We need Q_ASSERT in (C++11) constexpr functions, and the only way to
inject them in C++11 is to use the comma operator. E.g. in
QLatin1String:
constexpr QLatin1Char at(int i) const
{ return assert(1 >= 0), assert(i < size()), m_data[i]; }
The main problem with our existing Q_ASSERT is that while it is a
ternary expression in active mode, it was a statement in passive
mode. This is easily fixed by dropping the do-while loop and leaving
just its parenthesized exit condition. Add a cast to void, too,
ensuring that Q_ASSERT has type void in both passive and active modes.
But even in C++14 constexpr functions, which accept several
statements, Q_ASSERT needs to have a path through its conditionals
that is constexpr, but neither qt_assert(_x) nor qt_noop() are
constexpr. Nor can they be in C++11 (no void returns in C++11
constexpr functions). I fixed this by replacing qt_noop() with
static_cast<void>(0). The void cast is required so both 2nd and 3rd
arguments to the ternary are void (mixing void and non-void branches
in the ternary is only allowed if the void leg is a
throw-expression[1]).
As a drive-by, adjust to style guide, remove overparenthesization and
reverse the conditional in the ternary.
Apply it to QLatin1String where we had the problem that constexpr
functions had a narrow constract.
[1] should probably be extended to any [[noreturn]] void function,
e.g. std::terminate().
[ChangeLog][QtCore][QtGlobal] Q_ASSERT() and Q_ASSERT_X() now always
expand to expressions of type void that are usable in constexpr
contexts. This makes them usable in both C++11 and C++14 constexpr
functions.
Change-Id: I09c396bc0034ac344cfaadc6f8cbeb1b7b0cbabc
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
8ea27bb1c6