Make Q_ASSERT() usable in constexpr functions

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>

1 files changed, 6 insertions, 5 deletions

diff --git a/src/corelib/tools/qstring.h b/src/corelib/tools/qstring.h
index 37f32c8c43..d4a4f23c93 100644
--- a/src/corelib/tools/qstring.h
+++ b/src/corelib/tools/qstring.h
@@ -100,7 +100,8 @@ public:
     Q_DECL_CONSTEXPR bool isNull() const Q_DECL_NOTHROW { return !data(); }
     Q_DECL_CONSTEXPR bool isEmpty() const Q_DECL_NOTHROW { return !size(); }
 
-    Q_DECL_CONSTEXPR QLatin1Char at(int i) const { return QLatin1Char(m_data[i]); }
+    Q_DECL_CONSTEXPR QLatin1Char at(int i) const
+    { return Q_ASSERT(i >= 0), Q_ASSERT(i < size()), QLatin1Char(m_data[i]); }
     Q_DECL_CONSTEXPR QLatin1Char operator[](int i) const { return at(i); }
 
     using value_type = const char;
@@ -125,13 +126,13 @@ public:
     const_reverse_iterator crend() const Q_DECL_NOTHROW { return const_reverse_iterator(begin()); }
 
     Q_DECL_CONSTEXPR QLatin1String mid(int pos) const
-    { return QLatin1String(m_data + pos, m_size - pos); }
+    { return Q_ASSERT(pos >= 0), Q_ASSERT(pos <= size()), QLatin1String(m_data + pos, m_size - pos); }
     Q_DECL_CONSTEXPR QLatin1String mid(int pos, int n) const
-    { return QLatin1String(m_data + pos, n); }
+    { return Q_ASSERT(pos >= 0), Q_ASSERT(n >= 0), Q_ASSERT(pos + n <= size()), QLatin1String(m_data + pos, n); }
     Q_DECL_CONSTEXPR QLatin1String left(int n) const
-    { return QLatin1String(m_data, n); }
+    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data, n); }
     Q_DECL_CONSTEXPR QLatin1String right(int n) const
-    { return QLatin1String(m_data + m_size - n, n); }
+    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data + m_size - n, n); }
 
     inline bool operator==(const QString &s) const Q_DECL_NOTHROW;
     inline bool operator!=(const QString &s) const Q_DECL_NOTHROW;