Re: [PATCH] c++: detecting copy-init context during CTAD [PR102137]

[Jason Merrill <jason@redhat.com>]

On 3/8/22 17:17, Patrick Palka wrote:
> On Tue, 8 Mar 2022, Jason Merrill wrote:
> 
>> On 3/8/22 14:38, Patrick Palka wrote:
>>> On Tue, 8 Mar 2022, Jason Merrill wrote:
>>>
>>>> On 3/8/22 11:36, Patrick Palka wrote:
>>>>> On Mon, 7 Mar 2022, Jason Merrill wrote:
>>>>>
>>>>>> On 3/7/22 10:47, Patrick Palka wrote:
>>>>>>> On Fri, 4 Mar 2022, Jason Merrill wrote:
>>>>>>>
>>>>>>>> On 3/4/22 14:24, Patrick Palka wrote:
>>>>>>>>> Here we're failing to communicate to cp_finish_decl from
>>>>>>>>> tsubst_expr
>>>>>>>>> that we're in a copy-initialization context (via the
>>>>>>>>> LOOKUP_ONLYCONVERTING
>>>>>>>>> flag), which causes do_class_deduction to always consider
>>>>>>>>> explicit
>>>>>>>>> deduction guides when performing CTAD for a templated variable
>>>>>>>>> initializer.
>>>>>>>>>
>>>>>>>>> We could fix this by passing LOOKUP_ONLYCONVERTING appropriately
>>>>>>>>> when
>>>>>>>>> calling cp_finish_decl from tsubst_expr, but it seems
>>>>>>>>> do_class_deduction
>>>>>>>>> can determine if we're in a copy-init context by simply
>>>>>>>>> inspecting
>>>>>>>>> the
>>>>>>>>> initializer, and thus render its flags parameter unnecessary,
>>>>>>>>> which
>>>>>>>>> is
>>>>>>>>> what this patch implements.  (If we were to fix this in
>>>>>>>>> tsubst_expr
>>>>>>>>> instead, I think we'd have to inspect the initializer in the
>>>>>>>>> same
>>>>>>>>> way
>>>>>>>>> in order to detect a copy-init context?)
>>>>>>>>
>>>>>>>> Hmm, does this affect conversions as well?
>>>>>>>>
>>>>>>>> Looks like it does:
>>>>>>>>
>>>>>>>> struct A
>>>>>>>> {
>>>>>>>>       explicit operator int();
>>>>>>>> };
>>>>>>>>
>>>>>>>> template <class T> void f()
>>>>>>>> {
>>>>>>>>       T t = A();
>>>>>>>> }
>>>>>>>>
>>>>>>>> int main()
>>>>>>>> {
>>>>>>>>       f<int>(); // wrongly accepted
>>>>>>>> }
>>>>>>>>
>>>>>>>> The reverse, initializing via an explicit constructor, is caught
>>>>>>>> by
>>>>>>>> code
>>>>>>>> in
>>>>>>>> build_aggr_init much like the code your patch adds to
>>>>>>>> do_auto_deduction;
>>>>>>>> perhaps we should move/copy that code to cp_finish_decl?
>>>>>>>
>>>>>>> Ah, makes sense.  Moving that code from build_aggr_init to
>>>>>>> cp_finish_decl broke things, but using it in both spots seems to
>>>>>>> work
>>>>>>> well.  And I suppose we might as well use it in do_class_deduction
>>>>>>> too,
>>>>>>> since doing so lets us remove the flags parameter.
>>>>>>
>>>>>> Before removing the flags parameter please try asserting that it now
>>>>>> matches
>>>>>> is_copy_initialization and see if anything breaks.
>>>>>
>>>>> I added to do_class_deduction:
>>>>>
>>>>>      gcc_assert (bool(flags & LOOKUP_ONLYCONVERTING) ==
>>>>> is_copy_initialization
>>>>> (init));
>>>>>
>>>>> Turns out removing the flags parameter breaks CTAD for new-expressions
>>>>> of the form 'new TT(x)' because in this case build_new passes just 'x'
>>>>> as the initializer to do_auto_deduction (as opposed to a single
>>>>> TREE_LIST),
>>>>> for which is_copy_initialization returns true even though it's really
>>>>> direct initalization.
>>>>>
>>>>> Also turns out we're similarly not passing the right LOOKUP_* flags to
>>>>> cp_finish_decl from instantiate_body, which breaks consideration of
>>>>> explicit conversions/deduction guides when instantiating the initializer
>>>>> of a static data member.  I added some xfailed testcases for these
>>>>> situations.
>>>>
>>>> Maybe we want to check is_copy_initialization in cp_finish_decl?
>>>
>>> That seems to work nicely :) All xfailed tests for the static data
>>> member initialization case now also pass.
>>>
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>>> trunk?
>>>
>>> -- >8 --
>>>
>>> Subject: [PATCH] c++: detecting copy-init context during CTAD [PR102137]
>>>
>>> Here we're failing to communicate to cp_finish_decl from tsubst_expr
>>> that we're in a copy-initialization context (via the LOOKUP_ONLYCONVERTING
>>> flag), which causes us to always consider explicit deduction guides when
>>> performing CTAD for a templated variable initializer.
>>>
>>> It turns out this bug also affects consideration of explicit conversion
>>> operators for the same reason.  But consideration of explicit constructors
>>> seems to do the right thing thanks to code in build_aggr_init that sets
>>> LOOKUP_ONLYCONVERTING when the initializer represents copy-initialization.
>>>
>>> This patch fixes this by making cp_finish_decl set LOOKUP_ONLYCONVERTING
>>> by inspecting the initializer like build_aggr_init does, so that callers
>>> don't need to explicitly pass this flag.
>>>
>>> 	PR c++/102137
>>> 	PR c++/87820
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* cp-tree.h (is_copy_initialization): Declare.
>>> 	* decl.cc (cp_finish_decl): Set LOOKUP_ONLYCONVERTING
>>> 	when is_copy_initialization is true.
>>> 	* init.cc (build_aggr_init): Split out copy-initialization
>>> 	check into ...
>>> 	(is_copy_initialization): ... here.
>>> 	* pt.cc (instantiate_decl): Pass 0 instead of
>>> 	LOOKUP_ONLYCONVERTING as flags to cp_finish_decl.
>>
>> Any reason not to use LOOKUP_NORMAL, both here and in tsubst_expr?  OK either
>> way.
> 
> Thanks a lot.  I'm not really sure what the consequences of using
> LOOKUP_NORMAL (which implies LOOKUP_PROTECT) instead of 0 would be here,
> and there's a bunch of other callers of cp_finish_decl which pass 0 or
> some other value that excludes LOOKUP_PROTECT.  So I figure such a change
> would be better off as a separate patch that changes/audits all such
> cp_finish_decl at once.

Agreed.  Not important, just curious.

>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp0x/explicit15.C: New test.
>>> 	* g++.dg/cpp1z/class-deduction108.C: New test.
>>> ---
>>>    gcc/cp/cp-tree.h                              |  1 +
>>>    gcc/cp/decl.cc                                |  3 +
>>>    gcc/cp/init.cc                                | 20 +++--
>>>    gcc/cp/pt.cc                                  |  3 +-
>>>    gcc/testsuite/g++.dg/cpp0x/explicit15.C       | 83 +++++++++++++++++++
>>>    .../g++.dg/cpp1z/class-deduction108.C         | 78 +++++++++++++++++
>>>    6 files changed, 181 insertions(+), 7 deletions(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/explicit15.C
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction108.C
>>>
>>> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
>>> index ac723901098..fd76909ca75 100644
>>> --- a/gcc/cp/cp-tree.h
>>> +++ b/gcc/cp/cp-tree.h
>>> @@ -7039,6 +7039,7 @@ extern void emit_mem_initializers
>>> (tree);
>>>    extern tree build_aggr_init			(tree, tree, int,
>>>                                                     tsubst_flags_t);
>>>    extern int is_class_type			(tree, int);
>>> +extern bool is_copy_initialization		(tree);
>>>    extern tree build_zero_init			(tree, tree, bool);
>>>    extern tree build_value_init			(tree,
>>> tsubst_flags_t);
>>>    extern tree build_value_init_noctor		(tree, tsubst_flags_t);
>>> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
>>> index 199ac768d43..5452f7d9cb3 100644
>>> --- a/gcc/cp/decl.cc
>>> +++ b/gcc/cp/decl.cc
>>> @@ -7968,6 +7968,9 @@ cp_finish_decl (tree decl, tree init, bool
>>> init_const_expr_p,
>>>      if (type == error_mark_node)
>>>        return;
>>>    +  if (VAR_P (decl) && is_copy_initialization (init))
>>> +    flags |= LOOKUP_ONLYCONVERTING;
>>> +
>>>      /* Warn about register storage specifiers except when in GNU global
>>>         or local register variable extension.  */
>>>      if (VAR_P (decl) && DECL_REGISTER (decl) && asmspec_tree == NULL_TREE)
>>> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
>>> index 545d904c0f9..d5faa0dc519 100644
>>> --- a/gcc/cp/init.cc
>>> +++ b/gcc/cp/init.cc
>>> @@ -2019,11 +2019,7 @@ build_aggr_init (tree exp, tree init, int flags,
>>> tsubst_flags_t complain)
>>>          return stmt_expr;
>>>        }
>>>    -  if (init && init != void_type_node
>>> -      && TREE_CODE (init) != TREE_LIST
>>> -      && !(TREE_CODE (init) == TARGET_EXPR
>>> -	   && TARGET_EXPR_DIRECT_INIT_P (init))
>>> -      && !DIRECT_LIST_INIT_P (init))
>>> +  if (is_copy_initialization (init))
>>>        flags |= LOOKUP_ONLYCONVERTING;
>>>        is_global = begin_init_stmts (&stmt_expr, &compound_stmt);
>>> @@ -2331,6 +2327,20 @@ is_class_type (tree type, int or_else)
>>>      return 1;
>>>    }
>>>    +/* Returns true iff the variable initializer INIT represents
>>> +   copy-initialization (and therefore we must set LOOKUP_ONLYCONVERTING
>>> +   when processing it).  */
>>> +
>>> +bool
>>> +is_copy_initialization (tree init)
>>> +{
>>> +  return (init && init != void_type_node
>>> +	  && TREE_CODE (init) != TREE_LIST
>>> +	  && !(TREE_CODE (init) == TARGET_EXPR
>>> +	       && TARGET_EXPR_DIRECT_INIT_P (init))
>>> +	  && !DIRECT_LIST_INIT_P (init));
>>> +}
>>> +
>>>    /* Build a reference to a member of an aggregate.  This is not a C++
>>>       `&', but really something which can have its address taken, and
>>>       then act as a pointer to member, for example TYPE :: FIELD can have
>>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>>> index 402365285cf..e8b5d8fbb73 100644
>>> --- a/gcc/cp/pt.cc
>>> +++ b/gcc/cp/pt.cc
>>> @@ -26602,8 +26602,7 @@ instantiate_decl (tree d, bool defer_ok, bool
>>> expl_inst_class_mem_p)
>>>    	      const_init
>>>    		= DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (code_pattern);
>>>    	      cp_finish_decl (d, init, /*init_const_expr_p=*/const_init,
>>> -			      /*asmspec_tree=*/NULL_TREE,
>>> -			      LOOKUP_ONLYCONVERTING);
>>> +			      /*asmspec_tree=*/NULL_TREE, 0);
>>>    	    }
>>>    	  if (enter_context)
>>>    	    pop_nested_class ();
>>> diff --git a/gcc/testsuite/g++.dg/cpp0x/explicit15.C
>>> b/gcc/testsuite/g++.dg/cpp0x/explicit15.C
>>> new file mode 100644
>>> index 00000000000..076d8b31631
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/explicit15.C
>>> @@ -0,0 +1,83 @@
>>> +// PR c++/87820
>>> +// { dg-do compile { target c++11 } }
>>> +
>>> +struct A {
>>> +  constexpr explicit operator int() const { return 0; }
>>> +};
>>> +
>>> +template<class T>
>>> +void f() {
>>> +  A a;
>>> +  T t1 = a; // { dg-error "cannot convert" }
>>> +  T t2 = {a}; // { dg-error "cannot convert" }
>>> +  T t3(a);
>>> +  T t4{a};
>>> +  T t5 = T(a);
>>> +  T t6 = T{a};
>>> +  new T(a);
>>> +  new T{a};
>>> +}
>>> +
>>> +template<class T>
>>> +void g() {
>>> +  T t;
>>> +  int n1 = t; // { dg-error "cannot convert" }
>>> +  int n2 = {t}; // { dg-error "cannot convert" }
>>> +  int n3(t);
>>> +  int n4{t};
>>> +  int n5 = int(t);
>>> +  int n6 = int{t};
>>> +  new int(t);
>>> +  new int{t};
>>> +}
>>> +
>>> +template void f<int>();
>>> +template void g<A>();
>>> +
>>> +template<class T>
>>> +struct B {
>>> +  static constexpr A a{};
>>> +  static constexpr T t1 = a; // { dg-error "cannot convert" }
>>> +  static constexpr T t2 = {a}; // { dg-error "cannot convert" }
>>> +  static constexpr T t4{a}; // { dg-bogus "cannot convert" }
>>> +  static constexpr T t5 = T(a);
>>> +  static constexpr T t6 = T{a};
>>> +};
>>> +
>>> +template<class T>
>>> +struct C {
>>> +  static constexpr T t{};
>>> +  static constexpr int n1 = t; // { dg-error "cannot convert" }
>>> +  static constexpr int n2 = {t}; // { dg-error "cannot convert" }
>>> +  static constexpr int n4{t}; // { dg-bogus "cannot convert" }
>>> +  static constexpr int n5 = int(t);
>>> +  static constexpr int n6 = int{t};
>>> +};
>>> +
>>> +template struct B<int>;
>>> +template struct C<A>;
>>> +
>>> +#if __cpp_inline_variables
>>> +template<class T>
>>> +struct D {
>>> +  static inline A a;
>>> +  static inline T t1 = a; // { dg-error "cannot convert" "" { target c++17
>>> } }
>>> +  static inline T t2 = {a}; // { dg-error "cannot convert" "" { target
>>> c++17 } }
>>> +  static inline T t4{a};
>>> +  static inline T t5 = T(a);
>>> +  static inline T t6 = T{a};
>>> +};
>>> +
>>> +template<class T>
>>> +struct E {
>>> +  static inline T t;
>>> +  static inline int n1 = t; // { dg-error "cannot convert" "" { target
>>> c++17 } }
>>> +  static inline int n2 = {t}; // { dg-error "cannot convert" "" { target
>>> c++17 } }
>>> +  static inline int n4{t};
>>> +  static inline int n5 = int(t);
>>> +  static inline int n6 = int{t};
>>> +};
>>> +
>>> +template struct D<int>;
>>> +template struct E<A>;
>>> +#endif
>>> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction108.C
>>> b/gcc/testsuite/g++.dg/cpp1z/class-deduction108.C
>>> new file mode 100644
>>> index 00000000000..e82c4bafb04
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction108.C
>>> @@ -0,0 +1,78 @@
>>> +// PR c++/102137
>>> +// { dg-do compile { target c++17 } }
>>> +
>>> +template<class T>
>>> +struct A {
>>> +  constexpr A() { }
>>> +  constexpr A(int) { }
>>> +};
>>> +
>>> +explicit A(...) -> A<int>;
>>> +
>>> +template<template<class> class TT>
>>> +void f() {
>>> +  TT x1 = 0; // { dg-error "deduction|no match" }
>>> +  TT x2 = {0}; // { dg-error "explicit deduction guide" }
>>> +  TT x3(0);
>>> +  TT x4{0};
>>> +  TT x5;
>>> +  new TT(0);
>>> +  new TT{0};
>>> +  new TT();
>>> +  new TT{};
>>> +  new TT;
>>> +}
>>> +
>>> +template<class T>
>>> +void g(T t) {
>>> +  A a1 = t; // { dg-error "deduction|no match" }
>>> +  A a2 = {t}; // { dg-error "explicit deduction guide" }
>>> +  A a3(t);
>>> +  A a4{t};
>>> +  A a5;
>>> +  new A(t);
>>> +  new A{t};
>>> +}
>>> +
>>> +template void f<A>();
>>> +template void g(int);
>>> +
>>> +template<template<class> class TT>
>>> +struct B {
>>> +  static inline TT x1 = 0; // { dg-error "deduction|no match" }
>>> +  static inline TT x2 = {0}; // { dg-error "explicit deduction guide" }
>>> +  static inline TT x4{0};
>>> +  static inline TT x5;
>>> +};
>>> +
>>> +template<class T>
>>> +struct C {
>>> +  static inline T t;
>>> +  static inline A a1 = t; // { dg-error "deduction|no match" }
>>> +  static inline A a2 = {t}; // { dg-error "explicit deduction guide" }
>>> +  static inline A a4{t};
>>> +  static inline A a5{};
>>> +};
>>> +
>>> +template struct B<A>;
>>> +template struct C<int>;
>>> +
>>> +template<template<class> class TT>
>>> +struct E {
>>> +  static constexpr TT x1 = 0; // { dg-error "deduction|no match" }
>>> +  static constexpr TT x2 = {0}; // { dg-error "explicit deduction guide" }
>>> +  static constexpr TT x4{0};
>>> +  static constexpr TT x5{};
>>> +};
>>> +
>>> +template<class T>
>>> +struct F {
>>> +  static constexpr T t{};
>>> +  static constexpr A a1 = t; // { dg-error "deduction|no match" }
>>> +  static constexpr A a2 = {t}; // { dg-error "explicit deduction guide" }
>>> +  static constexpr A a4{t};
>>> +  static constexpr A a5{};
>>> +};
>>> +
>>> +template struct E<A>;
>>> +template struct F<int>;
>>
>>
>