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.
And if something is required to be a constant expression and we're
inside a template, then it seems at that point we're dealing with the
final templated form of that thing (which doesn't contain
NON_DEPENDENT_EXPR), which I suppose explains why the patch can get away
with asserting !(flags & tf_error) inside the NON_DEPENDENT_EXPR case of
p_c_e_1.
So I guess I don't fully understand the purpose of NON_DEPENDENT_EXPR
or how it should interact with fold_non_dependent_expr and constant
evaluation...
Actually, why are we seeing a NON_DEPENDENT_EXPR here? Did it leak into the
AST somehow? They should all be temporary within build_x_whatever functions.
Hmm yes, kind of. The unusual thing about this testcase is that
build_non_dependent_expr (called from grok_array_decl) for the
COND_EXPR
(bool)__r && __s ? 1 : 2
wraps only the condition operand, yielding
NON_DEPENDENT_EXPR<<<(bool)__r && __s>>> ? 1 : 2 // #1
rather than wrapping the whole thing i.e.
NON_DEPENDENT_EXPR<<<(bool)__r && __s ? 1 : 2>>> // #2
cp_build_array_ref then tries to speculatively fold the non-dependent #1
as a whole, during which the COND_EXPR case of tsubst_copy_and_build
tries to speculative fold #1's condition NON_DEPENDENT_EXPR<<<(bool)__r &&
__s>>>
on its own, which ends in a crash from p_c_e_1 because at this point
processing_template_decl is cleared so p_c_e_1 attempts trial evaluation
of the CAST_EXPR (bool)__r.
If instead build_non_dependent_expr yielded #2, speculative folding of
#2 as a whole just yield #2 since tsubst doesn't look through
NON_DEPENDENT_EXPR.
Alternatively p_c_e_1 could continue to recurse into NON_DEPENDENT_EXPR,
but with trial evaluation disabled by setting processing_template_decl,
but as mentioned it seems pointless for p_c_e_1 to ever return true for
NON_DEPENDENT_EXPR.
... Since we're not issuing a diagnostic in this case, I suppose we should
also assert that tf_error isn't set. Bootstrapped and regtested on
x86_64-pc-linux-gnu.
-- >8 --
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 | 5 ++++-
gcc/testsuite/g++.dg/template/non-dependent21.C | 9 +++++++++
2 files changed, 13 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..b363ef08411 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9065,6 +9065,10 @@ 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:
+ gcc_checking_assert (!(flags & tf_error));
+ return false;
+
case CLEANUP_POINT_EXPR:
case MUST_NOT_THROW_EXPR:
case TRY_CATCH_EXPR:
@@ -9072,7 +9076,6 @@ 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;
+}
