Bug 98675 - Accessing member of temporary outside its lifetime allowed in constexpr function
Summary: Accessing member of temporary outside its lifetime allowed in constexpr function
Status: NEW
Alias: None
Product: gcc
Classification: Unclassified
Component: c++ (show other bugs)
Version: 10.2.1
: P3 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
URL:
Keywords: accepts-invalid
: 109991 (view as bug list)
Depends on:
Blocks: constexpr
  Show dependency treegraph
 
Reported: 2021-01-14 11:53 UTC by Jonathan Wakely
Modified: 2023-07-26 01:45 UTC (History)
4 users (show)

See Also:
Host:
Target:
Build:
Known to work:
Known to fail:
Last reconfirmed: 2021-01-14 00:00:00


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Description Jonathan Wakely 2021-01-14 11:53:09 UTC
struct X {
    int i = 0;

    constexpr int& get() { return i; }
};

constexpr int g()
{
    const auto& i = X{}.get();
    return i;  // undefined
}

constexpr auto i = g();


G++ fails to diagnose the error here.

Clang says:

<source>:13:16: error: constexpr variable 'i' must be initialized by a constant expression
constexpr auto i = g();
               ^   ~~~
<source>:10:12: note: read of object outside its lifetime is not allowed in a constant expression
    return i;  // undefined
           ^
<source>:13:20: note: in call to 'g()'
constexpr auto i = g();
                   ^


Intel says:

<source>(13): error: expression must have a constant value
  constexpr auto i = g();
                     ^
<source>(10): note: attempt to access expired storage
      return i;  // undefined
             ^
Comment 1 Marek Polacek 2021-01-14 14:31:53 UTC
Confirmed.
Comment 2 Tobias Schlüter 2022-01-31 04:23:04 UTC
Here's another testcase (derived from code in the Eigen library ) that seems to illustrate the same issue. Please follow the compiler explorer link to see a non-constexpr version and its gimple that makes clear that there indeed is a lifetime issue and also clang's error message https://godbolt.org/z/4zoKaoKa5

struct B;

#define CONSTEXPR constexpr
#define CONSTEVAL consteval

struct A {
public:
    CONSTEXPR A() { m_val = 1; }
    CONSTEXPR A(const A& other) { m_val = other.m_val;}
    CONSTEXPR ~A() {};

    CONSTEXPR int val() { return m_val; }

    int m_val;

    CONSTEXPR B operator<<(int);
};

struct B {
    A& m_a;
    CONSTEXPR B(A& ref) : m_a(ref) {}
    CONSTEXPR B& operator,(int i) { m_a.m_val = i; return *this; }
    CONSTEXPR ~B() { finished(); }
    CONSTEXPR A finished() { return m_a; }
};

CONSTEXPR B A::operator<<(int i) {
    m_val = i;
    return B(*this);
}

CONSTEVAL int f()
{
    A a = (A() << 1, 2, 3, 4).finished();
    return a.val();
}

int g()
{
    return f();
}
Comment 3 Tobias Schlüter 2022-01-31 04:35:21 UTC
Sorry, in my example, I think actually clang is wrong.
Comment 4 Andrew Pinski 2022-01-31 04:43:06 UTC
(In reply to Tobias Schlüter from comment #3)
> Sorry, in my example, I think actually clang is wrong.

What is the order of destruction of tempories here in the following statement:
A() << 1

Is A() destoryed before the temp B that is returned from operator <<.

Note MSVC accepts the code so it might be a bug in clang.
Comment 5 Tobias Schlüter 2022-01-31 11:44:28 UTC
I've submitted this to clang's bug tracker as well.  https://github.com/llvm/llvm-project/issues/53494
Comment 6 Andrew Pinski 2023-05-26 15:00:38 UTC
*** Bug 109991 has been marked as a duplicate of this bug. ***
Comment 7 GCC Commits 2023-07-26 01:45:41 UTC
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:9fdbd7d6fa5e0a76898dd66658934e3184111680

commit r14-2773-g9fdbd7d6fa5e0a76898dd66658934e3184111680
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sun Jul 23 01:15:14 2023 +1000

    c++: Track lifetimes in constant evaluation [PR70331,PR96630,PR98675]
    
    This adds rudimentary lifetime tracking in C++ constexpr contexts,
    allowing the compiler to report errors with using values after their
    backing has gone out of scope. We don't yet handle other ways of
    accessing values outside their lifetime (e.g. following explicit
    destructor calls).
    
            PR c++/96630
            PR c++/98675
            PR c++/70331
    
    gcc/cp/ChangeLog:
    
            * constexpr.cc (constexpr_global_ctx::is_outside_lifetime): New
            function.
            (constexpr_global_ctx::get_value): Don't return expired values.
            (constexpr_global_ctx::get_value_ptr): Likewise.
            (constexpr_global_ctx::remove_value): Mark value outside
            lifetime.
            (outside_lifetime_error): New function.
            (cxx_eval_call_expression): No longer track save_exprs.
            (cxx_eval_loop_expr): Likewise.
            (cxx_eval_constant_expression): Add checks for outside lifetime
            values. Remove local variables at end of bind exprs, and
            temporaries after cleanup points.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/constexpr-lifetime1.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime2.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime3.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime4.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime5.C: New test.
            * g++.dg/cpp1y/constexpr-lifetime6.C: New test.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>