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

On 12/14/18 5:33 PM, Alexandre Oliva wrote:
On Dec 14, 2018, Jason Merrill <> wrote:

If inh is false, we're a copy constructor, which always has a parm,
so this hunk seems unnecessary.


-      int cvquals = cp_type_quals (TREE_TYPE (parm));
+      int cvquals = parm ? cp_type_quals (TREE_TYPE (parm)) : 0;

This could also check !inh.


And in the existing code, while I'm looking at it:

The "if (inh) continue" is odd, there's no reason to iterate through
the fields ignoring all of them when we could skip the loop entirely.

Heh, funny, an earlier version of the patch that added an if (inh) to
print an error on zero-args had an 'else fields = NULL;'.  That
improvement went away along with my course change.  But look!, it's back
in the version below ;-)

Testing...  Ok to install if it passes?

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

This patch started out from the testcase in PR88146, that attempted to
synthesize an inherited ctor without any args before a varargs
ellipsis and crashed while at that, because of the unguarded
dereferencing of the parm type list, that usually contains a
terminator.  The terminator is not there for varargs functions,
however, and without any other args, we ended up dereferencing a NULL
pointer.  Oops.

Guarding accesses to parm would be easy, but not necessary.  In
do_build_copy_constructor, non-inherited ctors are copy-ctors, that
always have at least one parm, so parm needs not be guarded when we
know the access will only take place when we're dealing with an
inherited ctor.  The only other problematic use was in the cvquals
initializer, a variable only used in a loop over fields, that we
skipped individually in inherited ctors.  I've arranged to skip the
entire loop over fields for inherited ctors, and to only initialize
cvquals otherwise.

Avoiding the crash from unguarded accesses was easy, but I thought we
should still produce the sorry message we got in other testcases that
passed arguments through the ellipsis in inherited ctors.  I put a
check in, and noticed the inherited ctors were synthesized with the
location assigned to the class name, although they were initially
assigned the location of the using declaration.  I decided the latter
was better, and arranged for the better location to be retained.

Further investigation revealed the lack of a sorry message had to do
with the call being in a non-evaluated context, in this case, a
noexcept expression.  The sorry would be correctly reported in other
contexts, so I rolled back the check I'd added, but retained the
source location improvement.

I was still concerned about issuing sorry messages while instantiating
template ctors even in non-evaluated contexts, e.g., if a template
ctor had a base initializer that used an inherited ctor with enough
arguments that they'd go through an ellipsis.  I wanted to defer the
instantiation of such template ctors, but that would have been wrong
for constexpr template ctors, and already done for non-constexpr ones.
So, I just consolidated multiple test variants into a single testcase
that explores and explains various of the possibilities I thought of.

for  gcc/cp/ChangeLog

	PR c++/88146
	* method.c (do_build_copy_constructor): Skip iteration over
	fields for inherited ctors, and initialize cvquals otherwise.
	(synthesize_method): Retain location of inherited ctor.

for  gcc/testsuite/ChangeLog

	PR c++/88146
	* g++.dg/cpp0x/inh-ctor32.C: New.
  gcc/cp/method.c                         |   14 +-
  gcc/testsuite/g++.dg/cpp0x/inh-ctor32.C |  229 +++++++++++++++++++++++++++++++
  2 files changed, 238 insertions(+), 5 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/inh-ctor32.C

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index fd023e200538..4cbdadbe3d26 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -677,7 +677,7 @@ do_build_copy_constructor (tree fndecl)
        tree fields = TYPE_FIELDS (current_class_type);
        tree member_init_list = NULL_TREE;
-      int cvquals = cp_type_quals (TREE_TYPE (parm));
+      int cvquals;
        int i;
        tree binfo, base_binfo;
        tree init;
@@ -704,6 +704,11 @@ do_build_copy_constructor (tree fndecl)
  						inh, member_init_list);
+ if (!inh)
+	cvquals = cp_type_quals (TREE_TYPE (parm));
+      else
+	fields = NULL;

Let's move the initialization of "fields" inside the 'then' block here with the initialization of "cvquals", rather than clear it in the 'else'. OK with that change.


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