libstdc++: Fix return value of std::ranges::advance [PR 100833]

The three-argument form of ranges::advance is supposed to return the
difference between the second argument and the distance the iterator was
advanced. When a non-random-access iterator is not advanced (because it
already equals the sentinel) we were returning 0 rather than n - 0.

libstdc++-v3/ChangeLog:

	PR libstdc++/100833
	* include/bits/ranges_base.h (ranges::advance(iter, n, sentinel)):
	Fix return value for no-op case.
	* testsuite/24_iterators/range_operations/advance.cc: Test
	return values of three-argument overload.

2 files changed, 33 insertions, 17 deletions

diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 11f05fa..32d664f 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -749,7 +749,7 @@ namespace ranges
 	      }
 	  }
 	else if (__it == __bound || __n == 0)
-	  return iter_difference_t<_It>(0);
+	  return __n;
 	else if (__n > 0)
 	  {
 	    iter_difference_t<_It> __m = 0;
diff --git a/libstdc++-v3/testsuite/24_iterators/range_operations/advance.cc b/libstdc++-v3/testsuite/24_iterators/range_operations/advance.cc
index e4abf83..feee08b 100644
--- a/libstdc++-v3/testsuite/24_iterators/range_operations/advance.cc
+++ b/libstdc++-v3/testsuite/24_iterators/range_operations/advance.cc
@@ -49,10 +49,12 @@ test01()
   std::ranges::advance(iter, r.begin());
   VERIFY( iter == r.begin() );
 
-  std::ranges::advance(iter, 99, r.end());
+  auto diff = std::ranges::advance(iter, 99, r.end());
   VERIFY( iter == r.end() );
-  std::ranges::advance(iter, -222, r.begin());
+  VERIFY( diff == 97 );
+  diff = std::ranges::advance(iter, -222, r.begin());
   VERIFY( iter == r.begin() );
+  VERIFY( diff == -220 );
 }
 
 void
@@ -77,10 +79,12 @@ test02()
   std::ranges::advance(iter, r.begin());
   VERIFY( iter == r.begin() );
 
-  std::ranges::advance(iter, 99, r.end());
+  auto diff = std::ranges::advance(iter, 99, r.end());
   VERIFY( iter == r.end() );
-  std::ranges::advance(iter, -222, r.begin());
+  VERIFY( diff == 97 );
+  diff = std::ranges::advance(iter, -222, r.begin());
   VERIFY( iter == r.begin() );
+  VERIFY( diff == -220 );
 }
 
 void
@@ -108,15 +112,19 @@ test03()
   std::ranges::advance(iter, r.end());
   VERIFY( iter == r.end() );
 
-  std::ranges::advance(iter, 99, r.end());
+  auto diff = std::ranges::advance(iter, 99, r.end());
   VERIFY( iter == r.end() );
-  std::ranges::advance(iter, 99, r.end());
+  VERIFY( diff == 99 ); // PR libstdc++/100833
+  diff = std::ranges::advance(iter, 99, r.end());
   VERIFY( iter == r.end() );
+  VERIFY( diff == 99 );
   iter = r.begin();
-  std::ranges::advance(iter, 99, r.end());
+  diff = std::ranges::advance(iter, 99, r.end());
   VERIFY( iter == r.end() );
-  std::ranges::advance(iter, 99, r.end());
+  VERIFY( diff == 97 );
+  diff = std::ranges::advance(iter, 99, r.end());
   VERIFY( iter == r.end() );
+  VERIFY( diff == 99 );
 }
 
 void
@@ -145,17 +153,21 @@ test04()
   std::ranges::advance(iter, r2.end());
   VERIFY( iter == r2.end() );
 
-  std::ranges::advance(iter, 99, r2.end());
+  auto diff = std::ranges::advance(iter, 99, r2.end());
   VERIFY( iter == r2.end() );
-  std::ranges::advance(iter, 99, r2.end());
+  VERIFY( diff == 99 );
+  diff = std::ranges::advance(iter, 99, r2.end());
   VERIFY( iter == r2.end() );
+  VERIFY( diff == 99 );
 
   test_range<int, input_iterator_wrapper> r3(a);
   iter = r3.begin();
-  std::ranges::advance(iter, 99, r3.end());
+  diff = std::ranges::advance(iter, 99, r3.end());
   VERIFY( iter == r3.end() );
-  std::ranges::advance(iter, 99, r3.end());
+  VERIFY( diff == 97 );
+  diff = std::ranges::advance(iter, 99, r3.end());
   VERIFY( iter == r3.end() );
+  VERIFY( diff == 99 );
 }
 
 void
@@ -180,17 +192,21 @@ test05()
   std::ranges::advance(iter, r2.end());
   VERIFY( iter == r2.end() );
 
-  std::ranges::advance(iter, 99, r2.end());
+  auto diff = std::ranges::advance(iter, 99, r2.end());
   VERIFY( iter == r2.end() );
-  std::ranges::advance(iter, 99, r2.end());
+  VERIFY( diff == 99 );
+  diff = std::ranges::advance(iter, 99, r2.end());
   VERIFY( iter == r2.end() );
+  VERIFY( diff == 99 );
 
   test_range<int, output_iterator_wrapper> r3(a);
   iter = r3.begin();
-  std::ranges::advance(iter, 99, r3.end());
+  diff = std::ranges::advance(iter, 99, r3.end());
   VERIFY( iter == r3.end() );
-  std::ranges::advance(iter, 99, r3.end());
+  VERIFY( diff == 97 );
+  diff = std::ranges::advance(iter, 99, r3.end());
   VERIFY( iter == r3.end() );
+  VERIFY( diff == 99 );
 }
 
 int