On Thu, Feb 27, 2025 at 10:42:07AM -0500, Jason Merrill wrote:
> On 2/26/25 2:16 PM, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > Yet another problem that started with r15-6052, compile time evaluation of
> > prvalues.
> >
> > cp_fold_r/TARGET_EXPR sees:
> >
> > TARGET_EXPR <D.2701, <<< Unknown tree: expr_stmt
> > D.2701.__p = TARGET_EXPR <D.2684, <<< Unknown tree: aggr_init_expr
> > 3
> > f1
> > D.2684 >>>> >>>>
> >
> > so when we call maybe_constant_init, the object we're initializing is
> > D.2701,
> > and the init is the expr_stmt. We unwrap the
> > EXPR_STMT/INIT_EXPR/TARGET_EXPR
> > in maybe_constant_init_1 and so end up evaluating the f1 call. But f1
> > returns
> > c2 whereas the type of D.2701 is ._anon_0 -- the closure.
>
> Sounds like the problem is with the maybe_constant_init_1 unwrapping, it
> probably shouldn't strip INIT_EXPR if the type doesn't match that of 'decl'.
Yes, that's where the types change. I thought that skipping the unpacking
here might cause other issues but it seems not.
> > So then we crash in replace_decl on:
> >
> > gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
> > (TREE_TYPE (decl), TREE_TYPE (replacement)));
> >
> > due to the mismatched types.
> >
> > cxx_eval_outermost_constant_expr is already ready for the types to be
> > different, in which case the result isn't constant. But replace_decl
> > is called before that check.
> >
> > I'm leaving the assert in replace_decl on purpose, maybe we'll find
> > another use for it.
> >
> > PR c++/118986
> >
> > gcc/cp/ChangeLog:
> >
> > * constexpr.cc (cxx_eval_call_expression): Check that the types match
> > before calling replace_decl.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp2a/constexpr-prvalue1.C: New test.
> > ---
> > gcc/cp/constexpr.cc | 4 +++-
> > .../g++.dg/cpp2a/constexpr-prvalue1.C | 23 +++++++++++++++++++
> > 2 files changed, 26 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue1.C
> >
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index 59dd0668af3..204cda2a222 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -3390,7 +3390,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> > tree t,
> > current object under construction. */
> > if (!*non_constant_p && ctx->object
> > && CLASS_TYPE_P (TREE_TYPE (res))
> > - && !is_empty_class (TREE_TYPE (res)))
> > + && !is_empty_class (TREE_TYPE (res))
> > + && same_type_ignoring_top_level_qualifiers_p
> > + (TREE_TYPE (res), TREE_TYPE (ctx->object)))
>
> If this happens, rather than just skip the replace_decl, I think we want to
> set *non_constant_p or I expect we'll end up with a wrong value somewhere.
Good point, I agree.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Yet another problem that started with r15-6052, compile time evaluation of
prvalues.
cp_fold_r/TARGET_EXPR sees:
TARGET_EXPR <D.2701, <<< Unknown tree: expr_stmt
D.2701.__p = TARGET_EXPR <D.2684, <<< Unknown tree: aggr_init_expr
3
f1
D.2684 >>>> >>>>
so when we call maybe_constant_init, the object we're initializing is D.2701,
and the init is the expr_stmt. We unwrap the EXPR_STMT/INIT_EXPR/TARGET_EXPR
in maybe_constant_init_1 and so end up evaluating the f1 call. But f1 returns
c2 whereas the type of D.2701 is ._anon_0 -- the closure.
So then we crash in replace_decl on:
gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
(TREE_TYPE (decl), TREE_TYPE (replacement)));
due to the mismatched types.
cxx_eval_outermost_constant_expr is already ready for the types to be
different, in which case the result isn't constant. But replace_decl
is called before that check.
I'm leaving the assert in replace_decl on purpose, maybe we'll find
another use for it.
PR c++/118986
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_call_expression): Check that the types match
before calling replace_decl, if not, set *non_constant_p.
(maybe_constant_init_1): Don't strip INIT_EXPR if it would change the
type of the expression.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-prvalue1.C: New test.
---
gcc/cp/constexpr.cc | 29 +++++++++++++------
.../g++.dg/cpp2a/constexpr-prvalue1.C | 23 +++++++++++++++
2 files changed, 43 insertions(+), 9 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue1.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index c68666cc5dd..5439b2ea8fe 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3388,16 +3388,22 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
tree t,
/* Rewrite all occurrences of the function's RESULT_DECL with the
current object under construction. */
- if (!*non_constant_p && ctx->object
+ if (!*non_constant_p
+ && ctx->object
&& CLASS_TYPE_P (TREE_TYPE (res))
&& !is_empty_class (TREE_TYPE (res)))
- if (replace_decl (&result, res, ctx->object))
- {
- cacheable = false;
- result = cxx_eval_constant_expression (ctx, result, lval,
- non_constant_p,
- overflow_p);
- }
+ {
+ if (!same_type_ignoring_top_level_qualifiers_p
+ (TREE_TYPE (res), TREE_TYPE (ctx->object)))
+ *non_constant_p = true;
+ else if (replace_decl (&result, res, ctx->object))
+ {
+ cacheable = false;
+ result = cxx_eval_constant_expression (ctx, result, lval,
+ non_constant_p,
+ overflow_p);
+ }
+ }
/* Only cache a permitted result of a constant expression. */
if (cacheable && !reduced_constant_expression_p (result))
@@ -9605,7 +9611,12 @@ maybe_constant_init_1 (tree t, tree decl, bool
allow_non_constant,
if (TREE_CODE (t) == CONVERT_EXPR
&& VOID_TYPE_P (TREE_TYPE (t)))
t = TREE_OPERAND (t, 0);
- if (TREE_CODE (t) == INIT_EXPR)
+ /* Doing the following stripping could change the type of the expression,
+ which then would not match DECL's type, and cause mischief later. */
+ if (TREE_CODE (t) == INIT_EXPR
+ && (!decl
+ || same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (decl),
+ TREE_TYPE (t))))
t = TREE_OPERAND (t, 1);
if (TREE_CODE (t) == TARGET_EXPR)
t = TARGET_EXPR_INITIAL (t);
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue1.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue1.C
new file mode 100644
index 00000000000..f4e704d9487
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-prvalue1.C
@@ -0,0 +1,23 @@
+// PR c++/118986
+// { dg-do compile { target c++20 } }
+// { dg-options "-O" }
+
+struct c1 {
+ constexpr c1(int *ptr) {}
+};
+struct c2 {
+ c1 _M_t;
+ constexpr ~c2() {}
+};
+constexpr inline
+c2 f1 ()
+{
+ return c2(new int);
+}
+
+void
+f ()
+{
+ auto l = [p = f1()](){};
+ [p = f1()](){};
+}
base-commit: 9792126ac769f2962c0f305991818c64f9e51221
--
2.48.1
