[BUG] Last use of copy/move/forward in for _expression_ not exercised

[JohelEGP]

Title: Last use of copy/move/forward in for expression not exercised.

Minimal reproducer (https://cpp2.godbolt.org/z/4PE6dTY8f):

f: (copy v: std::vector<int>) = { for v do (_) { } } // Doesn't move.
f: (move v: std::vector<int>) = { for v do (_) { } } // Doesn't move.
f: (forward r) = { for r do (_) { } } // Doesn't forward.
main: () = { }

Commands:

cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -I . main.cpp

Expected result:

auto f(std::vector<int> v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : std::move(v) ) {}}// Doesn't move.
#line 2 "/app/example.cpp2"
auto f(std::vector<int>&& v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : std::move(v) ) {}}// Doesn't move.
auto f(auto&& r) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : CPP2_FORWARD(r) ) {}}// Doesn't forward.

Actual result and error:

auto f(std::vector<int> v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : v ) {}}// Doesn't move.
#line 2 "/app/example.cpp2"
auto f(std::vector<int>&& v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : v ) {}}// Doesn't move.
auto f(auto&& r) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : r ) {}}// Doesn't forward.

Cpp2 lowered to Cpp1:

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"

#line 1 "/app/example.cpp2"


//=== Cpp2 type definitions and function declarations ===========================

#line 1 "/app/example.cpp2"
auto f(std::vector<int> v) -> void;
#line 2 "/app/example.cpp2"
auto f(std::vector<int>&& v) -> void;
auto f(auto&& r) -> void;
auto main() -> int;

//=== Cpp2 function definitions =================================================

#line 1 "/app/example.cpp2"
auto f(std::vector<int> v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : v ) {}}// Doesn't move.
#line 2 "/app/example.cpp2"
auto f(std::vector<int>&& v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : v ) {}}// Doesn't move.
auto f(auto&& r) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : r ) {}}// Doesn't forward.
auto main() -> int{}

Output:

Program returned: 0

See also:

• [BUG] Local copy/forward parameter not moved/forwarded #825.

[JohelEGP]

When I reported this, I think I was hoping to get a container cleared on move.
That can't happen because it initializes a auto&& variable (https://eel.is/c++draft/stmt.ranged).

It uses the variable as an lvalue to access the iterators, so const is already propagated.
Propagating rvalueness does nothing.
Except perhaps not confusing users in rare error messages
where the variable appears declared as an lvalue reference instead of an rvalue reference.
That might led users astray in their development process.

[JohelEGP]

Description:

Even if the expression isn't just the parameter name,
move/forward on last use doesn't happen.

Minimal reproducer (https://cpp2.godbolt.org/z/Tvana63M9):

copy: (copy x) x;
f: (copy v: std::vector<int>) = { for v.copy() do (_) { } } // Doesn't move.
f: (move v: std::vector<int>) = { for v.copy() do (_) { } } // Doesn't move.
f: (forward r) = { for r.copy() do (_) { } } // Doesn't forward.
main: () = { }

Commands:

cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -Werror=unused-value -Werror=unused-parameter -I . main.cpp

Expected result: Move/forward on last use in the expression of the for.

Actual result and error:

auto f(std::vector<int> v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : CPP2_UFCS_0(copy, v) ) {}}// Doesn't move.
auto f(std::vector<int>&& v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : CPP2_UFCS_0(copy, v) ) {}}// Doesn't move.
auto f(auto&& r) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : CPP2_UFCS_0(copy, r) ) {}}// Doesn't forward.
auto main() -> int{}

Cpp2 lowered to Cpp1:

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"

#line 1 "/app/example.cpp2"


//=== Cpp2 type definitions and function declarations ===========================

#line 1 "/app/example.cpp2"
[[nodiscard]] auto copy(auto x) -> auto;
#line 2 "/app/example.cpp2"
auto f(std::vector<int> v) -> void;
auto f(std::vector<int>&& v) -> void;
auto f(auto&& r) -> void;
auto main() -> int;

//=== Cpp2 function definitions =================================================

#line 1 "/app/example.cpp2"
[[nodiscard]] auto copy(auto x) -> auto { return std::move(x);  }
#line 2 "/app/example.cpp2"
auto f(std::vector<int> v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : CPP2_UFCS_0(copy, v) ) {}}// Doesn't move.
auto f(std::vector<int>&& v) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : CPP2_UFCS_0(copy, v) ) {}}// Doesn't move.
auto f(auto&& r) -> void{for ( [[maybe_unused]] auto const& unnamed_param_1 : CPP2_UFCS_0(copy, r) ) {}}// Doesn't forward.
auto main() -> int{}

[JohelEGP]

The same is true for the loop variable (https://cpp2.godbolt.org/z/KsYen7Y7h):

main: (args) = {
  for args do (copy x) _ = x; // Doesn't move.
  for args do (move x) _ = x; // Doesn't move.
  for args do (forward x) _ = x; // Doesn't forward.
}

auto main(int const argc_, char** argv_) -> int{
  auto const args = cpp2::make_args(argc_, argv_); 
  for ( auto x : args ) static_cast<void>(x); // Doesn't move.
  for ( auto&& x : args ) static_cast<void>(x); // Doesn't move.
  for ( auto&& x : args ) static_cast<void>(x); // Doesn't forward.
}