le;
 
   std::stringstream ss;
@@ -143,7 +143,7 @@ test04()
   ss.str("");
   ss.clear();
   sum = 0;
-  auto p2 = a + 8;
+  int* p2 = a + 8;
   std::less_equal<> le2;
   ss << !le2(p2, b) << ' ' << !le2(b, p2) << ' ' << (le2(p2, b) && le2(b, p2));
   while (ss >> n)
@@ -167,7 +167,7 @@ void
 test05()
 {
   std::less<const X*> lt;
-  auto p = y + 4;
+  X* p = y + 4;
   std::stringstream ss;
   ss << lt(x, p) << ' ' << lt(p, x) << ' ' << (!lt(p, x) && !lt(x, p));
   int sum = 0;
@@ -183,7 +183,7 @@ test05()
   ss.str("");
   ss.clear();
   sum = 0;
-  auto p2 = y + 4;
+  X* p2 = y + 4;
   std::less<> lt2;
   ss << lt2(x, p2) << ' ' << lt2(p2, x) << ' ' << (!lt2(x, p2) && !lt2(p2, x));
   while (ss >> n)