On Tue, Sep 17, 2024 at 05:16:48PM -0400, Jason Merrill wrote:
> Tested x86_64-pc-linux-gnu. Marek, thoughts?
Makes sense. OK, thanks.
> -- 8< --
>
> The -Wdangling-reference diagnostic talks about the full-expression, but
> prints one call, while the full-expression in a declaration is the entire
> initialization. It seems more useful to point out the temporary that the
> compiler thinks we might be getting a dangling reference to.
>
> gcc/cp/ChangeLog:
>
> * call.cc (do_warn_dangling_reference): Return temporary
> instead of the call it's passed to.
> (maybe_warn_dangling_reference): Adjust diagnostic.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference1.C: Adjust diagnostic.
> ---
> gcc/cp/call.cc | 23 +++++++++----------
> .../g++.dg/warn/Wdangling-reference1.C | 2 +-
> 2 files changed, 12 insertions(+), 13 deletions(-)
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 1ecf3aac705..3f753e2d2f9 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -14253,19 +14253,18 @@ reference_like_class_p (tree ctype)
> return false;
> }
>
> -/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> - that initializes the LHS (and at least one of its arguments represents
> - a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> +/* Helper for maybe_warn_dangling_reference to find a problematic temporary
> + in EXPR (as outlined in maybe_warn_dangling_reference), or NULL_TREE
> if none found. For instance:
>
> - const S& s = S().self(); // S::self (&TARGET_EXPR <...>)
> - const int& r = (42, f(1)); // f(1)
> - const int& t = b ? f(1) : f(2); // f(1)
> - const int& u = b ? f(1) : f(g); // f(1)
> - const int& v = b ? f(g) : f(2); // f(2)
> + const S& s = S().self(); // S()
> + const int& r = (42, f(1)); // temporary for passing 1 to f
> + const int& t = b ? f(1) : f(2); // temporary for 1
> + const int& u = b ? f(1) : f(g); // temporary for 1
> + const int& v = b ? f(g) : f(2); // temporary for 2
> const int& w = b ? f(g) : f(g); // NULL_TREE
> const int& y = (f(1), 42); // NULL_TREE
> - const int& z = f(f(1)); // f(f(1))
> + const int& z = f(f(1)); // temporary for 1
>
> EXPR is the initializer. If ARG_P is true, we're processing an argument
> to a function; the point is to distinguish between, for example,
> @@ -14365,7 +14364,7 @@ do_warn_dangling_reference (tree expr, bool arg_p)
> && !reference_related_p (TREE_TYPE (rettype),
> TREE_TYPE (arg)))
> continue;
> - return expr;
> + return arg;
> }
> /* Don't warn about member functions like:
> std::any a(...);
> @@ -14438,8 +14437,8 @@ maybe_warn_dangling_reference (const_tree decl, tree
> init)
> auto_diagnostic_group d;
> if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
> "possibly dangling reference to a temporary"))
> - inform (EXPR_LOCATION (call), "the temporary was destroyed at "
> - "the end of the full expression %qE", call);
> + inform (EXPR_LOCATION (call), "%qT temporary created here",
> + TREE_TYPE (call));
> }
> }
>
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> index a184317dd5c..5e60a415836 100644
> --- a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> @@ -117,7 +117,7 @@ const B& b10 = lox (H{}); // { dg-warning "dangling
> reference" }
>
> struct S {
> const int &r; // { dg-warning "dangling reference" }
> - S() : r(f(10)) { } // { dg-message "destroyed" }
> + S() : r(f(10)) { } // { dg-message "created" }
> };
>
> // From cppreference.
> --
> 2.46.0
>
Marek
