QFile::exists result reused across an unknown call

[Long Huang]

cfg/qt.cfg marks QFile::exists as <pure/>. When the argument is a local const QString, cppcheck reuses the first call's result after a call to an unknown function, and then reports a later identical call as always true. But QFile::exists reads filesystem state, which any unknown call (or another
process) can change, so the second result is not known.

Either QFile::exists (and probably QFileInfo::exists/QDir::exists) should not be <pure/>, or value flow should drop a pure call's known result after a call to an unknown function.

Reproduction

Save as pure-exists.cpp:

#include <QFile>
#include <QString>

void RemoveFile(const QString& path);

void Check()
{
   const QString path("x");
   if(!QFile::exists(path)) throw false;
   RemoveFile(path);
   if(QFile::exists(path))
   {
      throw 1;
   }
}

Actual:

pure-exists.cpp:11:knownConditionTrueFalse:Condition 'QFile::exists(path)' is always true

Expected:

No warning. RemoveFile is unknown and may delete the file, so the second QFile::exists(path) can return false.

Note:

• With path as a const QString& function parameter instead of a local, no warning is emitted.
• In googletest code the first call is usually ASSERT_TRUE(QFile::exists(p)). googletest.cfg expands it to {if (!(cond)) throw false;}, which produces the same pattern. A following retry loop such asfor(...; retries < 10 && !deleted; ...) { QTest::qWait(10); deleted = !QFile::exists(p); } is then reported as Condition '!deleted' is always true.

[CHR]

Thanks for reporting, ticket is here: https://trac.cppcheck.net/ticket/15173