This patch adds a check to detect changing the active union member during
initialization of the union. It uses the CONSTRUCTOR_NO_CLEARING flag as a
proxy for whether the non-empty CONSTRUCTOR of UNION_TYPE we're assigning to in
cxx_eval_store_expression is in the process of being initialized, which seems to
work well.
In order for this check to work reliably, we also have to adjust
cxx_eval_bare_aggregate to set the active union member before processing the
initializer.
Does this look OK to commit after testing?
gcc/cp/ChangeLog:
PR c++/94066
* constexpr.c (cxx_eval_bare_aggregate): When constructing a union,
always set the active union member before evaluating the initializer.
Relax assertion that verifies the index of the constructor element we're
initializing hasn't been changed.
(cxx_eval_store_expression): Diagnose changing the active union member
while the union is in the process of being initialized.
gcc/testsuite/ChangeLog:
PR c++/94066
* g++.dg/cpp1y/pr94066.C: New test.
* g++.dg/cpp1y/pr94066-2.C: New test.
* g++.dg/cpp1y/pr94066-3.C: New test.
---
gcc/cp/constexpr.c | 25 ++++++++++++++++++++++++-
gcc/testsuite/g++.dg/cpp1y/pr94066-2.C | 19 +++++++++++++++++++
gcc/testsuite/g++.dg/cpp1y/pr94066-3.C | 18 ++++++++++++++++++
gcc/testsuite/g++.dg/cpp1y/pr94066.C | 18 ++++++++++++++++++
4 files changed, 79 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/pr94066.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 192face9a3a..97fe5572f71 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3751,6 +3751,11 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree
t,
/* If we built a new CONSTRUCTOR, attach it now so that other
initializers can refer to it. */
CONSTRUCTOR_APPEND_ELT (*p, index, new_ctx.ctor);
+ else if (TREE_CODE (type) == UNION_TYPE)
+ /* If we're constructing a union, set the active union member now so
+ that we can later detect if the initializer attempts to activate
+ another member. */
+ CONSTRUCTOR_APPEND_ELT (*p, index, NULL_TREE);
tree elt = cxx_eval_constant_expression (&new_ctx, value,
lval,
non_constant_p, overflow_p);
@@ -3784,7 +3789,12 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree
t,
}
else
{
- if (new_ctx.ctor != ctx->ctor)
+ if (TREE_CODE (type) == UNION_TYPE
+ && (*p)->last().index != index)
+ /* The initializer may have erroneously changed the active union
+ member we were initializing. */
+ gcc_assert (*non_constant_p);
+ else if (new_ctx.ctor != ctx->ctor)
{
/* We appended this element above; update the value. */
gcc_assert ((*p)->last().index == index);
@@ -4647,6 +4657,19 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
index);
*non_constant_p = true;
}
+ else if (TREE_CODE (t) == MODIFY_EXPR
+ && CONSTRUCTOR_NO_CLEARING (*valp))
+ {
+ /* Diagnose changing the active union member while the union
+ is in the process of being initialized. */
+ if (!ctx->quiet)
+ error_at (cp_expr_loc_or_input_loc (t),
+ "change of the active member of a union "
+ "from %qD to %qD during initialization",
+ CONSTRUCTOR_ELT (*valp, 0)->index,
+ index);
+ *non_constant_p = true;
+ }
/* Changing active member. */
vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
no_zero_init = true;
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
b/gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
new file mode 100644
index 00000000000..1c00b650961
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
@@ -0,0 +1,19 @@
+// PR c++/94066
+// { dg-do compile { target c++14 } }
+
+struct A { long x; };
+
+union U;
+constexpr A foo(U *up);
+
+union U {
+ U() = default;
+ A a = foo(this); int y;
+};
+
+constexpr A foo(U *up) {
+ up->y = 11; // { dg-error "'U::a' to 'U::y'" }
+ return {42};
+}
+
+extern constexpr U u = {};
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
b/gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
new file mode 100644
index 00000000000..6bf1ec81885
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
@@ -0,0 +1,18 @@
+// PR c++/94066
+// { dg-do compile { target c++14 } }
+
+struct A { long x; };
+
+union U;
+constexpr int foo(U *up);
+
+union U {
+ int a = foo(this); int y;
+};
+
+constexpr int foo(U *up) {
+ up->y = 11; // { dg-error "'U::a' to 'U::y'" }
+ return {42};
+}
+
+extern constexpr U u = {};
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr94066.C
b/gcc/testsuite/g++.dg/cpp1y/pr94066.C
new file mode 100644
index 00000000000..6725c8c737f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr94066.C
@@ -0,0 +1,18 @@
+// PR c++/94066
+// { dg-do compile { target c++14 } }
+
+struct A { long x; };
+
+union U;
+constexpr A foo(U *up);
+
+union U {
+ A a = foo(this); int y;
+};
+
+constexpr A foo(U *up) {
+ up->y = 11; // { dg-error "'U::a' to 'U::y'" }
+ return {42};
+}
+
+extern constexpr U u = {};
--
2.26.0.rc1.11.g30e9940356
