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[BUG] No implicit move or forward on last use in initialization #749
Description
Title: No implicit move or forward on last use in initialization.
Minimal reproducer (https://cpp2.godbolt.org/z/Pb6YaYvsT):
f: (copy x) = { _ = x; } // OK, moves.
f: (forward x) = { _ = x; } // OK, forwards.
g: (copy x) = { i := x; } // Wrong, doesn't move.
g: (forward x) = { i := x; } // Wrong, 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.cppExpected result:
auto f(auto x) -> void{static_cast<void>(std::move(x));}// OK, moves.
auto f(auto&& x) -> void{static_cast<void>(CPP2_FORWARD(x)); }// OK, forwards.
auto g(auto x) -> void{auto i {std::move(x)}; }// Wrong, doesn't move.
auto g(auto&& x) -> void{auto i {CPP2_FORWARD(x)}; }// Wrong, doesn't forward.Actual result and error:
Program returned: 0
auto f(auto x) -> void{static_cast<void>(std::move(x));}// OK, moves.
auto f(auto&& x) -> void{static_cast<void>(CPP2_FORWARD(x)); }// OK, forwards.
auto g(auto x) -> void{auto i {x}; }// Wrong, doesn't move.
auto g(auto&& x) -> void{auto i {x}; }// Wrong, doesn't forward.Cpp2 lowered to Cpp1:
//=== Cpp2 type declarations ====================================================
#include "cpp2util.h"
//=== Cpp2 type definitions and function declarations ===========================
auto f(auto x) -> void;
auto f(auto&& x) -> void;
auto g(auto x) -> void;
auto g(auto&& x) -> void;
auto main() -> int;
//=== Cpp2 function definitions =================================================
auto f(auto x) -> void{static_cast<void>(std::move(x));}// OK, moves.
auto f(auto&& x) -> void{static_cast<void>(CPP2_FORWARD(x)); }// OK, forwards.
auto g(auto x) -> void{auto i {x}; }// Wrong, doesn't move.
auto g(auto&& x) -> void{auto i {x}; }// Wrong, doesn't forward.
auto main() -> int{}
Activity
hsutter commentedon Nov 14, 2023
Thanks! Repeating rationale here from #792 comment thread: The issue is that I was trying out suppressing move-from-last-use if the last use was in an initializer, because of examples like
view: std::span = local_variable;when that was the last use oflocal_variable, which is an error for an rvalue. But that also suppresses a righteous move in examples likeresult := local_variable;. So I now agree those should be treated as rvalues too, and if that causes an error for an rvalue because there is no further us of that variable in scope, then the programmer must write an explicit discard of the variable.Re-enable move from last use in initializers, closes hsutter#749
Re-enable move from last use in initializers, closes hsutter#749