c++: Fix CTAD for aggregates in template [PR95568]

95568 complains that CTAD for aggregates doesn't work within
requires-clause and it turned out that it doesn't work when we try
the deduction in a template.  The reason is that maybe_aggr_guide
creates a guide that can look like this

  template<class T> X(decltype (X<T>::x))-> X<T>

where the parameter is a decltype, which is a non-deduced context.  So
the subsequent build_new_function_call fails because unify_one_argument
can't deduce anything from it ([temp.deduct.type]: "If a template
parameter is used only in non-deduced contexts and is not explicitly
specified, template argument deduction fails.")

Those decltypes come from finish_decltype_type.  We can just use
TREE_TYPE instead.  I pondered using unlowered_expr_type, but that
didn't make any difference for the FIELD_DECLs I saw in
class-deduction-aggr6.C.

gcc/cp/ChangeLog:

	PR c++/95568
	* pt.c (collect_ctor_idx_types): Use TREE_TYPE.

gcc/testsuite/ChangeLog:

	PR c++/95568
	* g++.dg/cpp2a/class-deduction-aggr5.C: New test.
	* g++.dg/cpp2a/class-deduction-aggr6.C: New test.

gcc/cp/pt.c

@@ -28329,7 +28329,7 @@ collect_ctor_idx_types (tree ctor, tree list, tree elt = NULL_TREE)
   tree idx, val; unsigned i;
   FOR_EACH_CONSTRUCTOR_ELT (v, i, idx, val)
     {
-      tree ftype = elt ? elt : finish_decltype_type (idx, true, tf_none);
+      tree ftype = elt ? elt : TREE_TYPE (idx);
       if (BRACE_ENCLOSED_INITIALIZER_P (val)
 	  && CONSTRUCTOR_NELTS (val)
 	  /* As in reshape_init_r, a non-aggregate or array-of-dependent-bound

gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C

@@ -0,0 +1,20 @@
+// PR c++/95568
+// { dg-do compile { target c++20 } }
+
+template<typename T> struct X { T x; };
+template<typename T, typename U> struct X2 { T x; U y; };
+template<typename T> concept Y = requires { X{0}; };
+
+template<typename T>
+void g()
+{
+  X{0};
+  X2{1, 2.2};
+  Y auto y = X{1};
+}
+
+void
+fn ()
+{
+  g<int>();
+}

gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr6.C

@@ -0,0 +1,35 @@
+// PR c++/95568
+// { dg-do compile { target c++20 } }
+// CTAD with aggregates containing bit-fields.
+
+template<class, class> struct same_type;
+template<class T> struct same_type<T, T> {};
+
+enum E { e };
+enum class F { f };
+
+template<typename T>
+struct X {
+  T a : 5;
+};
+
+template<typename>
+void g()
+{
+  auto x = X{ 0 };
+  same_type<decltype(x.a), int>();
+  auto x2 = X{ E::e };
+  same_type<decltype(x2.a), E>();
+  auto x3 = X{ false };
+  same_type<decltype(x3.a), bool>();
+  auto x4 = X{ 0u };
+  same_type<decltype(x4.a), unsigned int>();
+  auto x5 = X{ F::f };
+  same_type<decltype(x5.a), F>();
+}
+
+void
+fn ()
+{
+  g<int>();
+}