On Wed, 1 Apr 2020, Jason Merrill wrote:
> We represent 'this' in a default member initializer with a PLACEHOLDER_EXPR.
> Normally in constexpr evaluation when we encounter one it refers to
> ctx->ctor, but when we're creating a temporary of class type, that replaces
> ctx->ctor, so a PLACEHOLDER_EXPR that refers to the type of the member being
> initialized needs to be replaced before that happens.
>
> This patch fixes the testcase below, but not the original testcase from the
> PR,
> which is still broken due to PR94219.
>
> Tested x86_64-pc-linux-gnu, applying to trunk and 9.
>
> gcc/cp/ChangeLog
> 2020-03-31 Jason Merrill <ja...@redhat.com>
>
> PR c++/94205
> * constexpr.c (cxx_eval_constant_expression) [TARGET_EXPR]: Call
> replace_placeholders.
> * typeck2.c (store_init_value): Fix arguments to
> fold_non_dependent_expr.
> ---
> gcc/cp/constexpr.c | 6 ++++++
> gcc/cp/tree.c | 2 +-
> gcc/cp/typeck2.c | 2 +-
> gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C | 20 +++++++++++++++++++
> gcc/testsuite/g++.dg/cpp1z/lambda-this4.C | 13 ++++++++++++
> 5 files changed, 41 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1z/lambda-this4.C
>
> diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> index e85b3c113f0..91f0c3ba269 100644
> --- a/gcc/cp/constexpr.c
> +++ b/gcc/cp/constexpr.c
> @@ -5553,6 +5553,12 @@ cxx_eval_constant_expression (const constexpr_ctx
> *ctx, tree t,
> tree init = TARGET_EXPR_INITIAL (t);
> if ((AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
> {
> + if (ctx->object)
> + /* If the initializer contains any PLACEHOLDER_EXPR, we need to
> + resolve them before we create a new CONSTRUCTOR for the
> + temporary. */
> + init = replace_placeholders (init, ctx->object);
I think I'm running into an issue due to this call to replace_placeholders.
After this change, the following (compiled with -std=c++17) ICEs
struct S
{
int a = [this] { return 6; } ();
};
S
foo()
{
return {};
}
with
internal compiler error: in gimplify_var_or_parm_decl, at gimplify.c:2830
Unsharing 'init' before before doing replace_placeholders seems to fix
this ICE.
(This came up while working on PR94034, which seems like it may admit a
fix that's similar to this one for PR94205.)
> +
> /* We're being expanded without an explicit target, so start
> initializing a new object; expansion with an explicit target
> strips the TARGET_EXPR before we get here. */
> diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
> index a2172dea0d8..5eb0dcd717a 100644
> --- a/gcc/cp/tree.c
> +++ b/gcc/cp/tree.c
> @@ -3239,7 +3239,7 @@ replace_placeholders_r (tree* t, int* walk_subtrees,
> void* data_)
> a PLACEHOLDER_EXPR has been encountered. */
>
> tree
> -replace_placeholders (tree exp, tree obj, bool *seen_p)
> +replace_placeholders (tree exp, tree obj, bool *seen_p /*= NULL*/)
> {
> /* This is only relevant for C++14. */
> if (cxx_dialect < cxx14)
> diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
> index bff4ddbcf81..cf1cb5ace66 100644
> --- a/gcc/cp/typeck2.c
> +++ b/gcc/cp/typeck2.c
> @@ -871,7 +871,7 @@ store_init_value (tree decl, tree init, vec<tree,
> va_gc>** cleanups, int flags)
> {
> bool const_init;
> tree oldval = value;
> - value = fold_non_dependent_expr (value);
> + value = fold_non_dependent_expr (value, tf_warning_or_error, true,
> decl);
> if (DECL_DECLARED_CONSTEXPR_P (decl)
> || (DECL_IN_AGGR_P (decl)
> && DECL_INITIALIZED_IN_CLASS_P (decl)))
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> new file mode 100644
> index 00000000000..c51f734a134
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> @@ -0,0 +1,20 @@
> +// PR c++/94205
> +// { dg-do compile { target c++14 } }
> +
> +struct S
> +{
> + struct A
> + {
> + S *p;
> + constexpr A(S* p): p(p) {}
> + constexpr operator int() { p->i = 5; return 6; }
> + };
> + int i;
> + int a = A(this);
> +};
> +
> +
> +constexpr S s = {};
> +
> +#define SA(X) static_assert((X),#X)
> +SA(s.i == 5 && s.a == 6);
> diff --git a/gcc/testsuite/g++.dg/cpp1z/lambda-this4.C
> b/gcc/testsuite/g++.dg/cpp1z/lambda-this4.C
> new file mode 100644
> index 00000000000..5d968791491
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/lambda-this4.C
> @@ -0,0 +1,13 @@
> +// PR c++/94205
> +// { dg-do compile { target c++17 } }
> +
> +struct S
> +{
> + int i;
> + int a = [this] { this->i = 5; return 6; } ();
> +};
> +
> +
> +constexpr S s = {};
> +
> +static_assert (s.i == 5 && s.a == 6);
>
> base-commit: f14b41d27124601284347a10d496362c8b4b8e1c
> --
> 2.18.1
>
>
