c++: CTAD and deduction guide selection [PR86439]

c++: CTAD and deduction guide selection [PR86439]

During CTAD, we select the best viable deduction guide using
build_new_function_call, which performs overload resolution on the set
of candidate guides and then forms a call to the guide.  As the PR
points out, this latter step is unnecessary and occasionally incorrect
since a call to the selected guide may be ill-formed, or forming the
call may have side effects such as prematurely deducing the type of a {}.

So this patch introduces a specialized subroutine based on
build_new_function_call that stops short of building a call to the
selected function, and makes do_class_deduction use this subroutine
instead.  And since a call is no longer built, do_class_deduction
doesn't need to set tf_decltype or cp_unevaluated_operand anymore.

This change causes us to reject some container CTAD examples in the
libstdc++ testsuite due to deduction failure for {}, which AFAICT is the
correct behavior.  Previously in e.g. the first removed example

  std::map{{std::pair{1, 2.0}, {2, 3.0}, {3, 4.0}}, {}},

the type of the {} would get deduced to less<int> as a side effect of
forming a call to the chosen guide

  template<typename _Key, typename _Tp, typename _Compare = less<_Key>,
           typename _Allocator = allocator<pair<const _Key, _Tp>>>
      map(initializer_list<pair<_Key, _Tp>>,
          _Compare = _Compare(), _Allocator = _Allocator())
      -> map<_Key, _Tp, _Compare, _Allocator>;

which made later overload resolution for the constructor call
unambiguous.  Now, the type of the {} remains undeduced until
constructor overload resolution, and we complain about ambiguity
for the two equally good constructor candidates

  map(initializer_list<value_type>,
      const _Compare& = _Compare(),
      const allocator_type& = allocator_type())

  map(initializer_list<value_type>, const allocator_type&).

This patch fixes these problematic container CTAD examples by giving
the {} an appropriate concrete type.  Two of these adjusted CTAD
examples (one for std::set and one for std::multiset) end up triggering
an unrelated CTAD bug on trunk, PR101174, so these two adjusted examples
are commented out for now.

	PR c++/86439

gcc/cp/ChangeLog:

	* call.c (print_error_for_call_failure): Constify 'args' parameter.
	(perform_dguide_overload_resolution): Define.
	* cp-tree.h: (perform_dguide_overload_resolution): Declare.
	* pt.c (do_class_deduction): Use perform_dguide_overload_resolution
	instead of build_new_function_call.  Don't use tf_decltype or
	set cp_unevaluated_operand.  Remove unnecessary NULL_TREE tests.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/map/cons/deduction.cc: Replace ambiguous
	CTAD examples.
	* testsuite/23_containers/multimap/cons/deduction.cc: Likewise.
	* testsuite/23_containers/multiset/cons/deduction.cc: Likewise.
	Mention one of the replaced examples is broken due to PR101174.
	* testsuite/23_containers/set/cons/deduction.cc: Likewise.
	* testsuite/23_containers/unordered_map/cons/deduction.cc: Replace
	ambiguous CTAD examples.
	* testsuite/23_containers/unordered_multimap/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/cons/deduction.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/cons/deduction.cc: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction88.C: New test.
	* g++.dg/cpp1z/class-deduction89.C: New test.
	* g++.dg/cpp1z/class-deduction90.C: New test.