[PATCH v2] c++: fixes for derived-to-base reference binding [PR107085]

[Marek Polacek <polacek@redhat.com>]

On Wed, Oct 05, 2022 at 08:25:29PM -0400, Jason Merrill wrote:
> On 10/5/22 17:27, Marek Polacek wrote:
> > This PR reports that
> > 
> >    struct Base {};
> >    struct Derived : Base {};
> >    static_assert(__reference_constructs_from_temporary(Base const&, Derived));
> > 
> > doesn't pass, which it should: it's just like
> > 
> >    const Base& b(Derived{});
> > 
> > where we bind 'b' to the Base subobject of a temporary object of type
> > Derived.  The ck_base conversion didn't have ->need_temporary_p set because
> > we didn't need to create a temporary object just for the base, but the whole
> > object is a temporary so we're still binding to a temporary.  Fixed by
> > the conv_is_prvalue hunk.
> > 
> > That broke a bunch of tests.  I've distilled the issue into a simple test
> > in elision4.C.  Essentially, we have
> > 
> >    struct B { /* ... */ };
> >    struct D : B { };
> >    B b = D();
> > 
> > and we set force_elide in build_over_call, but we're unable to actually
> > elide the B::B(B&&) call, and crash on gcc_assert (!force_elide);.
> > 
> > <https://en.cppreference.com/w/cpp/language/copy_elision> says that copy
> > elision "can only apply when the object being initialized is known not to be
> > a potentially-overlapping subobject".  So I suppose we shouldn't force_elide
> > the B::B(B&&) call.  I don't belive the CWG 2327 code was added to handle
> > derived-to-base conversions, at that time conv_binds_ref_to_prvalue wasn't
> > checking ck_base at all.
> > 
> > Does that make sense?  If so...
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > 	PR c++/107085
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* call.cc (conv_is_prvalue): Return true if the base subobject is part
> > 	of a temporary object.
> 
> No, the base subobject of a prvalue is an xvalue.

Ah, so this is just like T().m where T() is a prvalue but the whole thing
is an xvalue.  Duly noted.
 
> I think the problem is that an expression being a prvalue is a subset of
> binding a reference to a temporary, and we shouldn't try to express both of
> those using the same function: you need a separate
> conv_binds_ref_to_temporary.

Ack, so how about this?  Thanks,

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
This PR reports that

  struct Base {};
  struct Derived : Base {};
  static_assert(__reference_constructs_from_temporary(Base const&, Derived));

doesn't pass, which it should: it's just like

  const Base& b(Derived{});

where we bind 'b' to the Base subobject of a temporary object of type
Derived.  The ck_base conversion didn't have ->need_temporary_p set because
we didn't need to create a temporary object just for the base, but the whole
object is a temporary so we're still binding to a temporary.  Since the
Base subobject is an xvalue, a new function is introduced.

	PR c++/107085

gcc/cp/ChangeLog:

	* call.cc (conv_binds_ref_to_temporary): New.
	(ref_conv_binds_directly): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/reference_constructs_from_temporary1.C: Adjust expected
	result.
	* g++.dg/ext/reference_converts_from_temporary1.C: Likewise.
	* g++.dg/cpp0x/elision4.C: New test.
---
 gcc/cp/call.cc                                | 23 ++++++++++++++++++-
 gcc/testsuite/g++.dg/cpp0x/elision4.C         | 15 ++++++++++++
 .../reference_constructs_from_temporary1.C    |  2 +-
 .../ext/reference_converts_from_temporary1.C  |  2 +-
 4 files changed, 39 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/elision4.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index bd04a1d309a..715a83f5a69 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -9210,6 +9210,27 @@ conv_binds_ref_to_prvalue (conversion *c)
   return conv_is_prvalue (next_conversion (c));
 }
 
+/* True iff C is a conversion that binds a reference to a temporary.
+   This is a superset of conv_binds_ref_to_prvalue: here we're also
+   interested in xvalues.  */
+
+static bool
+conv_binds_ref_to_temporary (conversion *c)
+{
+  if (conv_binds_ref_to_prvalue (c))
+    return true;
+  if (c->kind != ck_ref_bind)
+    return false;
+  c = next_conversion (c);
+  /* This is the case for
+       struct Base {};
+       struct Derived : Base {};
+       const Base& b(Derived{});
+     where we bind 'b' to the Base subobject of a temporary object of type
+     Derived.  The subobject is an xvalue; the whole object is a prvalue.  */
+  return (c->kind == ck_base && conv_is_prvalue (next_conversion (c)));
+}
+
 /* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE does
    not involve creating a temporary.  Return tristate::TS_FALSE if converting
    EXPR to a reference type TYPE binds the reference to a temporary.  If the
@@ -9230,7 +9251,7 @@ ref_conv_binds_directly (tree type, tree expr, bool direct_init_p /*= false*/)
 					  /*c_cast_p=*/false, flags, tf_none);
   tristate ret (tristate::TS_UNKNOWN);
   if (conv && !conv->bad_p)
-    ret = tristate (!conv_binds_ref_to_prvalue (conv));
+    ret = tristate (!conv_binds_ref_to_temporary (conv));
 
   /* Free all the conversions we allocated.  */
   obstack_free (&conversion_obstack, p);
diff --git a/gcc/testsuite/g++.dg/cpp0x/elision4.C b/gcc/testsuite/g++.dg/cpp0x/elision4.C
new file mode 100644
index 00000000000..3cc2e3afa5d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/elision4.C
@@ -0,0 +1,15 @@
+// PR c++/107085
+// { dg-do compile { target c++11 } }
+
+struct X {
+  X();
+  X(X&&);
+};
+struct Z : X {};
+X x1 = Z();
+X x2 = X(Z());
+
+struct B { };
+struct D : B { };
+B b1 = D();
+B b2 = B(D());
diff --git a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
index 76de905a35d..5354b1dc4e6 100644
--- a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
+++ b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
@@ -201,7 +201,7 @@ SA(!__reference_constructs_from_temporary(const int&, H));
 SA(!__reference_constructs_from_temporary(int&&, G2));
 SA(!__reference_constructs_from_temporary(const int&, H2));
 
-SA(!__reference_constructs_from_temporary(const Base&, Der));
+SA(__reference_constructs_from_temporary(const Base&, Der));
 
 // This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
 SA(!__reference_constructs_from_temporary(int&&, id<int[3]>));
diff --git a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
index 90196c38742..e6c159e9b00 100644
--- a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
+++ b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
@@ -201,7 +201,7 @@ SA( __reference_converts_from_temporary(const int&, H));
 SA(!__reference_converts_from_temporary(int&&, G2));
 SA(!__reference_converts_from_temporary(const int&, H2));
 
-SA(!__reference_converts_from_temporary(const Base&, Der));
+SA(__reference_converts_from_temporary(const Base&, Der));
 
 // This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
 SA(!__reference_converts_from_temporary(int&&, id<int[3]>));

base-commit: 3ec926d36fbf7cb3ff45759471139f3a71d1c4de
-- 
2.37.3