Re: [PATCH] c++: get_nsdmi in template context [PR108116]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 22 Dec 2022, Patrick Palka wrote:

> On Thu, 22 Dec 2022, Jason Merrill wrote:
> 
> > On 12/22/22 16:41, Patrick Palka wrote:
> > > On Thu, 22 Dec 2022, Jason Merrill wrote:
> > > 
> > > > On 12/22/22 11:31, Patrick Palka wrote:
> > > > > On Wed, 21 Dec 2022, Jason Merrill wrote:
> > > > > 
> > > > > > On 12/21/22 09:52, Patrick Palka wrote:
> > > > > > > Here during ahead of time checking of C{}, we indirectly call
> > > > > > > get_nsdmi
> > > > > > > for C::m from finish_compound_literal, which in turn calls
> > > > > > > break_out_target_exprs for C::m's (non-templated) initializer,
> > > > > > > during
> > > > > > > which we end up building a call to A::~A and checking
> > > > > > > expr_noexcept_p
> > > > > > > for it (from build_vec_delete_1).  But this is all done with
> > > > > > > processing_template_decl set, so the built A::~A call is templated
> > > > > > > (whose form r12-6897-gdec8d0e5fa00ceb2 recently changed) which
> > > > > > > expr_noexcept_p doesn't expect and we crash.
> > > > > > > 
> > > > > > > In r10-6183-g20afdcd3698275 we fixed a similar issue by guarding a
> > > > > > > expr_noexcept_p call with !processing_template_decl, which works
> > > > > > > here
> > > > > > > too.  But it seems to me since the initializer we obtain in
> > > > > > > get_nsdmi is
> > > > > > > always non-templated, it should be calling break_out_target_exprs
> > > > > > > with
> > > > > > > processing_template_decl cleared since otherwise the function might
> > > > > > > end
> > > > > > > up mixing templated and non-templated trees.
> > > > > > > 
> > > > > > > I'm not sure about this though, perhaps this is not the best fix
> > > > > > > here.
> > > > > > > Alternatively, when processing_template_decl we could make get_nsdmi
> > > > > > > avoid calling break_out_target_exprs at all or something.
> > > > > > > Additionally,
> > > > > > > perhaps break_out_target_exprs should be a no-op more generally when
> > > > > > > processing_template_decl since we shouldn't see any TARGET_EXPRs
> > > > > > > inside
> > > > > > > a template?
> > > > > > 
> > > > > > Hmm.
> > > > > > 
> > > > > > Any time we would call break_out_target_exprs we're dealing with
> > > > > > non-dependent
> > > > > > expressions; if we're in a template, we're building up an initializer
> > > > > > or a
> > > > > > call that we'll soon throw away, just for the purpose of checking or
> > > > > > type
> > > > > > computation.
> > > > > > 
> > > > > > Furthermore, as you say, the argument is always a non-template tree,
> > > > > > whether
> > > > > > in get_nsdmi or convert_default_arg.  So having
> > > > > > processing_template_decl
> > > > > > cleared would be correct.
> > > > > > 
> > > > > > I don't think we can get away with not calling break_out_target_exprs
> > > > > > at
> > > > > > all
> > > > > > in a template; if nothing else, we would lose immediate invocation
> > > > > > expansion.
> > > > > > However, we could probably skip the bot_manip tree walk, which should
> > > > > > avoid
> > > > > > the problem.
> > > > > > 
> > > > > > Either way we end up returning non-template trees, as we do now, and
> > > > > > callers
> > > > > > have to deal with transient CONSTRUCTORs containing such (as we do in
> > > > > > massage_init_elt).
> > > > > 
> > > > > Ah I see, makes sense.
> > > > > 
> > > > > > 
> > > > > > Does convert_default_arg not run into the same problem, e.g. when
> > > > > > calling
> > > > > > 
> > > > > >     void g(B = {0});
> > > > > 
> > > > > In practice it seems not, because we don't call convert_default_arg
> > > > > when processing_template_decl is set (verified with an assert to
> > > > > that effect).  In build_over_call for example we exit early when
> > > > > processing_template_decl is set, and return a templated CALL_EXPR
> > > > > that doesn't include default arguments at all.  A consequence of
> > > > > this is that we don't reject ahead of time a call that would use
> > > > > an ill-formed dependent default argument, e.g.
> > > > > 
> > > > >     template<class T>
> > > > >     void g(B = T{0});
> > > > > 
> > > > >     template<class>
> > > > >     void f() {
> > > > >       g<void>();
> > > > >     }
> > > > > 
> > > > > since the default argument instantiation would be the responsibility
> > > > > of convert_default_arg.
> > > > > 
> > > > > Thinking hypothetically here, if we do in the future want to include
> > > > > default
> > > > > arguments in the templated form of a CALL_EXPR,
> > > > 
> > > > We definitely do not want to; the templated form should be as close as
> > > > possible to the source.
> > > 
> > > Ah, sounds good.
> > > 
> > > > 
> > > > We might want to perform non-dependent conversions to get any errors (such
> > > > as
> > > > this one) before throwing away the result.  Which would be parallel to
> > > > what we
> > > > currently do in calling get_nsdmi, and would want the same behavior.
> > > 
> > > *nod*
> > > 
> > > > 
> > > > > [snip]
> > > > 
> > > > > shall we go with the original approach to clear
> > > > > processing_template_decl directly from get_nsdmi?
> > > > 
> > > > OK, but then we should also checking_assert !processing_template_decl in
> > > > b_o_t_e.
> > > 
> > > Unfortunately we'd trigger that assert from maybe_constant_value, which
> > > potentially calls b_o_t_e with processing_template_decl set.
> > 
> > maybe_constant_value could also clear processing_template_decl; entries in
> > cv_cache are non-templated.
> 
> Aha!  I'll try that.

How does this look?  Bootstrapped and regtested on x86_64-pc-linux-gnu.

-- >8 --

Subject: [PATCH] c++: get_nsdmi in template context [PR108116]

Here during ahead of time checking of C{}, we indirectly call get_nsdmi
for C::m from finish_compound_literal, which in turn calls
break_out_target_exprs for C::m's (non-templated) initializer, during
which we build a call to A::~A and check expr_noexcept_p for it (from
build_vec_delete_1).  But this is all done with processing_template_decl
set, so the built A::~A call is templated (whose form was recently
changed by r12-6897-gdec8d0e5fa00ceb2) which expr_noexcept_p doesn't
expect, and we crash.

This patch fixes this by clearing processing_template_decl before
the call to break_out_target_exprs from get_nsdmi.  And since it more
generally seems we shouldn't be seeing (or producing) non-templated
trees from break_out_target_exprs, this patch also adds an assert to
that effect.

	PR c++/108116

gcc/cp/ChangeLog:

	* constexpr.cc (maybe_constant_value): Clear
	processing_template_decl before calling break_out_target_exprs.
	* init.cc (get_nsdmi): Likewise.
	* tree.cc (break_out_target_exprs): Assert processing_template_decl
	is cleared.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/nsdmi-template24.C: New test.
---
 gcc/cp/constexpr.cc                           |  4 ++++
 gcc/cp/init.cc                                |  4 ++++
 gcc/cp/tree.cc                                |  4 ++++
 gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C | 22 +++++++++++++++++++
 4 files changed, 34 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index d99c49bdbe2..414af7a6d4c 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8507,6 +8507,10 @@ maybe_constant_value (tree t, tree decl /* = NULL_TREE */,
       r = *cached;
       if (r != t)
 	{
+	  /* Clear processing_template_decl for sake of break_out_target_exprs;
+	     entries in the cv_cache are non-templated.  */
+	  processing_template_decl_sentinel ptds;
+
 	  r = break_out_target_exprs (r, /*clear_loc*/true);
 	  protected_set_expr_location (r, EXPR_LOCATION (t));
 	}
diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index 73e6547c076..b49a7ca9169 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -670,6 +670,10 @@ get_nsdmi (tree member, bool in_ctor, tsubst_flags_t complain)
       current_class_ptr = build_address (current_class_ref);
     }
 
+  /* Clear processing_template_decl for sake of break_out_target_exprs;
+     INIT is always non-templated.  */
+  processing_template_decl_sentinel ptds;
+
   /* Strip redundant TARGET_EXPR so we don't need to remap it, and
      so the aggregate init code below will see a CONSTRUCTOR.  */
   bool simple_target = (init && SIMPLE_TARGET_EXPR_P (init));
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 33bde16f128..faf01616f87 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -3342,6 +3342,10 @@ break_out_target_exprs (tree t, bool clear_location /* = false */)
   static int target_remap_count;
   static splay_tree target_remap;
 
+  /* We shouldn't be called on templated trees, nor do we want to
+     produce them.  */
+  gcc_checking_assert (!processing_template_decl);
+
   if (!target_remap_count++)
     target_remap = splay_tree_new (splay_tree_compare_pointers,
 				   /*splay_tree_delete_key_fn=*/NULL,
diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C
new file mode 100644
index 00000000000..202c67d7321
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C
@@ -0,0 +1,22 @@
+// PR c++/108116
+// { dg-do compile { target c++11 } }
+
+#include <initializer_list>
+
+struct A {
+  A(int);
+  ~A();
+};
+
+struct B {
+  B(std::initializer_list<A>);
+};
+
+struct C {
+  B m{0};
+};
+
+template<class>
+void f() {
+  C c = C{};
+};
-- 
2.39.0.95.g7c2ef319c5