[committed] libstdc++: Improve diagnostics for misuses of output iterators

[Jonathan Wakely]

This adds deleted overloads so that the errors for invalid uses of
std::advance and std::distance are easier to understand (see for example
PR 102181).

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator_base_funcs.h (__advance): Add
	deleted overload to improve diagnostics.
	(__distance): Likewise.

Tested powerpc64le-linux. Committed to trunk.

-------------- next part --------------
commit d9dfd7ad3e0196f60a3fc6df6d65a40fb905409f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 29 21:19:36 2021

    libstdc++: Improve diagnostics for misuses of output iterators
    
    This adds deleted overloads so that the errors for invalid uses of
    std::advance and std::distance are easier to understand (see for example
    PR 102181).
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_iterator_base_funcs.h (__advance): Add
            deleted overload to improve diagnostics.
            (__distance): Likewise.

diff --git a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
index e5afab7f4fd..fc6e9880de3 100644
--- a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
+++ b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
@@ -119,6 +119,13 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 	       input_iterator_tag);
 #endif
 
+#if __cplusplus >= 201103L
+  // Give better error if std::distance called with a non-Cpp17InputIterator.
+  template<typename _OutputIterator>
+    void
+    __distance(_OutputIterator, _OutputIterator, output_iterator_tag) = delete;
+#endif
+
   /**
    *  @brief A generalization of pointer arithmetic.
    *  @param  __first  An input iterator.
@@ -186,6 +193,13 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 	__i += __n;
     }
 
+#if __cplusplus >= 201103L
+  // Give better error if std::advance called with a non-Cpp17InputIterator.
+  template<typename _OutputIterator, typename _Distance>
+    void
+    __advance(_OutputIterator&, _Distance, output_iterator_tag) = delete;
+#endif
+
   /**
    *  @brief A generalization of pointer arithmetic.
    *  @param  __i  An input iterator.