Re: [C++ Patch] [PR c++/88146] do not crash synthesizing inherited ctor(...)

[Alexandre Oliva <aoliva at redhat dot com>]

Christophe,

Thanks again for the report.  This was quite an adventure to figure
out ;-)  See below.


[PR88146] avoid diagnostics diffs if cdtor_returns_this

Diagnostics for testsuite/g++.dg/cpp0x/inh-ctor32.C varied across
platforms.  Specifically, on ARM, the diagnostics within the subtest
derived_ctor::inherited_derived_ctor::constexpr_noninherited_ctor did
not match those displayed on other platforms, and the test failed.

The difference seemed to have to do with locations assigned to ctors,
but it was more subtle: on ARM, the instantiation of bor's template
ctor was nested within the instantiation of bar's template ctor
inherited from bor.  The reason turned out to be related with the
internal return type of ctors: arm_cxx_cdtor_returns_this is enabled
for because of AAPCS, while cxx.cdtor_returns_this is disabled on most
other platforms.  While convert_to_void returns early with a VOID
expr, the non-VOID return type of the base ctor CALL_EXPR causes
convert_to_void to inspect the called decl for nodiscard attributes:
maybe_warn_nodiscard -> cp_get_fndecl_from_callee ->
maybe_constant_init -> cxx_eval_outermost_constant_expr ->
instantiate_constexpr_fns -> nested instantiation.

The internal return type assigned to a cdtor should not affect
instantiation (constexpr or template) decisions, IMHO.  We know it
affects diagnostics, but I have a hunch this might bring deeper issues
with it, so I've arranged for the CALL_EXPR handler in convert_to_void
to disregard cdtors, regardless of the ABI.


The patch is awkward on purpose: it's meant to illustrate both
portions of the affected code, to draw attention to a potential
problem, and to get bootstrap-testing coverage for the path that will
be taken on ARM.  I envision removing the first hunk, and the else
from the second hunk, once testing is done.

The first hunk is there to highlight where convert_to_void returns
early on x86, instead of handling the CALL_EXPR.

BTW (here's the potential problem), shouldn't we go into the CALL_EXPR
case for the volatile void mentioned in comments next to the case, or
won't that match VOID_TYPE_P?

Finally, I shall mention the possibility of taking the opposite
direction, and actually looking for nodiscard in cdtor calls so as to
trigger the constexpr side effects that we've inadvertently triggered
and observed with the inh-ctor32.C testcase.  It doesn't feel right to
me, but I've been wrong many times before ;-)

Would a rearranged version of the patch, dropping the redundant tests
and retaining only the addition of the test for cdtor identifiers, be
ok to install, provided that it passes regression testing?


Note this patch does NOT carry a ChangeLog entry.  That's also on
purpose, to indicate it's not meant to be included as is.
---
 gcc/cp/cvt.c |   21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index eb1687377c3e..1a15af8a6e99 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -1112,7 +1112,8 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
         error_at (loc, "pseudo-destructor is not called");
       return error_mark_node;
     }
-  if (VOID_TYPE_P (TREE_TYPE (expr)))
+  if (VOID_TYPE_P (TREE_TYPE (expr))
+      && TREE_CODE (expr) != CALL_EXPR)
     return expr;
   switch (TREE_CODE (expr))
     {
@@ -1169,6 +1170,24 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
       break;
 
     case CALL_EXPR:   /* We have a special meaning for volatile void fn().  */
+      /* cdtors may return this or void, depending on
+	 targetm.cxx.cdtor_returns_this, but this shouldn't affect our
+	 decisions here: nodiscard cdtors are nonsensical, and we
+	 don't want to call maybe_warn_nodiscard because it may
+	 trigger constexpr or template instantiation in a way that
+	 changes their instantiaton nesting.  This changes the way
+	 contexts are printed in diagnostics, with bad consequences
+	 for the testsuite, but there may be other undesirable
+	 consequences of visiting referenced ctors too soon.  */
+      if (DECL_P (TREE_OPERAND (expr, 0))
+	  && IDENTIFIER_CDTOR_P (DECL_NAME (TREE_OPERAND (expr, 0))))
+	return expr;
+      /* FIXME: Move this test before the one above, after a round of
+	 testing as it is, to get coverage of the behavior we'd get on
+	 ARM.  */
+      else if (VOID_TYPE_P (TREE_TYPE (expr)))
+	return expr;
+
       maybe_warn_nodiscard (expr, implicit);
       break;
 


-- 
Alexandre Oliva, freedom fighter   https://FSFLA.org/blogs/lxo
Be the change, be Free!         FSF Latin America board member
GNU Toolchain Engineer                Free Software Evangelist
Hay que enGNUrecerse, pero sin perder la terGNUra jamás-GNUChe