On Tue, 15 Feb 2022, Jason Merrill wrote:
> On 2/15/22 17:00, Patrick Palka wrote:
> > On Tue, 15 Feb 2022, Jason Merrill wrote:
> >
> > > On 2/15/22 15:13, Patrick Palka wrote:
> > > > On Tue, 15 Feb 2022, Patrick Palka wrote:
> > > >
> > > > > Here we're crashing from potential_constant_expression because it
> > > > > tries
> > > > > to perform trial evaluation of the first operand '(bool)__r' of the
> > > > > conjunction (which is overall wrapped in a NON_DEPENDENT_EXPR), but
> > > > > cxx_eval_constant_expression ICEs on unhandled trees (of which
> > > > > CAST_EXPR
> > > > > is one).
> > > > >
> > > > > Since cxx_eval_constant_expression always treats NON_DEPENDENT_EXPR
> > > > > as non-constant, and since NON_DEPENDENT_EXPR is also opaque to
> > > > > instantiate_non_dependent_expr, it seems futile to have p_c_e_1 ever
> > > > > return true for NON_DEPENDENT_EXPR, so let's just instead return false
> > > > > and avoid recursing.
> > >
> > > Well, in a template we use pce1 to decide whether to complain about
> > > something
> > > that needs to be constant but can't be. We aren't trying to get a value
> > > yet.
> >
> > Makes sense.. though for NON_DEPENDENT_EXPR in particular, ISTM this
> > tree is always used in a context where a constant expression isn't
> > required, e.g. in the build_x_* functions.
>
> Fair enough. The patch is OK with a comment to that effect.
Thanks, I committed the following as r12-7264:
-- >8 --
Subject: [PATCH] c++: treat NON_DEPENDENT_EXPR as not potentially constant
[PR104507]
Here we're crashing from potential_constant_expression because it tries
to perform trial evaluation of the first operand '(bool)__r' of the
conjunction (which is overall wrapped in a NON_DEPENDENT_EXPR), but
cxx_eval_constant_expression ICEs on unsupported trees (of which CAST_EXPR
is one). The sequence of events is:
1. build_non_dependent_expr for the array subscript yields
NON_DEPENDENT_EXPR<<<(bool)__r && __s>>> ? 1 : 2
2. cp_build_array_ref calls fold_non_dependent_expr on this subscript
(after this point, processing_template_decl is cleared)
3. during which, the COND_EXPR case of tsubst_copy_and_build calls
fold_non_dependent_expr on the first operand
4. during which, we crash from p_c_e_1 because it attempts trial
evaluation of the CAST_EXPR '(bool)__r'.
Note that even if this crash didn't happen, fold_non_dependent_expr
from cp_build_array_ref would still ultimately be one big no-op here
since neither constexpr evaluation nor tsubst handle NON_DEPENDENT_EXPR.
In light of this and of the observation that we should never see
NON_DEPENDENT_EXPR in a context where a constant expression is needed
(it's used primarily in the build_x_* family of functions), it seems
futile for p_c_e_1 to ever return true for NON_DEPENDENT_EXPR. And the
otherwise inconsistent handling of NON_DEPENDENT_EXPR between p_c_e_1,
cxx_evaluate_constexpr_expression and tsubst apparently leads to weird
bugs such as this one.
PR c++/104507
gcc/cp/ChangeLog:
* constexpr.cc (potential_constant_expression_1)
<case NON_DEPENDENT_EXPR>: Return false instead of recursing.
Assert tf_error isn't set.
gcc/testsuite/ChangeLog:
* g++.dg/template/non-dependent21.C: New test.
---
gcc/cp/constexpr.cc | 9 ++++++++-
gcc/testsuite/g++.dg/template/non-dependent21.C | 9 +++++++++
2 files changed, 17 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/template/non-dependent21.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 7274c3b760e..4716694cb71 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9065,6 +9065,14 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
case BIND_EXPR:
return RECUR (BIND_EXPR_BODY (t), want_rval);
+ case NON_DEPENDENT_EXPR:
+ /* Treat NON_DEPENDENT_EXPR as non-constant: it's not handled by
+ constexpr evaluation or tsubst, so fold_non_dependent_expr can't
+ do anything useful with it. And we shouldn't see it in a context
+ where a constant expression is strictly required, hence the assert. */
+ gcc_checking_assert (!(flags & tf_error));
+ return false;
+
case CLEANUP_POINT_EXPR:
case MUST_NOT_THROW_EXPR:
case TRY_CATCH_EXPR:
@@ -9072,7 +9080,6 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
case EH_SPEC_BLOCK:
case EXPR_STMT:
case PAREN_EXPR:
- case NON_DEPENDENT_EXPR:
/* For convenience. */
case LOOP_EXPR:
case EXIT_EXPR:
diff --git a/gcc/testsuite/g++.dg/template/non-dependent21.C
b/gcc/testsuite/g++.dg/template/non-dependent21.C
new file mode 100644
index 00000000000..89900837b8b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent21.C
@@ -0,0 +1,9 @@
+// PR c++/104507
+
+extern const char *_k_errmsg[];
+
+template<class>
+const char* DoFoo(int __r, int __s) {
+ const char* n = _k_errmsg[(bool)__r && __s ? 1 : 2];
+ return n;
+}
--
2.35.1.129.gb80121027d
