<spanstream>: Fix ambiguity among constructors of basic_ispanstream (#5309)

Author: frederick-vs-ja

stl/inc/spanstream

@@ -190,6 +190,11 @@ void swap(basic_spanbuf<_Elem, _Traits>& _Left, basic_spanbuf<_Elem, _Traits>& _
     _Left.swap(_Right);
 }
 
+template <class _Elem, size_t _Extent>
+span<_Elem, _Extent> _As_nonconst_span(const span<const _Elem, _Extent> _Span) noexcept {
+    return span<_Elem, _Extent>{const_cast<_Elem*>(_Span.data()), _Span.size()};
+}
+
 _EXPORT_STD template <class _Elem, class _Traits>
 class basic_ispanstream : public basic_istream<_Elem, _Traits> {
 private:
@@ -207,9 +212,6 @@ public:
     explicit basic_ispanstream(_STD span<_Elem> _Span, ios_base::openmode _Which = ios_base::in)
         : _Mybase(_STD addressof(_Buf)), _Buf(_Span, _Which | ios_base::in) {}
 
-    explicit basic_ispanstream(_STD span<const _Elem> _Span)
-        : basic_ispanstream(_STD span<_Elem>{const_cast<_Elem*>(_Span.data()), _Span.size()}) {}
-
     basic_ispanstream(const basic_ispanstream&) = delete;
 
     basic_ispanstream(basic_ispanstream&& _Right) : _Mybase(_STD move(_Right)), _Buf(_STD move(_Right._Buf)) {
@@ -220,7 +222,8 @@ public:
         requires (
             !convertible_to<_ReadOnlyRange, _STD span<_Elem>> && convertible_to<_ReadOnlyRange, _STD span<const _Elem>>)
     explicit basic_ispanstream(_ReadOnlyRange&& _Range)
-        : basic_ispanstream(static_cast<_STD span<const _Elem>>(_STD forward<_ReadOnlyRange>(_Range))) {}
+        : basic_ispanstream(
+              _STD _As_nonconst_span(static_cast<_STD span<const _Elem>>(_STD forward<_ReadOnlyRange>(_Range)))) {}
 
     basic_ispanstream& operator=(const basic_ispanstream&) = delete;
 
@@ -254,7 +257,7 @@ public:
             !convertible_to<_ReadOnlyRange, _STD span<_Elem>> && convertible_to<_ReadOnlyRange, _STD span<const _Elem>>)
     void span(_ReadOnlyRange&& _Range) noexcept {
         const auto _Sp = static_cast<_STD span<const _Elem>>(_STD forward<_ReadOnlyRange>(_Range));
-        this->span(_STD span<_Elem>{const_cast<_Elem*>(_Sp.data()), _Sp.size()});
+        rdbuf()->span(_STD _As_nonconst_span(_Sp));
     }
 
 private:

tests/std/tests/P0448R4_spanstream/test.cpp

@@ -674,6 +674,28 @@ void test_ispanstream() {
         assert(static_cast<test_buf*>(special_range_constructed.rdbuf())->epptr() == nullptr);
     }
 
+    { // GH-5308: <spanstream>: Constructing ispanstream from string no longer compiles since VS 17.13
+        basic_string<CharT> str(42, static_cast<CharT>('*'));
+        basic_ispanstream<CharT> from_string_constructed{str};
+        assert(from_string_constructed.span().data() == str.data());
+        assert(static_cast<test_buf*>(from_string_constructed.rdbuf())->eback() == str.data());
+        assert(static_cast<test_buf*>(from_string_constructed.rdbuf())->gptr() == str.data());
+        assert(static_cast<test_buf*>(from_string_constructed.rdbuf())->egptr() == str.data() + str.size());
+        assert(static_cast<test_buf*>(from_string_constructed.rdbuf())->pbase() == nullptr);
+        assert(static_cast<test_buf*>(from_string_constructed.rdbuf())->pptr() == nullptr);
+        assert(static_cast<test_buf*>(from_string_constructed.rdbuf())->epptr() == nullptr);
+
+        basic_ispanstream<CharT> from_string_reset{span<CharT>{}};
+        from_string_reset.span(str);
+        assert(from_string_reset.span().data() == str.data());
+        assert(static_cast<test_buf*>(from_string_reset.rdbuf())->eback() == str.data());
+        assert(static_cast<test_buf*>(from_string_reset.rdbuf())->gptr() == str.data());
+        assert(static_cast<test_buf*>(from_string_reset.rdbuf())->egptr() == str.data() + str.size());
+        assert(static_cast<test_buf*>(from_string_reset.rdbuf())->pbase() == nullptr);
+        assert(static_cast<test_buf*>(from_string_reset.rdbuf())->pptr() == nullptr);
+        assert(static_cast<test_buf*>(from_string_reset.rdbuf())->epptr() == nullptr);
+    }
+
     { // span
         CharT buffer[10];
         basic_ispanstream<CharT> is{span<CharT>{buffer}};