Don't clobber std::string_view equality with char *

Make the wxString(std::string_view) constructor explicit.

Otherwise, when comparing a std::string_view with a const char *, the
cast to wxString will be considered as a candidate for the comparison,
ultimately causing an "ambiguous overload for 'operator=='" error.

For example, this sample only builds if the constructor is explicit:

  #include <wx/string.h>
  #include <string_view>

  int main() {
    std::string_view view = "abc";
    const char *str = "abc";
    return view == str;
  }

However, making the constructor explicit will break assignment:

    std::string_view view = "abc";
    wxString s;
    s = view; // Error: no match for "operator="

That we can fix by implementing operator=(std::string_view)

That, however, introduces another ambiguity:

    std::string str = "abc";
    wxString s;
    s = str; // Ambiguous between s = wxString(str)
                              and s = std::string_view(str)

That we can fix by implementing operator=(std::string)

Finally, note that some rather obscure ambiguities remain, such as:

    wxString s;
    s = {"abc", 2}; // Ambiguous between s = wxString("abc", 2)
                                     and s = std::string_view("abc", 2)

Avoiding them is not simple (https://cplusplus.github.io/LWG/issue2946)
and doesn't add much value.

Closes #23834.

docs/changes.txt

@@ -108,6 +108,11 @@ Changes in behaviour which may result in build errors
   compatible with the previous wxWidgets versions, but now compare values, and
   not pointers, in their Index() member function.
 
+- Due to the possibility to construct wxString from std::string_view some
+  previously valid code, such as "wxstr = {"Hello", 2}", is now ambiguous.
+  Please use explicit class name, e.g. "wxstr = wxString{"Hello", 2}" to
+  preserve the previous behaviour.
+
 - Generic wxSearchCtrl doesn't provide methods that make sense only for
   multiline text controls any longer, for consistency with the other ports.
 

include/wx/string.h

@@ -1263,7 +1263,7 @@ public:
   #endif
 
 #ifdef wxHAS_STD_STRING_VIEW
-    wxString(std::wstring_view view)
+    explicit wxString(std::wstring_view view)
         { assign(view.data(), view.length()); }
 #endif  // wxHAS_STD_STRING_VIEW
 
@@ -1272,7 +1272,7 @@ public:
       { assign(str.c_str(), str.length()); }
 
     #ifdef wxHAS_STD_STRING_VIEW
-        wxString(std::string_view view)
+        explicit wxString(std::string_view view)
             { assign(view.data(), view.length()); }
     #endif  // wxHAS_STD_STRING_VIEW
 #endif // wxNO_IMPLICIT_WXSTRING_ENCODING
@@ -1883,6 +1883,29 @@ public:
     { return assign(s); }
 #endif // wxNO_IMPLICIT_WXSTRING_ENCODING
 
+#if wxUSE_UNICODE_WCHAR
+  wxString& operator=(const std::wstring& str) { m_impl = str; return *this; }
+  wxString& operator=(std::wstring&& str) noexcept { m_impl = std::move(str); return *this; }
+#else // wxUSE_UNICODE_UTF8
+  wxString& operator=(const std::wstring& str)
+    { return assign(str.c_str(), str.length()); }
+#endif
+
+#ifdef wxHAS_STD_STRING_VIEW
+  wxString& operator=(std::wstring_view view)
+    { return assign(view.data(), view.length()); }
+#endif  // wxHAS_STD_STRING_VIEW
+
+#ifndef wxNO_IMPLICIT_WXSTRING_ENCODING
+  wxString& operator=(const std::string& str)
+    { return assign(str.c_str(), str.length()); }
+
+    #ifdef wxHAS_STD_STRING_VIEW
+      wxString& operator=(std::string_view view)
+        { return assign(view.data(), view.length()); }
+    #endif  // wxHAS_STD_STRING_VIEW
+#endif // wxNO_IMPLICIT_WXSTRING_ENCODING
+
   // string concatenation
     // in place concatenation
     /*

tests/strings/stdstrings.cpp

@@ -690,5 +690,14 @@ TEST_CASE("StdString::View", "[stdstring]")
     std::string_view strViewInvalidUTF(strInvalidUTF);
 
     CHECK( "" == wxString::FromUTF8(strViewInvalidUTF) );
+
+    /* Ensure we don't clobber comparisons on base types */
+    std::string_view view = "abc";
+    const char *str = "abc";
+    CHECK( view == str );
+
+    std::wstring_view wview = L"abc";
+    const wchar_t *wstr = L"abc";
+    CHECK( wview == wstr );
 }
 #endif // wxHAS_STD_STRING_VIEW