[PATCH v2] c++: ICE with constexpr lambda [PR107280]

[Marek Polacek <polacek@redhat.com>]

On Fri, Mar 10, 2023 at 01:47:46PM -0500, Jason Merrill wrote:
> On 3/10/23 11:17, Marek Polacek wrote:
> > We crash here since r10-3661, the store_init_value hunk in particular.
> > Before, we called cp_fully_fold_init, so e.g.
> > 
> >    {.str=VIEW_CONVERT_EXPR<char[8]>("")}
> > 
> > was folded into
> > 
> >    {.str=""}
> > 
> > but now we don't fold and keep the VCE around, and it causes trouble in
> > cxx_eval_store_expression: in the !refs->is_empty () loop we descend on
> > .str's initializer but since it's wrapped in a VCE, we skip the STRING_CST
> > check and then crash on the CONSTRUCTOR_NO_CLEARING.
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/12?
> > 
> > 	PR c++/107280
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* constexpr.cc (cxx_eval_store_expression): Strip location wrappers.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp1z/constexpr-lambda28.C: New test.
> > ---
> >   gcc/cp/constexpr.cc                             |  3 ++-
> >   gcc/testsuite/g++.dg/cpp1z/constexpr-lambda28.C | 15 +++++++++++++++
> >   2 files changed, 17 insertions(+), 1 deletion(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-lambda28.C
> > 
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index 8683c00596a..abf6ee560c5 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -6033,7 +6033,8 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
> >   	  *valp = build_constructor (type, NULL);
> >   	  CONSTRUCTOR_NO_CLEARING (*valp) = no_zero_init;
> >   	}
> > -      else if (TREE_CODE (*valp) == STRING_CST)
> > +      else if (STRIP_ANY_LOCATION_WRAPPER (*valp),
> > +	       TREE_CODE (*valp) == STRING_CST)
> 
> Seems like this is stripping the location wrapper when we try to modify the
> string; I think we want to strip it earlier, when we first initialize the
> array member.

Hmm, I suppose we don't want to do the stripping too early.  I could
have put it in get_nsdmi, for instance, but maybe here is good as well?

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
We crash here since r10-3661, the store_init_value hunk in particular.
Before, we called cp_fully_fold_init, so e.g.

  {.str=VIEW_CONVERT_EXPR<char[8]>("")}

was folded into

  {.str=""}

but now we don't fold and keep the VCE around, and it causes trouble in
cxx_eval_store_expression: in the !refs->is_empty () loop we descend on
.str's initializer but since it's wrapped in a VCE, we skip the STRING_CST
check and then crash on the CONSTRUCTOR_NO_CLEARING.

	PR c++/107280

gcc/cp/ChangeLog:

	* typeck2.cc (process_init_constructor_record): Strip location wrappers.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-lambda28.C: New test.
---
 gcc/cp/typeck2.cc                               |  1 +
 gcc/testsuite/g++.dg/cpp1z/constexpr-lambda28.C | 15 +++++++++++++++
 2 files changed, 16 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-lambda28.C

diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index c56b69164e2..d0984910ce4 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1889,6 +1889,7 @@ process_init_constructor_record (tree type, tree init, int nested, int flags,
 	  CONSTRUCTOR_PLACEHOLDER_BOUNDARY (init) = 1;
 	  CONSTRUCTOR_PLACEHOLDER_BOUNDARY (next) = 0;
 	}
+      STRIP_ANY_LOCATION_WRAPPER (next);
       CONSTRUCTOR_APPEND_ELT (v, field, next);
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda28.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda28.C
new file mode 100644
index 00000000000..aafbfddd8b9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-lambda28.C
@@ -0,0 +1,15 @@
+// PR c++/107280
+// { dg-do compile { target c++17 } }
+
+struct string {
+  char str[8] = "";
+};
+template <int, int> constexpr void
+test ()
+{
+  string str{};
+  auto append = [&](const char *s) { *str.str = *s; };
+  append("");
+}
+
+static_assert ((test<true, true>(), true), "");

base-commit: 40c1352c5a4530350012d6a922435cf491663daa
-- 
2.39.2