[gcc r14-9332] c++: Fix template deduction for conversion operators with xobj parameters [PR113629]

[Nathaniel Shead <nshead@gcc.gnu.org>]

https://gcc.gnu.org/g:49d83e963aa453600088380aebd507e172eb80ad

commit r14-9332-g49d83e963aa453600088380aebd507e172eb80ad
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Wed Mar 6 00:43:22 2024 +1100

    c++: Fix template deduction for conversion operators with xobj parameters [PR113629]
    
    Unification for conversion operators (DEDUCE_CONV) doesn't perform
    transformations like handling forwarding references. This is correct in
    general, but not for xobj parameters, which should be handled "normally"
    for the purposes of deduction: [temp.deduct.conv] only applies to the
    return type of the conversion function.
    
            PR c++/113629
    
    gcc/cp/ChangeLog:
    
            * pt.cc (type_unification_real): Only use DEDUCE_CONV for the
            return type of a conversion function.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp23/explicit-obj-conv-op.C: New test.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/pt.cc                                      | 12 +++++-
 gcc/testsuite/g++.dg/cpp23/explicit-obj-conv-op.C | 49 +++++++++++++++++++++++
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index c4bc54a8fdb..a6e6c804130 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23312,10 +23312,18 @@ type_unification_real (tree tparms,
 	   parameter pack is a non-deduced context.  */
 	continue;
 
+      /* [temp.deduct.conv] only applies to the deduction of the return
+	 type, which is always the first argument here.  Other arguments
+	 (notably, explicit object parameters) should undergo normal
+	 call-like unification.  */
+      unification_kind_t kind = strict;
+      if (strict == DEDUCE_CONV && ia > 0)
+	kind = DEDUCE_CALL;
+
       arg = args[ia];
       ++ia;
 
-      if (unify_one_argument (tparms, full_targs, parm, arg, subr, strict,
+      if (unify_one_argument (tparms, full_targs, parm, arg, subr, kind,
 			      explain_p))
 	return 1;
     }
@@ -23324,6 +23332,8 @@ type_unification_real (tree tparms,
       && parms != void_list_node
       && TREE_CODE (TREE_VALUE (parms)) == TYPE_PACK_EXPANSION)
     {
+      gcc_assert (strict != DEDUCE_CONV);
+
       /* Unify the remaining arguments with the pack expansion type.  */
       tree argvec;
       tree parmvec = make_tree_vec (1);
diff --git a/gcc/testsuite/g++.dg/cpp23/explicit-obj-conv-op.C b/gcc/testsuite/g++.dg/cpp23/explicit-obj-conv-op.C
new file mode 100644
index 00000000000..a6ae4ea1dda
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/explicit-obj-conv-op.C
@@ -0,0 +1,49 @@
+// PR c++/113629
+// { dg-do compile { target c++23 } }
+
+template <typename T> constexpr bool is_lvalue = false;
+template <typename T> constexpr bool is_lvalue<T&> = true;
+
+struct A {
+  constexpr operator bool(this auto&& self) {
+    return is_lvalue<decltype(self)>;
+  }
+};
+
+constexpr A a;
+static_assert(static_cast<bool>(a));
+static_assert((bool)a);
+static_assert(!static_cast<bool>(A{}));
+static_assert(!(bool)A{});
+
+struct B : A {};
+
+constexpr B b;
+static_assert(static_cast<bool>(b));
+static_assert((bool)b);
+static_assert(!static_cast<bool>(B{}));
+static_assert(!(bool)B{});
+
+struct C {
+  template <typename R, typename T>
+  explicit constexpr operator R(this T&&) {
+    return is_lvalue<T>;
+  }
+};
+
+constexpr C c;
+static_assert(static_cast<bool>(c));
+static_assert((bool)c);
+static_assert(!static_cast<bool>(C{}));
+static_assert(!(bool)C{});
+
+struct D {
+  explicit constexpr operator bool(this const D&) { return true; }
+  explicit constexpr operator bool(this const D&&) { return false; }
+};
+
+constexpr D d;
+static_assert(static_cast<bool>(d));
+static_assert((bool)d);
+static_assert(!static_cast<bool>(D{}));
+static_assert(!(bool)D{});