[Bug c++/109232] New: Using deduced return type in an unevaluated context leads to codegen

[Bug c++/109232] New: Using deduced return type in an unevaluated context leads to codegen

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109232

            Bug ID: 109232
           Summary: Using deduced return type in an unevaluated context
                    leads to codegen
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com
  Target Milestone: ---

auto begin(auto&& r) {
    return r.begin();
}

namespace {
    struct R {
        int* begin();
    };
}

static_assert(__is_same(decltype(begin(R())), int*));
int main() {}

Even though ::begin is only used in an unevaluated context, GCC at -O0
generates code for begin<R>, leading to a warning and a failure to link
(https://gcc.godbolt.org/z/cdajoP7f3):

<source>:10:14: warning: 'int* {anonymous}::R::begin()' used but never defined
   10 |         int* begin();
      |              ^~~~~
/opt/compiler-explorer/gcc-trunk-20230320/bin/../lib/gcc/x86_64-linux-gnu/13.0.1/../../../../x86_64-linux-gnu/bin/ld:
/tmp/cc7gqEhh.o: in function `auto begin<(anonymous namespace)::R&>((anonymous
namespace)::R&)':
<source>:2: undefined reference to `(anonymous namespace)::R::begin()'
collect2: error: ld returned 1 exit status
Execution build compiler returned: 1

[Bug c++/109232] Using deduced return type in an unevaluated context leads to codegen

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109232

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |link-failure

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
maybe it's implementation defined or unspecified whether instantiation happens?

[Bug c++/109232] Using deduced return type in an unevaluated context leads to codegen

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109232

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang does not emit the function but does emit the warning ...

[Bug c++/109232] Using deduced return type in an unevaluated context leads to codegen

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109232

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
What seems to be happening is determine_visibility correctly notices that
begin<R> should have internal linkage due to R having internal linkage, so it
proceeds to clear TREE_PUBLIC, DECL_COMDAT, DECL_EXTERN, etc for begin<R>.

Later in cgraph_node::finalize for begin<R>, we force its definition to be
outputted due to

  /* When not optimizing, also output the static functions. (see
     PR24561), but don't do so for always_inline functions, functions
     declared inline and nested functions.  These were optimized out
     in the original implementation and it is unclear whether we want
     to change the behavior here.  */
  if (((!opt_for_fn (decl, optimize) || flag_keep_static_functions
        || node->no_reorder)
       && !node->cpp_implicit_alias
       && !DECL_DISREGARD_INLINE_LIMITS (decl)
       && !DECL_DECLARED_INLINE_P (decl)
       && !(DECL_CONTEXT (decl)
            && TREE_CODE (DECL_CONTEXT (decl)) == FUNCTION_DECL))
      && !DECL_COMDAT (decl) && !DECL_EXTERNAL (decl))
    node->force_output = 1;

I guess we need to somehow refine the above test to exclude implicit
instantiations with internal linkage?

[Bug c++/109232] Using deduced return type in an unevaluated context leads to codegen

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109232

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Here's another perhaps simpler version that explicitly gives begin<R> internal
linkage, which also fails to link:

namespace {
    template<class T>
    auto begin(T&& r) {
        return r.begin();
    }
}

struct R {
    int* begin();
};

static_assert(__is_same(decltype(begin(R())), int*));
int main() { }

[Bug c++/109232] Using deduced return type in an unevaluated context leads to codegen

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109232

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
So one workaround is to explicitly declare begin inline, which works because
the problematic test in cgraph_node::finalize_function excludes
DECL_DECLARED_INLINE_P functions.