[gcc r14-10110] c++: constexpr union member access folding [PR114709]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:0844170e9ef60a8b2f6fba6786672f30ce1c2749

commit r14-10110-g0844170e9ef60a8b2f6fba6786672f30ce1c2749
Author: Patrick Palka <ppalka@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 <jason@redhat.com>

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 302b266809f..2e83d24dfda 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -5799,6 +5799,9 @@ 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 00000000000..34c264944b6
--- /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'" }