String-like containers: add implicit conversions towards std:: string views

In C++20 std::basic_string_view has gained a range constructor (like
QStringView always had), but that range constructor has been made
explicit. This means we can't just pass a QString(View) to a function
taking a u16string_view. The consensus seems to be that that types that
should implictly convert towards stdlib's string views should do that
via implicit conversion operators. This patch adds them for

* QByteArrayView => std::string_view
* QString(View) => std::u16string_view
* QUtf8StringView => std::string_view or std::u8string_view, depending
  on the storage_type

QLatin1StringView doesn't have a matching std:: view so I'm not enabling
its conversion.

QByteArray poses a challenge, in that it already defines a conversion
towards const char *. (One can disable that conversion with a macro.)
That conversion makes it impossible to support:

  QByteArray ba;
  std::string_view sv1(ba);  // 1
  std::string_view sv2 = ba; // 2

because:

* if only operator const char *() is defined, then (2) doesn't work
  (situation right now);

* if both conversions to const char * and string_view are defined, then
  (1) is ambiguous on certain compilers (MSVC, QCC). Interestingly
  enough, not on GCC/Clang, but only in C++17 and later modes.

I can't kill the conversion towards const char * (API break, and we use
it *everywhere* in Qt), hence, QByteArray does not get the implicit
conversion, at least not in this patch.

[ChangeLog][QtCore][QByteArrayView] Added an implicit conversion
operator towards std::string_view.

[ChangeLog][QtCore][QString] Added an implicit conversion operator
towards std::u16string_view.

[ChangeLog][QtCore][QStringView] Added an implicit conversion operator
towards std::u16string_view.

[ChangeLog][QtCore][QUtf8StringView] Added an implicit conversion
operator towards std::string_view (QUtf8StringView is using char
as its storage type in Qt 6). Note that QUtf8StringView is planned to
use char8_t in Qt 7, therefore it is expected that the conversion will
change towards std::u8string_view in Qt 7.

Change-Id: I6d3b64d211a386241ae157765cd1b03f531f909a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/corelib/text/qbytearrayview.h b/src/corelib/text/qbytearrayview.h
index f822a2ca20..a056c7836e 100644
--- a/src/corelib/text/qbytearrayview.h
+++ b/src/corelib/text/qbytearrayview.h
@@ -301,6 +301,9 @@ public:
     [[nodiscard]] constexpr char front() const { Q_ASSERT(!empty()); return m_data[0]; }
     [[nodiscard]] constexpr char back()  const { Q_ASSERT(!empty()); return m_data[m_size - 1]; }
 
+    [[nodiscard]] Q_IMPLICIT operator std::string_view() const noexcept
+    { return std::string_view(m_data, size_t(m_size)); }
+
     //
     // Qt compatibility API:
     //