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Re: [v3 PATCH] Implement LWG 2758.


On 16/08/16 12:52 +0100, Jonathan Wakely wrote:
On 10/08/16 10:50 +0300, Ville Voutilainen wrote:
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 59f1c64..89e2100 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -1227,9 +1227,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
      *  @param __n   The number of characters to append from the string_view.
      *  @return  Reference to this string.
      */
-      basic_string& append(__sv_type __sv,
+      template <typename _Tp,
+		enable_if_t<is_convertible_v<const _Tp&, __sv_type>,
+			    bool> = true>
+      basic_string& append(const _Tp& __svt,
			   size_type __pos, size_type __n = npos)

I would prefer the enable_if constraint to be on the return type, so
there is only one template parameter:

    template <typename _Tp>
      enable_if_t<is_convertible_v<const _Tp&, __sv_type>, basic_string&>
      append(const _Tp& __svt,
             size_type __pos, size_type __n = npos)

However these function templates appear to have a design problem, see
PR 77264.


This patch constrains the new overloads using:

 template<typename _Tp, typename _Res>
   using _If_sv = enable_if_t<
     __and_<is_convertible<const _Tp&, __sv_type>,
	     __not_<is_convertible<const _Tp&, const _CharT*>>>::value,
     _Res>;

so that anything convertible to const _CharT* goes to the same
overload as would have been chosen prior to C++17.

I'm testing this now but won't commit it until it's clear this is a
sensible resolution for LWG 2758.

commit 80e14013c4581fba08099f7a627c63cebc2a378e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Aug 16 13:50:55 2016 +0100

    PR 77264 constrain new std::basic_string overloads
    
    	PR libstdc++/77264
    	* include/bits/basic_string.h (operator=(__sv_type)
    	(append(__sv_type), assign(__sv_type), insert(size_type, __sv_type))
    	(replace(size_type, size_type, __sv_type))
    	(replace(const_iterator, const_iterator, __sv_type))
    	(find(__sv_type, size_type), rfind(__sv_type, size_type))
    	(compare(size_type, size_type, __sv_type)): Reformat.
    	(_If_sv): Define helper for SFINAE constaints.
    	(append(const _Tp&, size_type, size_type))
    	(assign(const _Tp&, size_type, size_type))
    	(insert(size_type, const _Tp&, size_type, size_type))
    	(replace(size_type, size_type, const _Tp&, size_type, size_type)):
    	Use _If_sv.
    	* testsuite/21_strings/basic_string/modifiers/replace/char/7.cc: Test
    	SFINAE constraints.
    	* testsuite/21_strings/basic_string/modifiers/replace/wchar_t/7.cc:
    	Likewise.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 89e2100..68cfc99 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -709,7 +709,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @brief  Set value to string constructed from a string_view.
        *  @param  __sv  A string_view.
        */
-      basic_string& operator=(__sv_type __sv)
+      basic_string&
+      operator=(__sv_type __sv)
       {	return this->assign(__sv); }
 
       /**
@@ -1217,9 +1218,16 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __sv  The string_view to be appended.
        *  @return  Reference to this string.
        */
-      basic_string& append(__sv_type __sv)
+      basic_string&
+      append(__sv_type __sv)
       { return this->append(__sv.data(), __sv.size()); }
 
+      template<typename _Tp, typename _Res>
+	using _If_sv = enable_if_t<
+	  __and_<is_convertible<const _Tp&, __sv_type>,
+		 __not_<is_convertible<const _Tp&, const _CharT*>>>::value,
+	  _Res>;
+
       /**
        *  @brief  Append a range of characters from a string_view.
        *  @param __sv  The string_view to be appended from.
@@ -1227,17 +1235,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __n   The number of characters to append from the string_view.
        *  @return  Reference to this string.
        */
-      template <typename _Tp,
-		enable_if_t<is_convertible_v<const _Tp&, __sv_type>,
-			    bool> = true>
-      basic_string& append(const _Tp& __svt,
-			   size_type __pos, size_type __n = npos)
-      {
-	__sv_type __sv = __svt;
-	return _M_append(__sv.data()
-			 + __sv._M_check(__pos, "basic_string::append"),
-			 __sv._M_limit(__pos, __n));
-      }
+      template <typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	append(const _Tp& __svt, size_type __pos, size_type __n = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return _M_append(__sv.data()
+			   + __sv._M_check(__pos, "basic_string::append"),
+			   __sv._M_limit(__pos, __n));
+	}
 #endif // C++17
 
       /**
@@ -1386,7 +1392,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __sv  The source string_view.
        *  @return  Reference to this string.
        */
-      basic_string& assign(__sv_type __sv)
+      basic_string&
+      assign(__sv_type __sv)
       {	return this->assign(__sv.data(), __sv.size()); }
 
       /**
@@ -1396,18 +1403,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __n  The number of characters to assign.
        *  @return  Reference to this string.
        */
-      template <typename _Tp,
-		enable_if_t<is_convertible_v<const _Tp&, __sv_type>,
-			    bool> = true>
-      basic_string&
-      assign(const _Tp& __svt,
-	     size_type __pos, size_type __n = npos)
-      {
-	__sv_type __sv = __svt;
-	return _M_replace(size_type(0), this->size(), __sv.data()
-			  + __sv._M_check(__pos, "basic_string::assign"),
-			  __sv._M_limit(__pos, __n));
-      }
+      template <typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	assign(const _Tp& __svt, size_type __pos, size_type __n = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return _M_replace(size_type(0), this->size(), __sv.data()
+			    + __sv._M_check(__pos, "basic_string::assign"),
+			    __sv._M_limit(__pos, __n));
+	}
 #endif // C++17
 
 #if __cplusplus >= 201103L
@@ -1647,8 +1651,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __sv   The string_view to insert.
        *  @return  Reference to this string.
       */
-      basic_string& insert(size_type __pos,
-			   __sv_type __sv)
+      basic_string&
+      insert(size_type __pos, __sv_type __sv)
       {	return this->insert(__pos, __sv.data(), __sv.size()); }
 
       /**
@@ -1660,17 +1664,16 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __n    The number of characters to insert.
        *  @return  Reference to this string.
       */
-      template <typename _Tp,
-		enable_if_t<is_convertible_v<const _Tp&, __sv_type>,
-			    bool> = true>
-      basic_string& insert(size_type __pos1, const _Tp& __svt,
-			   size_type __pos2, size_type __n = npos)
-      {
-	__sv_type __sv = __svt;
-	return this->replace(__pos1, size_type(0), __sv.data()
-			     + __sv._M_check(__pos2, "basic_string::insert"),
-			     __sv._M_limit(__pos2, __n));
-      }
+      template <typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	insert(size_type __pos1, const _Tp& __svt,
+	       size_type __pos2, size_type __n = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return this->replace(__pos1, size_type(0), __sv.data()
+			       + __sv._M_check(__pos2, "basic_string::insert"),
+			       __sv._M_limit(__pos2, __n));
+	}
 #endif // C++17
 
       /**
@@ -2070,8 +2073,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __sv  The string_view to insert.
        *  @return  Reference to this string.
       */
-      basic_string& replace(size_type __pos, size_type __n,
-			    __sv_type __sv)
+      basic_string&
+      replace(size_type __pos, size_type __n, __sv_type __sv)
       {	return this->replace(__pos, __n, __sv.data(), __sv.size()); }
 
       /**
@@ -2083,18 +2086,16 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __n2    The number of characters to insert.
        *  @return  Reference to this string.
       */
-      template <typename _Tp,
-		enable_if_t<is_convertible_v<const _Tp&, __sv_type>,
-			    bool> = true>
-      basic_string& replace(size_type __pos1, size_type __n1,
-			    const _Tp& __svt,
-			    size_type __pos2, size_type __n2 = npos)
-      {
-	__sv_type __sv = __svt;
-	return this->replace(__pos1, __n1, __sv.data()
-			     + __sv._M_check(__pos2, "basic_string::replace"),
-			     __sv._M_limit(__pos2, __n2));
-      }
+      template <typename _Tp>
+	_If_sv<_Tp, basic_string&>
+	replace(size_type __pos1, size_type __n1, const _Tp& __svt,
+		size_type __pos2, size_type __n2 = npos)
+	{
+	  __sv_type __sv = __svt;
+	  return this->replace(__pos1, __n1, __sv.data()
+			       + __sv._M_check(__pos2, "basic_string::replace"),
+			       __sv._M_limit(__pos2, __n2));
+	}
 
       /**
        *  @brief  Replace range of characters with string_view.
@@ -2105,8 +2106,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __sv    The string_view to insert from.
        *  @return  Reference to this string.
       */
-      basic_string& replace(const_iterator __i1, const_iterator __i2,
-			    __sv_type __sv)
+      basic_string&
+      replace(const_iterator __i1, const_iterator __i2, __sv_type __sv)
       {	return this->replace(__i1 - begin(), __i2 - __i1, __sv); }
 #endif // C++17
 
@@ -2241,8 +2242,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Index of start of first occurrence.
       */
       size_type
-      find(__sv_type __sv,
-	    size_type __pos = 0) const noexcept
+      find(__sv_type __sv, size_type __pos = 0) const noexcept
       {	return this->find(__sv.data(), __pos, __sv.size()); }
 #endif // C++17
 
@@ -2299,8 +2299,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Index of start of last occurrence.
       */
       size_type
-      rfind (__sv_type __sv,
-	     size_type __pos = npos) const noexcept
+      rfind(__sv_type __sv, size_type __pos = npos) const noexcept
       {	return this->rfind(__sv.data(), __pos, __sv.size()); }
 #endif // C++17
 
@@ -2721,11 +2720,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @return  Integer < 0, 0, or > 0.
        */
       int
-      compare(size_type __pos, size_type __n,
-	      __sv_type __sv) const
-      {
-	return __sv_type(*this).substr(__pos, __n).compare(__sv);
-      }
+      compare(size_type __pos, size_type __n, __sv_type __sv) const
+      { return __sv_type(*this).substr(__pos, __n).compare(__sv); }
 
       /**
        *  @brief  Compare to a string_view.
@@ -2736,17 +2732,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        *  @param __n2  The number of characters to compare.
        *  @return  Integer < 0, 0, or > 0.
        */
-      template <typename _Tp,
-		enable_if_t<is_convertible_v<const _Tp&, __sv_type>,
-			    bool> = true>
-      int compare(size_type __pos1, size_type __n1,
-		  const _Tp& __svt,
-		  size_type __pos2, size_type __n2 = npos) const
-      {
-	__sv_type __sv = __svt;
-	return __sv_type(*this)
-	  .substr(__pos1, __n1).compare(__sv.substr(__pos2, __n2));
-      }
+      template <typename _Tp>
+	_If_sv<_Tp, int>
+	compare(size_type __pos1, size_type __n1, const _Tp& __svt,
+		size_type __pos2, size_type __n2 = npos) const
+	{
+	  __sv_type __sv = __svt;
+	  return __sv_type(*this)
+	    .substr(__pos1, __n1).compare(__sv.substr(__pos2, __n2));
+	}
 #endif // C++17
 
       /**
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/char/7.cc b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/char/7.cc
index 004b235..2a2858b 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/char/7.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/char/7.cc
@@ -47,8 +47,30 @@ test03()
   VERIFY (str2 == "foofoo");
 }
 
+// PR libstdc++/77264
+void
+test04()
+{
+  bool test __attribute__((unused)) = true;
+
+  std::string str("a");
+  char c = 'b';
+  str.replace(0, 1, &c, 1);
+  VERIFY (str[0] == 'b');
+
+  struct S {
+    operator char*() { return &c; }
+    operator std::string_view() { return "!"; }
+    char c = 'a';
+  };
+
+  str.replace(0, 1, S{}, 1);
+  VERIFY (str[0] == 'a');
+}
+
 int main()
 { 
   test03();
+  test04();
   return 0;
 }
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/wchar_t/7.cc b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/wchar_t/7.cc
index 88d2666..1e2505e 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/wchar_t/7.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/replace/wchar_t/7.cc
@@ -47,8 +47,30 @@ test03()
   VERIFY (str2 == L"foofoo");
 }
 
+// PR libstdc++/77264
+void
+test04()
+{
+  bool test __attribute__((unused)) = true;
+
+  std::wstring str(L"a");
+  wchar_t c = L'b';
+  str.replace(0, 1, &c, 1);
+  VERIFY (str[0] == L'b');
+
+  struct S {
+    operator wchar_t*() { return &c; }
+    operator std::wstring_view() { return L"!"; }
+    wchar_t c = L'a';
+  };
+
+  str.replace(0, 1, S{}, 1);
+  VERIFY (str[0] == L'a');
+}
+
 int main()
 { 
   test03();
+  test04();
   return 0;
 }

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