Re: [PATCH] c++: equivalence of non-dependent calls [PR107461]

[Jason Merrill <jason@redhat.com>]

On 2/4/23 20:08, Patrick Palka wrote:
> On Sat, 4 Feb 2023, Jason Merrill wrote:
> 
>> On 2/4/23 15:31, Patrick Palka wrote:
>>> After r13-5684-g59e0376f607805 the (pruned) callee of a non-dependent
>>> CALL_EXPR is a bare FUNCTION_DECL rather than ADDR_EXPR of FUNCTION_DECL.
>>> This innocent change revealed that cp_tree_equal doesn't first check
>>> dependentness of a CALL_EXPR before treating the callee as a dependent
>>> name, which manifests as us incorrectly accepting the first two
>>> testcases below and rejecting the third:
>>>
>>>    * In the first testcase, cp_tree_equal incorrectly returns true for
>>>      the two non-dependent CALL_EXPRs f(0) and f(0) (whose CALL_EXPR_FN
>>>      are different FUNCTION_DECLs) and so we treat #2 as a redeclaration
>>>      of #1.
>>>
>>>    * Same issue in the second testcase, for f<int*>() and f<char>().
>>>
>>>    * In the third testcase, cp_tree_equal incorrectly returns true for
>>>      f<int>() and f<void(*)(int)>() which causes us to conflate the two
>>>      dependent specializations A<decltype(f<int>()(U()))> and
>>>      A<decltype(f<void(*)(int)>()(U()))>, leading to a bogus error.
>>>
>>> This patch fixes this by making called_fns_equal treat two callees as
>>> dependent names only if the CALL_EXPRs in question are dependent.
>>>
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
>>> trunk/12?  Patch generated with -w to ignore noisy whitespace changes.
>>>
>>> 	PR c++/107461
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* pt.cc (iterative_hash_template_arg) <case CALL_EXPR>: Treat
>>> 	the callee as a dependent name only if the CALL_EXPR is
>>> 	dependent.
>>> 	* tree.cc (called_fns_equal): Take two CALL_EXPRs instead of
>>> 	CALL_EXPR_FNs thereof.  As above.
>>> 	(cp_tree_equal) <case CALL_EXPR>: Adjust call to called_fns_equal.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp0x/overload5.C: New test.
>>> 	* g++.dg/cpp0x/overload5a.C: New test.
>>> 	* g++.dg/cpp0x/overload6.C: New test.
>>> ---
>>>    gcc/cp/pt.cc                            |  1 +
>>>    gcc/cp/tree.cc                          | 33 ++++++++++++++-----------
>>>    gcc/testsuite/g++.dg/cpp0x/overload5.C  | 12 +++++++++
>>>    gcc/testsuite/g++.dg/cpp0x/overload5a.C | 10 ++++++++
>>>    gcc/testsuite/g++.dg/cpp0x/overload6.C  | 16 ++++++++++++
>>>    5 files changed, 58 insertions(+), 14 deletions(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/overload5.C
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/overload5a.C
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/overload6.C
>>>
>>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>>> index 255332dc0c1..c9360240cd2 100644
>>> --- a/gcc/cp/pt.cc
>>> +++ b/gcc/cp/pt.cc
>>> @@ -1841,6 +1841,7 @@ iterative_hash_template_arg (tree arg, hashval_t val)
>>>        case CALL_EXPR:
>>>          {
>>>    	tree fn = CALL_EXPR_FN (arg);
>>> +	if (TREE_TYPE (arg) == NULL_TREE)
>>
>> How about changing dependent_name to take the CALL_EXPR rather than the
>> CALL_EXPR_FN?  That would mean some changes to write_expression to move the
>> dependent_name handling into the CALL_EXPR handling, but that doesn't seem
>> like a bad thing.  Other callers seem like a trivial change.
> 
> Indeed changing dependent_name seems best, but I'm worried about such a
> refactoring to write_expression causing unintended mangling changes at
> this stage.  Because it seems the CALL_EXPR case of write_expression
> isn't the user of the dependent_name branch of write_expression, at
> least according to the following patch which causes us to ICE on
> mangle{37,57,58,76}.C:

Yeah, I tried the same thing.  Maybe for GCC 13 better to add a new 
function rather than change the current one.

> diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
> index f2cda3be2cf..700857f8f3c 100644
> --- a/gcc/cp/mangle.cc
> +++ b/gcc/cp/mangle.cc
> @@ -3450,6 +3450,7 @@ write_expression (tree expr)
>       }
>     else if (dependent_name (expr))
>       {
> +      gcc_unreachable ();
>         tree name = dependent_name (expr);
>         if (IDENTIFIER_ANY_OP_P (name))
>   	{
> @@ -3554,7 +3555,19 @@ write_expression (tree expr)
>   		&& type_dependent_expression_p_push (expr))
>   	      fn = OVL_NAME (fn);
>   
> -	    write_expression (fn);
> +	    if (tree name = dependent_name (fn))
> +	      {
> +		if (IDENTIFIER_ANY_OP_P (name))
> +		  {
> +		    if (abi_version_at_least (16))
> +		      write_string ("on");
> +		    if (abi_warn_or_compat_version_crosses (16))
> +		      G.need_abi_warning = 1;
> +		  }
> +		write_unqualified_id (name);
> +	      }
> +	    else
> +	      write_expression (fn);
>   	  }
>   
>   	  for (i = 0; i < call_expr_nargs (expr); ++i)
> 
> And since the CALL_EXPR case of write_expression looks through an
> ADDR_EXPR callee before recursing, IIUC the refactoring would need to
> make dependent_name look through an ADDR_EXPR callee as well, which
> seems like a desirable/correct change but I'm worried that might have
> unintended consequences as well.
> 
>>
>>>    	  if (tree name = dependent_name (fn))
>>>    	    {
>>>    	      if (TREE_CODE (fn) == TEMPLATE_ID_EXPR)
>>> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
>>> index c1da868732b..3a57e71b76e 100644
>>> --- a/gcc/cp/tree.cc
>>> +++ b/gcc/cp/tree.cc
>>> @@ -3870,16 +3870,21 @@ decl_internal_context_p (const_tree decl)
>>>      return !TREE_PUBLIC (decl);
>>>    }
>>>    -/* Subroutine of cp_tree_equal: t1 and t2 are the CALL_EXPR_FNs of two
>>> -   CALL_EXPRS.  Return whether they are equivalent.  */
>>> +/* Subroutine of cp_tree_equal: t1 and t2 are two CALL_EXPRs.
>>> +   Return whether their CALL_EXPR_FNs are equivalent.  */
>>>      static bool
>>>    called_fns_equal (tree t1, tree t2)
>>> +{
>>> +  tree fn1 = CALL_EXPR_FN (t1);
>>> +  tree fn2 = CALL_EXPR_FN (t2);
>>> +  if (TREE_TYPE (t1) == NULL_TREE
>>> +      && TREE_TYPE (t2) == NULL_TREE)
>>>        {
>>>          /* Core 1321: dependent names are equivalent even if the overload
>>> sets
>>>    	 are different.  But do compare explicit template arguments.  */
>>> -  tree name1 = dependent_name (t1);
>>> -  tree name2 = dependent_name (t2);
>>> +      tree name1 = dependent_name (fn1);
>>> +      tree name2 = dependent_name (fn2);
>>>          if (name1 || name2)
>>>    	{
>>>    	  tree targs1 = NULL_TREE, targs2 = NULL_TREE;
>>> @@ -3891,19 +3896,19 @@ called_fns_equal (tree t1, tree t2)
>>>    	     of whether the function was named with a qualified- or
>>> unqualified-id.
>>>    	     Until that's fixed, check that we aren't looking at overload sets
>>> from
>>>    	     different scopes.  */
>>> -      if (is_overloaded_fn (t1) && is_overloaded_fn (t2)
>>> -	  && (DECL_CONTEXT (get_first_fn (t1))
>>> -	      != DECL_CONTEXT (get_first_fn (t2))))
>>> +	  if (is_overloaded_fn (fn1) && is_overloaded_fn (fn2)
>>> +	      && (DECL_CONTEXT (get_first_fn (fn1))
>>> +		  != DECL_CONTEXT (get_first_fn (fn2))))
>>>    	    return false;
>>>    -      if (TREE_CODE (t1) == TEMPLATE_ID_EXPR)
>>> -	targs1 = TREE_OPERAND (t1, 1);
>>> -      if (TREE_CODE (t2) == TEMPLATE_ID_EXPR)
>>> -	targs2 = TREE_OPERAND (t2, 1);
>>> +	  if (TREE_CODE (fn1) == TEMPLATE_ID_EXPR)
>>> +	    targs1 = TREE_OPERAND (fn1, 1);
>>> +	  if (TREE_CODE (fn2) == TEMPLATE_ID_EXPR)
>>> +	    targs2 = TREE_OPERAND (fn2, 1);
>>>    	  return cp_tree_equal (targs1, targs2);
>>>    	}
>>> -  else
>>> -    return cp_tree_equal (t1, t2);
>>> +    }
>>> +  return cp_tree_equal (fn1, fn2);
>>>    }
>>>      bool comparing_override_contracts;
>>> @@ -4037,7 +4042,7 @@ cp_tree_equal (tree t1, tree t2)
>>>    	if (KOENIG_LOOKUP_P (t1) != KOENIG_LOOKUP_P (t2))
>>>    	  return false;
>>>    -	if (!called_fns_equal (CALL_EXPR_FN (t1), CALL_EXPR_FN (t2)))
>>> +	if (!called_fns_equal (t1, t2))
>>>    	  return false;
>>>      	call_expr_arg_iterator iter1, iter2;
>>> diff --git a/gcc/testsuite/g++.dg/cpp0x/overload5.C
>>> b/gcc/testsuite/g++.dg/cpp0x/overload5.C
>>> new file mode 100644
>>> index 00000000000..e05b1594f51
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/overload5.C
>>> @@ -0,0 +1,12 @@
>>> +// PR c++/107461
>>> +// { dg-do compile { target c++11 } }
>>> +
>>> +int f(...);
>>> +template<class T> decltype(T() + f(0)) g(); // #1
>>> +
>>> +char f(int);
>>> +template<class T> decltype(T() + f(0)) g(); // #2, distinct from #1
>>> +
>>> +int main() {
>>> +  g<int>(); // { dg-error "ambiguous" }
>>> +}
>>> diff --git a/gcc/testsuite/g++.dg/cpp0x/overload5a.C
>>> b/gcc/testsuite/g++.dg/cpp0x/overload5a.C
>>> new file mode 100644
>>> index 00000000000..037114f199c
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/overload5a.C
>>> @@ -0,0 +1,10 @@
>>> +// PR c++/107461
>>> +// { dg-do compile { target c++11 } }
>>> +
>>> +template<class T> T f();
>>> +template<class T> decltype(T() + f<int*>()) g(); // #1
>>> +template<class T> decltype(T() + f<char>()) g(); // #2, distinct from #1
>>> +
>>> +int main() {
>>> +  g<int>(); // { dg-error "ambiguous" }
>>> +}
>>> diff --git a/gcc/testsuite/g++.dg/cpp0x/overload6.C
>>> b/gcc/testsuite/g++.dg/cpp0x/overload6.C
>>> new file mode 100644
>>> index 00000000000..1fbee0501de
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/overload6.C
>>> @@ -0,0 +1,16 @@
>>> +// PR c++/107461
>>> +// { dg-do compile { target c++11 } }
>>> +
>>> +template<class T> T f();
>>> +
>>> +template<class> struct A { };
>>> +
>>> +template<class T> struct B {
>>> +  template<class U, class = A<decltype(f<T>()(U()))>>
>>> +  static void g(U);
>>> +};
>>> +
>>> +int main() {
>>> +  B<int> b;
>>> +  B<void(*)(int)>::g(0); // { dg-bogus "no match" }
>>> +}
>>
>>
>