QChar: make construction from integral explicit

QChar should not be convertible from any integral type except from
char16_t, short and possibly char (since it's a direct superset).

David provided the perfect example:

  if (str == 123) { ~~~ }

compiles, with 123 implicitly converted to QChar (str == "123"
was meant instead). But similarly one can construct other
scenarios where QString(123) gets accidentally used (instead of
QString::number(123)), like QString s; s += 123;.

Add a macro to revert to the implicit constructors, for backwards
compatibility.

The breaks are mostly in tests that "abuse" of integers (arithmetic,
etc.). Maybe it's time for user-defined literals for QChar/QString,
but that is left for another commit.

[ChangeLog][Potentially Source-Incompatible Changes][QChar] QChar
constructors from integral types are now by default explicit.
It is recommended to use explicit conversions, QLatin1Char,
QChar::fromUcs4 instead of implicit conversions. The old behavior
can be restored by defining the QT_IMPLICIT_QCHAR_CONSTRUCTION
macro.

Change-Id: I6175f6ab9bcf1956f6f97ab0c9d9d5aaf777296d
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 13 insertions, 6 deletions

diff --git a/src/corelib/text/qchar.h b/src/corelib/text/qchar.h
index 982abe9346..f66671e27c 100644
--- a/src/corelib/text/qchar.h
+++ b/src/corelib/text/qchar.h
@@ -101,12 +101,18 @@ public:
         LastValidCodePoint = 0x10ffff
     };
 
+#ifdef QT_IMPLICIT_QCHAR_CONSTRUCTION
+#define QCHAR_MAYBE_IMPLICIT Q_IMPLICIT
+#else
+#define QCHAR_MAYBE_IMPLICIT explicit
+#endif
+
     constexpr Q_IMPLICIT QChar() noexcept : ucs(0) {}
     constexpr Q_IMPLICIT QChar(ushort rc) noexcept : ucs(rc) {}
-    constexpr Q_IMPLICIT QChar(uchar c, uchar r) noexcept : ucs(char16_t((r << 8) | c)) {}
+    constexpr QCHAR_MAYBE_IMPLICIT QChar(uchar c, uchar r) noexcept : ucs(char16_t((r << 8) | c)) {}
     constexpr Q_IMPLICIT QChar(short rc) noexcept : ucs(char16_t(rc)) {}
-    constexpr Q_IMPLICIT QChar(uint rc) noexcept : ucs(char16_t(rc & 0xffff)) {}
-    constexpr Q_IMPLICIT QChar(int rc) noexcept : ucs(char16_t(rc & 0xffff)) {}
+    constexpr QCHAR_MAYBE_IMPLICIT QChar(uint rc) noexcept : ucs(char16_t(rc & 0xffff)) {}
+    constexpr QCHAR_MAYBE_IMPLICIT QChar(int rc) noexcept : ucs(char16_t(rc & 0xffff)) {}
     constexpr Q_IMPLICIT QChar(SpecialCharacter s) noexcept : ucs(char16_t(s)) {} // implicit
     constexpr Q_IMPLICIT QChar(QLatin1Char ch) noexcept : ucs(ch.unicode()) {} // implicit
     constexpr Q_IMPLICIT QChar(char16_t ch) noexcept : ucs(ch) {} // implicit
@@ -114,18 +120,19 @@ public:
     static_assert(sizeof(wchar_t) == sizeof(char16_t));
 #endif
 #if defined(Q_OS_WIN) || defined(Q_CLANG_QDOC)
-#   if !defined(_WCHAR_T_DEFINED) || defined(_NATIVE_WCHAR_T_DEFINED)
     constexpr Q_IMPLICIT QChar(wchar_t ch) noexcept : ucs(char16_t(ch)) {} // implicit
-#   endif
 #endif
 
 #ifndef QT_NO_CAST_FROM_ASCII
+    // Always implicit -- allow for 'x' => QChar conversions
     QT_ASCII_CAST_WARN constexpr Q_IMPLICIT QChar(char c) noexcept : ucs(uchar(c)) { }
 #ifndef QT_RESTRICTED_CAST_FROM_ASCII
-    QT_ASCII_CAST_WARN constexpr Q_IMPLICIT QChar(uchar c) noexcept : ucs(c) { }
+    QT_ASCII_CAST_WARN constexpr QCHAR_MAYBE_IMPLICIT QChar(uchar c) noexcept : ucs(c) { }
 #endif
 #endif
 
+#undef QCHAR_MAYBE_IMPLICIT
+
     static constexpr QChar fromUcs2(char16_t c) noexcept { return QChar{c}; }
     static constexpr inline auto fromUcs4(char32_t c) noexcept;