Re: [PATCH] c++: Fix ICE from op_unqualified_lookup [PR97582]

[Patrick Palka via Gcc-patches]

On Wed, 10 Feb 2021, Jason Merrill wrote:

> On 2/10/21 12:32 PM, Patrick Palka wrote:
> > On Wed, 10 Feb 2021, Jason Merrill wrote:
> > 
> > > On 2/9/21 5:12 PM, Patrick Palka wrote:
> > > > On Tue, 2 Feb 2021, Jason Merrill wrote:
> > > > 
> > > > > On 2/2/21 12:19 AM, Patrick Palka wrote:
> > > > > > In this testcase, we're crashing because the lookup of operator+
> > > > > > from
> > > > > > within the generic lambda via lookup_name finds multiple bindings
> > > > > > (namely C1::operator+ and C2::operator+) and returns a TREE_LIST
> > > > > > thereof, something which maybe_save_operator_binding isn't prepared
> > > > > > to
> > > > > > handle.
> > > > > > 
> > > > > > Since we already discard the result of lookup_name when it returns a
> > > > > > class-scope binding here, it seems cleaner (and equivalent) to
> > > > > > instead
> > > > > > communicate to lookup_name that we don't want such bindings in the
> > > > > > first
> > > > > > place.  While this change seems like an improvement on its own, it
> > > > > > also
> > > > > > fixes the mentioned PR, because the call to lookup_name now returns
> > > > > > NULL_TREE rather than a TREE_LIST of (unwanted) class-scope
> > > > > > bindings.
> > > > > > 
> > > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
> > > > > > for
> > > > > > trunk/9/10?
> > > > > > 
> > > > > > gcc/cp/ChangeLog:
> > > > > > 
> > > > > >     PR c++/97582
> > > > > >     * name-lookup.c (op_unqualified_lookup): Pass BLOCK_NAMESPACE
> > > > > > to
> > > > > >     lookup_name in order to ignore class-scope bindings, rather
> > > > > >     than discarding them after the fact.
> > > > > > 
> > > > > > gcc/testsuite/ChangeLog:
> > > > > > 
> > > > > >     PR c++/97582
> > > > > >     * g++.dg/cpp0x/lambda/lambda-template17.C: New test.
> > > > > > ---
> > > > > >     gcc/cp/name-lookup.c                                  | 11
> > > > > > +++--------
> > > > > >     gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C |  8
> > > > > > ++++++++
> > > > > >     2 files changed, 11 insertions(+), 8 deletions(-)
> > > > > >     create mode 100644
> > > > > > gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C
> > > > > > 
> > > > > > diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
> > > > > > index 52e4a630e25..46d6cc0dfa4 100644
> > > > > > --- a/gcc/cp/name-lookup.c
> > > > > > +++ b/gcc/cp/name-lookup.c
> > > > > > @@ -9213,17 +9213,12 @@ op_unqualified_lookup (tree fnname)
> > > > > >             return NULL_TREE;
> > > > > >         }
> > > > > >     -  tree fns = lookup_name (fnname);
> > > > > > +  /* We don't need to remember class-scope functions or
> > > > > > declarations,
> > > > > > +     normal unqualified lookup will find them again.  */
> > > > > > +  tree fns = lookup_name (fnname, LOOK_where::BLOCK_NAMESPACE);
> > > > > 
> > > > > Hmm, I'd expect this to look past class-scope declarations to find
> > > > > namespace-scope declarations, but we want class decls to hide decls in
> > > > > an
> > > > > outer scope.
> > > > 
> > > > D'oh, good point.  But IIUC, even if we did return (and later inject at
> > > > instantiation time) namespace-scope declarations that were hidden by
> > > > class-scope declarations, wouldn't the lookup at instantiation time
> > > > still find and prefer the class-scope bindings (as desired)?  It seems
> > > > to me that the end result might be the same, but I'm not sure.
> > > 
> > > The injection happens in the function parameter binding level, so I'd
> > > expect
> > > it to be found before class bindings.
> > 
> > Oops, I didn't look at push_operator_bindings closely enough.  Never
> > mind about that idea then.
> > 
> > > 
> > > > Alternatively, would it be safe to assume that if lookup_name returns an
> > > > ambiguous result, then the result must consist of class-scope
> > > > declarations and so we can discard it?
> > > 
> > > That isn't true in general:
> > > 
> > > inline namespace A { int i; }
> > > inline namespace B { int i; }
> > > int main() { return i; } // ambiguous lookup
> > > 
> > > though I think it is true for functions.  But if the result is ambiguous,
> > > you
> > > can look at the first element to see if it's from class scope.
> > 
> > I see, that's good to know.  So something like this?
> > 
> > -- >8 --
> > 
> > Subject: [PATCH] c++: Fix ICE from op_unqualified_lookup [PR97582]
> > 
> > In this testcase, we're crashing because the lookup of operator+ from
> > within the generic lambda via lookup_name finds multiple bindings
> > (namely C1::operator+ and C2::operator+) and returns a TREE_LIST
> > thereof, something which op_unqualified_lookup (and
> > push_operator_bindings) isn't prepared to handle.
> > 
> > This patch makes op_unqualified_lookup and push_operator_bindings handle
> > an ambiguous lookup result appropriately.
> > 
> > gcc/cp/ChangeLog:
> > 
> >     PR c++/97582
> >     * name-lookup.c (op_unqualified_lookup): Handle an ambiguous
> >     lookup result by discarding it if the first element is
> >     a class-scope declaration, otherwise return it.
> >     (push_operator_bindings): Handle an ambiguous lookup result by
> >     doing push_local_binding on each element in the list.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> >     PR c++/97582
> >     * g++.dg/cpp0x/lambda/lambda-template17.C: New test.
> > ---
> >   gcc/cp/name-lookup.c                             | 16 ++++++++++++----
> >   .../g++.dg/cpp0x/lambda/lambda-template17.C      |  8 ++++++++
> >   2 files changed, 20 insertions(+), 4 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C
> > 
> > diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
> > index 1f4a7ac1d0c..27af21d9ac9 100644
> > --- a/gcc/cp/name-lookup.c
> > +++ b/gcc/cp/name-lookup.c
> > @@ -9219,11 +9219,15 @@ op_unqualified_lookup (tree fnname)
> >       /* Remember we found nothing!  */
> >       return error_mark_node;
> >   -  tree d = is_overloaded_fn (fns) ? get_first_fn (fns) : fns;
> > -  if (DECL_CLASS_SCOPE_P (d))
> > +  tree fn = fns;
> > +  if (TREE_CODE (fn) == TREE_LIST)
> > +    fn = TREE_VALUE (fn);
> > +  if (is_overloaded_fn (fn))
> > +    fn = get_first_fn (fn);
> > +  if (DECL_CLASS_SCOPE_P (fn))
> >       /* We don't need to remember class-scope functions or declarations,
> >          normal unqualified lookup will find them again.  */
> > -    fns = NULL_TREE;
> > +    return NULL_TREE;
> >       return fns;
> >   }
> > @@ -9302,7 +9306,11 @@ push_operator_bindings ()
> >         if (tree val = TREE_VALUE (binds))
> >     {
> >       tree name = TREE_PURPOSE (binds);
> > -     push_local_binding (name, val, /*using*/true);
> > +     if (TREE_CODE (val) == TREE_LIST)
> > +       for (tree v = val; v; v = TREE_CHAIN (v))
> > +         push_local_binding (name, TREE_VALUE (v), /*using*/true);
> > +     else
> > +       push_local_binding (name, val, /*using*/true);
> >     }
> >   }
> >   diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C
> > b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C
> > new file mode 100644
> > index 00000000000..6cafbab8cb0
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C
> > @@ -0,0 +1,8 @@
> > +// PR c++/97582
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct C1 { void operator+(); };
> > +struct C2 { void operator+(); };
> > +struct C3 : C1, C2 {
> > +  template <class T> void get() { [] (T x) { +x; }; }
> > +};
> 
> The testcase should instantiate get() and check that we get the appropriate
> error.

Done in the patch below:

-- >8 --

Subject: [PATCH] c++: Fix ICE from op_unqualified_lookup [PR97582]

In this testcase, we're crashing because the lookup of operator+ from
within the generic lambda via lookup_name finds multiple bindings
(namely C1::operator+ and C2::operator+) and returns a TREE_LIST
thereof, something which op_unqualified_lookup (and
push_operator_bindings) isn't prepared to handle.

This patch makes op_unqualified_lookup and push_operator_bindings handle
an ambiguous lookup result appropriately.

gcc/cp/ChangeLog:

        PR c++/97582
        * name-lookup.c (op_unqualified_lookup): Handle an ambiguous
        lookup result by discarding it if the first element is a
        class-scope declaration, otherwise return it.
        (push_operator_bindings): Handle an ambiguous lookup result by
        doing push_local_binding on each element in the list.

gcc/testsuite/ChangeLog:

        PR c++/97582
        * g++.dg/cpp0x/lambda/lambda-template17.C: New test.
---
 gcc/cp/name-lookup.c                             | 16 ++++++++++++----
 .../g++.dg/cpp0x/lambda/lambda-template17.C      | 10 ++++++++++
 2 files changed, 22 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C

diff --git a/gcc/cp/name-lookup.c b/gcc/cp/name-lookup.c
index 1f4a7ac1d0c..27af21d9ac9 100644
--- a/gcc/cp/name-lookup.c
+++ b/gcc/cp/name-lookup.c
@@ -9219,11 +9219,15 @@ op_unqualified_lookup (tree fnname)
     /* Remember we found nothing!  */
     return error_mark_node;
 
-  tree d = is_overloaded_fn (fns) ? get_first_fn (fns) : fns;
-  if (DECL_CLASS_SCOPE_P (d))
+  tree fn = fns;
+  if (TREE_CODE (fn) == TREE_LIST)
+    fn = TREE_VALUE (fn);
+  if (is_overloaded_fn (fn))
+    fn = get_first_fn (fn);
+  if (DECL_CLASS_SCOPE_P (fn))
     /* We don't need to remember class-scope functions or declarations,
        normal unqualified lookup will find them again.  */
-    fns = NULL_TREE;
+    return NULL_TREE;
 
   return fns;
 }
@@ -9302,7 +9306,11 @@ push_operator_bindings ()
       if (tree val = TREE_VALUE (binds))
        {
          tree name = TREE_PURPOSE (binds);
-         push_local_binding (name, val, /*using*/true);
+         if (TREE_CODE (val) == TREE_LIST)
+           for (tree v = val; v; v = TREE_CHAIN (v))
+             push_local_binding (name, TREE_VALUE (v), /*using*/true);
+         else
+           push_local_binding (name, val, /*using*/true);
        }
 }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C 
b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C
new file mode 100644
index 00000000000..34dd07c983d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-template17.C
@@ -0,0 +1,10 @@
+// PR c++/97582
+// { dg-do compile { target c++11 } }
+
+struct C1 { void operator+(); };
+struct C2 { void operator+(); };
+struct C3 : C1, C2 {
+  template <class T> void get() { [] (T x) { +x; }; } // { dg-error 
"ambiguous" }
+};
+
+template void C3::get<C3>();
-- 
2.30.0.452.gfb7fa4a1fd