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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug analyzer/106385] New: Support for std::optional in -fanalyzer
Date: Thu, 21 Jul 2022 16:26:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106385

            Bug ID: 106385
           Summary: Support for std::optional in -fanalyzer
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 97110
  Target Milestone: ---

Check that std::optional::has_value() (or equivalent) is checked before
accessing the contained value of a std::optional.

See
https://devblogs.microsoft.com/cppblog/new-stdoptional-checks-in-visual-stu=
dio-2022-version-17-3-preview-3/

std::optional either contains a value, or is empty. Some accessors for the
value are safe and will throw if no value is present, some have a precondit=
ion
that a value is present and do not check. The analyzer could flag when the
unchecked accessor is used without a preceeding check for a value.

void f(std::optional<int> o)
{
  int i =3D 0;

  if (o.has_value())
    i =3D *o; // safe
  if (o) // conversion to bool, equivalent to has_value()
    i =3D *o; // safe

  i =3D o.value_or(1);  // safe

  if (rand() % 2)
    i =3D o.value(); // safe, throws if no value
  else
    i =3D *o; // unsafe!

  i =3D *o; // "safe" because we already accessed it once.
}


Similarly for std::expected, which is a union of two types with accessors f=
or
the result value or error value, only one of which is present. Some accesso=
rs
are safe and will throw an exception, others are unchecked and have
preconditions.

We don't need to do this for std::variant, as there are no unchecked access=
ors
for it (std::get will throw if the alternative you ask for isn't active, and
std::visit handles any alternative being active).


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97110
[Bug 97110] [meta-bug] tracker bug for supporting C++ in -fanalyzer=