https://gcc.gnu.org/g:d3659e2dfcc6db83391cd2c6d70097cba35eb4b9
commit r13-8765-gd3659e2dfcc6db83391cd2c6d70097cba35eb4b9 Author: Patrick Palka <ppa...@redhat.com> Date: Wed Apr 24 17:49:56 2024 -0400 c++: constexpr union member access folding [PR114709] The object/offset canonicalization performed in cxx_fold_indirect_ref is undesirable for union member accesses because it loses information about the member being accessed which we may later need to diagnose an inactive-member access. So this patch restricts the canonicalization accordingly. PR c++/114709 gcc/cp/ChangeLog: * constexpr.cc (cxx_fold_indirect_ref): Restrict object/offset canonicalization to RECORD_TYPE member accesses. gcc/testsuite/ChangeLog: * g++.dg/cpp0x/constexpr-union8.C: New test. Reviewed-by: Jason Merrill <ja...@redhat.com> (cherry picked from commit 0844170e9ef60a8b2f6fba6786672f30ce1c2749) Diff: --- gcc/cp/constexpr.cc | 3 +++ gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C | 8 ++++++++ 2 files changed, 11 insertions(+) diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc index acb5496085bb..12dd9010148e 100644 --- a/gcc/cp/constexpr.cc +++ b/gcc/cp/constexpr.cc @@ -5547,6 +5547,9 @@ cxx_fold_indirect_ref (const constexpr_ctx *ctx, location_t loc, tree type, more folding opportunities. */ auto canonicalize_obj_off = [] (tree& obj, tree& off) { while (TREE_CODE (obj) == COMPONENT_REF + /* We need to preserve union member accesses so that we can + later properly diagnose accessing the wrong member. */ + && TREE_CODE (TREE_TYPE (TREE_OPERAND (obj, 0))) == RECORD_TYPE && (tree_int_cst_sign_bit (off) || integer_zerop (off))) { tree field = TREE_OPERAND (obj, 1); diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C new file mode 100644 index 000000000000..34c264944b68 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union8.C @@ -0,0 +1,8 @@ +// PR c++/114709 +// { dg-do compile { target c++11 } } + +struct T1 { int a, b; }; +struct T2 { int c; double d; }; +union U { T1 t1; T2 t2; }; + +constexpr int v = U{{1,2}}.t2.*&T2::c; // { dg-error "accessing 'U::t2'" }