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

On Dec 28, 2018, Alexandre Oliva <> wrote:

> diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c

And here's a cleaned-up patch based on my initial approach.

[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.

Regstrapped on x86_64- and i686-linux-gnu.  Ok to install?

for  gcc/cp/ChangeLog

	PR c++/88146
	* cvt.c (convert_to_void): Handle all cdtor calls as if
	returning void.
 gcc/cp/cvt.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index f758f2d9bc8f..092c81434e33 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -1175,6 +1175,18 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
     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: neither nodiscard warnings (nodiscard cdtors
+	 are nonsensical), nor should any constexpr or template
+	 instantiations be affected by an ABI property that is, or at
+	 least ought to be transparent to the language.  */
+      if (TREE_OPERAND (expr, 1)
+	  && TREE_CODE (TREE_OPERAND (expr, 1)) == ADDR_EXPR
+	  && DECL_P (TREE_OPERAND (TREE_OPERAND (expr, 1), 0))
+	return expr;
       maybe_warn_nodiscard (expr, implicit);

Alexandre Oliva, freedom fighter
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

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