| Message ID | Y0BPdGc2AH9/gUtn@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v4] c++: fixes for derived-to-base reference binding [PR107085] | expand |
On 10/7/22 12:10, Marek Polacek wrote: > On Thu, Oct 06, 2022 at 06:03:57PM -0400, Jason Merrill wrote: >> On 10/6/22 17:43, Marek Polacek wrote: >>> On Thu, Oct 06, 2022 at 02:00:40PM -0400, Jason Merrill wrote: >>>> On 10/6/22 13:51, Marek Polacek wrote: >>>>> On Thu, Oct 06, 2022 at 10:58:44AM -0400, Jason Merrill wrote: >>>>>> On 10/6/22 10:49, Marek Polacek wrote: >>>>>>> On Wed, Oct 05, 2022 at 08:25:29PM -0400, Jason Merrill wrote: >>>>>>>> On 10/5/22 17:27, Marek Polacek wrote: >>>>>>> --- 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))); >>>>>> >>>>>> I think you also want to check for the case of c->u.expr being a >>>>>> COMPONENT_REF/ARRAY_REF around a TARGET_EXPR, as you mentioned. >>>>> >>>>> I see. So this would be achieved using e.g. >>>>> >>>>> struct B { }; >>>>> struct D : B { }; >>>>> struct C { >>>>> D d; >>>>> }; >>>>> >>>>> const B& b = C{}.d; >>>> >>>> Yes. >>>> >>>>> Except I'm not sure how to trigger this via the built-in, which takes two types. >>>>> Am I missing something obvious? >>>> >>>> Indeed, it can't be triggered by the built-in. But I see >>>> ref_conv_binds_directly is also called from warn_for_range_copy, which ought >>>> to be able to trigger it. >>> >>> Even there, I think, it's not possible to trigger it with, say (d is an >>> array of D): >>> >>> for (const B &b : C{}.d) >>> // ... >>> >>> because warn_for_range_copy gets *__for_begin as the EXPR, which means that >>> we won't get to the TARGET_EXPR. It's like >>> >>> auto&& __for_begin = C{}.d; >>> const B &b = *__for_begin; >>> >>> and the conversion warn_for_range_copy sees is D -> const B -> const B&, the >>> original .u.expr is *__for_begin. >> >> Ah, right, and so the C{} temporary gets lifetime-extended and there's no >> problem to warn about. >> >>> I could add some checking assert to conv_binds_ref_to_temporary to see if >>> we ever encounter a COMPONENT_REF/ARRAY_REF around a TARGET_EXPR... >> >> Or try to handle it properly even if we can't exercise the code yet. > > Okay, done. Thanks. >> I'm thinking again about warning for e.g. >> >> const B& b = frotz(C{}.d); >> >> where frotz passes through its reference argument, which becomes a dangling >> reference. Handling the C{}.d case seems relevant to such a warning. >> >> I wonder how many false positives there would be for warning about this for >> any function that both takes and returns a reference? > > I suppose we'd have to ask if any (or all?) of the parameters is/are > a reference, to handle std::max? I was thinking to warn if, in a context that would extend temporary lifetime, a call argument binds a reference to a temporary and the function returns a reference. >> Do you want to try that, or shall I? > > So this is like the first example in https://gcc.gnu.org/PR106393, which > is assigned to...me. Let me take a stab at the simpler example. If I > can't make any progress within, say, a week, I'll unassign so as not to > block it. > > Meanwhile, here's the updated patch: > > 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): Rename to... > (ref_conv_binds_to_temporary): ...this. Use > conv_binds_ref_to_temporary. > * cp-tree.h (ref_conv_binds_directly): Rename to... > (ref_conv_binds_to_temporary): ...this. > * method.cc (ref_xes_from_temporary): Use ref_conv_binds_to_temporary. > * parser.cc (warn_for_range_copy): Likewise. > > 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 | 45 ++++++++++++++++--- > gcc/cp/cp-tree.h | 2 +- > gcc/cp/method.cc | 2 +- > gcc/cp/parser.cc | 5 ++- > gcc/testsuite/g++.dg/cpp0x/elision4.C | 15 +++++++ > .../reference_constructs_from_temporary1.C | 2 +- > .../ext/reference_converts_from_temporary1.C | 2 +- > 7 files changed, 61 insertions(+), 12 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..833cd014b7d 100644 > --- a/gcc/cp/call.cc > +++ b/gcc/cp/call.cc > @@ -9210,15 +9210,48 @@ conv_binds_ref_to_prvalue (conversion *c) > return 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 > - conversion is invalid or bad, return tristate::TS_UNKNOWN. DIRECT_INIT_P > +/* 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. */ > + if (c->kind != ck_base) > + return false; > + c = next_conversion (c); > + if (c->kind == ck_identity && c->u.expr) > + { > + if (TREE_CODE (c->u.expr) == TARGET_EXPR) > + return true; > + if ((TREE_CODE (c->u.expr) == COMPONENT_REF > + || TREE_CODE (c->u.expr) == ARRAY_REF) This should be a loop. > + && TREE_CODE (TREE_OPERAND (c->u.expr, 0)) == TARGET_EXPR) > + return true; > + } > + return false; > +} > + > +/* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE binds > + the reference to a temporary. Return tristate::TS_FALSE if converting > + EXPR to a reference type TYPE doesn't bind the reference to a temporary. If > + the conversion is invalid or bad, return tristate::TS_UNKNOWN. DIRECT_INIT_P > says whether the conversion should be done in direct- or copy-initialization > context. */ > > tristate > -ref_conv_binds_directly (tree type, tree expr, bool direct_init_p /*= false*/) > +ref_conv_binds_to_temporary (tree type, tree expr, bool direct_init_p/*=false*/) > { > gcc_assert (TYPE_REF_P (type)); > > @@ -9230,7 +9263,7 @@ 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/cp/cp-tree.h b/gcc/cp/cp-tree.h > index 8bc1c2dc7fd..469eb2fdb25 100644 > --- a/gcc/cp/cp-tree.h > +++ b/gcc/cp/cp-tree.h > @@ -6534,7 +6534,7 @@ extern bool sufficient_parms_p (const_tree); > extern tree type_decays_to (tree); > extern tree extract_call_expr (tree); > extern tree build_trivial_dtor_call (tree, bool = false); > -extern tristate ref_conv_binds_directly (tree, tree, bool = false); > +extern tristate ref_conv_binds_to_temporary (tree, tree, bool = false); > extern tree build_user_type_conversion (tree, tree, int, > tsubst_flags_t); > extern tree build_new_function_call (tree, vec<tree, va_gc> **, > diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc > index 55af5c43c18..622e1b9802e 100644 > --- a/gcc/cp/method.cc > +++ b/gcc/cp/method.cc > @@ -2233,7 +2233,7 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p) > tree val = build_stub_object (from); > if (!TYPE_REF_P (from) && TREE_CODE (from) != FUNCTION_TYPE) > val = CLASS_TYPE_P (from) ? force_rvalue (val, tf_none) : rvalue (val); > - return ref_conv_binds_directly (to, val, direct_init_p).is_false (); > + return ref_conv_binds_to_temporary (to, val, direct_init_p).is_true (); > } > > /* Worker for is_{,nothrow_}convertible. Attempt to perform an implicit > diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc > index 555476e42e7..dc3d17c416c 100644 > --- a/gcc/cp/parser.cc > +++ b/gcc/cp/parser.cc > @@ -13731,7 +13731,8 @@ warn_for_range_copy (tree decl, tree expr) > > if (TYPE_REF_P (type)) > { > - if (glvalue_p (expr) && ref_conv_binds_directly (type, expr).is_false ()) > + if (glvalue_p (expr) > + && ref_conv_binds_to_temporary (type, expr).is_true ()) > { > auto_diagnostic_group d; > if (warning_at (loc, OPT_Wrange_loop_construct, > @@ -13762,7 +13763,7 @@ warn_for_range_copy (tree decl, tree expr) > /* If we can initialize a reference directly, suggest that to avoid the > copy. */ > tree rtype = cp_build_reference_type (type, /*rval*/false); > - if (ref_conv_binds_directly (rtype, expr).is_true ()) > + if (ref_conv_binds_to_temporary (rtype, expr).is_false ()) > { > auto_diagnostic_group d; > if (warning_at (loc, OPT_Wrange_loop_construct, > 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: f8ba88b6a811ca9bb4b8411d3f65c329fb480ee1
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index bd04a1d309a..833cd014b7d 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -9210,15 +9210,48 @@ conv_binds_ref_to_prvalue (conversion *c) return 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 - conversion is invalid or bad, return tristate::TS_UNKNOWN. DIRECT_INIT_P +/* 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. */ + if (c->kind != ck_base) + return false; + c = next_conversion (c); + if (c->kind == ck_identity && c->u.expr) + { + if (TREE_CODE (c->u.expr) == TARGET_EXPR) + return true; + if ((TREE_CODE (c->u.expr) == COMPONENT_REF + || TREE_CODE (c->u.expr) == ARRAY_REF) + && TREE_CODE (TREE_OPERAND (c->u.expr, 0)) == TARGET_EXPR) + return true; + } + return false; +} + +/* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE binds + the reference to a temporary. Return tristate::TS_FALSE if converting + EXPR to a reference type TYPE doesn't bind the reference to a temporary. If + the conversion is invalid or bad, return tristate::TS_UNKNOWN. DIRECT_INIT_P says whether the conversion should be done in direct- or copy-initialization context. */ tristate -ref_conv_binds_directly (tree type, tree expr, bool direct_init_p /*= false*/) +ref_conv_binds_to_temporary (tree type, tree expr, bool direct_init_p/*=false*/) { gcc_assert (TYPE_REF_P (type)); @@ -9230,7 +9263,7 @@ 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/cp/cp-tree.h b/gcc/cp/cp-tree.h index 8bc1c2dc7fd..469eb2fdb25 100644 --- a/gcc/cp/cp-tree.h +++ b/gcc/cp/cp-tree.h @@ -6534,7 +6534,7 @@ extern bool sufficient_parms_p (const_tree); extern tree type_decays_to (tree); extern tree extract_call_expr (tree); extern tree build_trivial_dtor_call (tree, bool = false); -extern tristate ref_conv_binds_directly (tree, tree, bool = false); +extern tristate ref_conv_binds_to_temporary (tree, tree, bool = false); extern tree build_user_type_conversion (tree, tree, int, tsubst_flags_t); extern tree build_new_function_call (tree, vec<tree, va_gc> **, diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc index 55af5c43c18..622e1b9802e 100644 --- a/gcc/cp/method.cc +++ b/gcc/cp/method.cc @@ -2233,7 +2233,7 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p) tree val = build_stub_object (from); if (!TYPE_REF_P (from) && TREE_CODE (from) != FUNCTION_TYPE) val = CLASS_TYPE_P (from) ? force_rvalue (val, tf_none) : rvalue (val); - return ref_conv_binds_directly (to, val, direct_init_p).is_false (); + return ref_conv_binds_to_temporary (to, val, direct_init_p).is_true (); } /* Worker for is_{,nothrow_}convertible. Attempt to perform an implicit diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc index 555476e42e7..dc3d17c416c 100644 --- a/gcc/cp/parser.cc +++ b/gcc/cp/parser.cc @@ -13731,7 +13731,8 @@ warn_for_range_copy (tree decl, tree expr) if (TYPE_REF_P (type)) { - if (glvalue_p (expr) && ref_conv_binds_directly (type, expr).is_false ()) + if (glvalue_p (expr) + && ref_conv_binds_to_temporary (type, expr).is_true ()) { auto_diagnostic_group d; if (warning_at (loc, OPT_Wrange_loop_construct, @@ -13762,7 +13763,7 @@ warn_for_range_copy (tree decl, tree expr) /* If we can initialize a reference directly, suggest that to avoid the copy. */ tree rtype = cp_build_reference_type (type, /*rval*/false); - if (ref_conv_binds_directly (rtype, expr).is_true ()) + if (ref_conv_binds_to_temporary (rtype, expr).is_false ()) { auto_diagnostic_group d; if (warning_at (loc, OPT_Wrange_loop_construct, 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]>));