On Sat, Jan 24, 2026 at 09:41:51PM +1100, Nathaniel Shead wrote:
> On Sat, Jan 24, 2026 at 04:57:44PM +0800, Jason Merrill wrote:
> > On 1/24/26 4:41 PM, Nathaniel Shead wrote:
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?
> > >
> > > -- >8 --
> > >
> > > This fixes two issues with cp_fold_non_odr_use_1:
> > >
> > > - When called to fold away a reference, it considers the use to not be
> > > an lvalue and so 'maybe_constant_value' folds all the way through the
> > > nested INDIRECT_REF to a prvalue. Fixed by forcing the expression
> > > to be an ADDR_EXPR before calling 'maybe_constant_value', and then
> > > folding it away afterwards.
> >
> > For the REFERENCE_REF case it would make sense to pass its op0 to
> > fold_non_odr rather than the ref itself; we already convert_from_reference
> > the result.
> >
>
> Right, makes sense; thanks. Here's an updated patch that does this
> which passes bootstrap and regtest on x86_64-pc-linux-gnu, OK for trunk?
>
Actually, ignore this: I just noticed that this fails with modules in
some cases with wrong-looking GIMPLE; for cases like
export module M;
static const int& ref = 123;
constexpr int foo() { return ref; }
we no longer fold 'ref' to 123 but instead return
'(int)*(const int&)&_ZGRL3ref_'
which then can fail calls because the local symbol is not defined in
importers. Even my original patch will break on this case; I'll need to
think more about what to do here.
> -- >8 --
>
> This fixes two issues with cp_fold_non_odr_use_1:
>
> - When called to fold away a reference, it considers the use to not be
> an lvalue and so 'maybe_constant_value' folds all the way through the
> nested INDIRECT_REF to a prvalue. Fixed by folding the op0 of the
> INDIRECT_REF rather than the ref itself.
>
> - When used to fold away the initializing expression for an INIT_EXPR,
> it doesn't mark any new TARGET_EXPRs as eliding. Fixed by reapplying
> 'set_target_expr_eliding' for the initializer of an INIT_EXPR if it
> got modified during ff_only_non_odr walk.
>
> PR c++/123557
> PR c++/123738
>
> gcc/cp/ChangeLog:
>
> * cp-gimplify.cc (cp_fold): Pass op0 to cp_fold_non_odr_use_1
> when folding a reference. Reapply set_target_expr_eliding on
> the initializing expression of an INIT_EXPR.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/constexpr-ice22.C: New test.
> * g++.dg/cpp2a/constexpr-ref2.C: New test.
>
> Signed-off-by: Nathaniel Shead <[email protected]>
> Reviewed-by: Jason Merrill <[email protected]>
> ---
> gcc/cp/cp-gimplify.cc | 12 ++++++++----
> gcc/testsuite/g++.dg/cpp0x/constexpr-ice22.C | 14 ++++++++++++++
> gcc/testsuite/g++.dg/cpp2a/constexpr-ref2.C | 19 +++++++++++++++++++
> 3 files changed, 41 insertions(+), 4 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-ice22.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-ref2.C
>
> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> index 3c0f415fa87..1189c100527 100644
> --- a/gcc/cp/cp-gimplify.cc
> +++ b/gcc/cp/cp-gimplify.cc
> @@ -3411,9 +3411,9 @@ cp_fold (tree x, fold_flags_t flags)
> used as lvalues. */
> if ((flags & ff_only_non_odr) && REFERENCE_REF_P (x))
> {
> - tree r = cp_fold_non_odr_use_1 (x);
> - if (r != x)
> - return convert_from_reference (cp_fold (r, flags));
> + op0 = cp_fold_non_odr_use_1 (TREE_OPERAND (x, 0));
> + if (op0 != TREE_OPERAND (x, 0))
> + return convert_from_reference (cp_fold (op0, flags));
> }
> goto unary;
>
> @@ -3560,7 +3560,11 @@ cp_fold (tree x, fold_flags_t flags)
> if (op0 == error_mark_node || op1 == error_mark_node)
> x = error_mark_node;
> else if (op0 != TREE_OPERAND (x, 0) || op1 != TREE_OPERAND (x, 1))
> - x = build2_loc (loc, code, TREE_TYPE (x), op0, op1);
> + {
> + if (code == INIT_EXPR && op1 != TREE_OPERAND (x, 1))
> + set_target_expr_eliding (op1);
> + x = build2_loc (loc, code, TREE_TYPE (x), op0, op1);
> + }
> break;
> }
>
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ice22.C
> b/gcc/testsuite/g++.dg/cpp0x/constexpr-ice22.C
> new file mode 100644
> index 00000000000..900c8053a92
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ice22.C
> @@ -0,0 +1,14 @@
> +// PR c++/123557
> +// { dg-do compile { target c++11 } }
> +
> +struct a {
> + a() = default;
> + a(a const &) = default;
> + a(a &&);
> +};
> +struct c {
> + c();
> + a e;
> +};
> +constexpr a b;
> +c::c() : e(b) {}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-ref2.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-ref2.C
> new file mode 100644
> index 00000000000..659beed3a25
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-ref2.C
> @@ -0,0 +1,19 @@
> +// PR c++/123738
> +// { dg-do compile { target c++20 } }
> +
> +struct OStringLiteral {
> + int str = 0;
> +};
> +
> +template<auto L> struct OStringHolder {
> + static constexpr auto & literal = L;
> +};
> +
> +struct OString {
> + template<auto L> constexpr OString(OStringHolder<L> const &):
> + p(&OStringHolder<L>::literal.str) {}
> + int const * p;
> +};
> +
> +
> +constexpr OString s = OStringHolder<OStringLiteral{}>{};
> --
> 2.51.0
>
