c++: inherited CTAD fixes [PR116276]

This implements the overlooked inherited vs non-inherited guide
tiebreaker from P2582R1.  This requires tracking inherited-ness of a
guide, for which it seems natural to reuse the lang_decl_fn::context
field which for a constructor tracks its inherited-ness.

This patch also works around CLASSTYPE_CONSTRUCTORS not reliably
returning all inherited constructors (due to some using-decl handling
quirks in in push_class_level_binding) by iterating over TYPE_FIELDS
instead.

This patch also makes us recognize another written form of inherited
constructor, 'using Base<T>::Base::Base' whose USING_DECL_SCOPE is a
TYPENAME_TYPE.

	PR c++/116276

gcc/cp/ChangeLog:

	* call.cc (joust): Implement P2582R1 inherited vs non-inherited
	guide tiebreaker.
	* cp-tree.h (lang_decl_fn::context): Document usage in
	deduction_guide_p FUNCTION_DECLs.
	(inherited_guide_p): Declare.
	* pt.cc (inherited_guide_p): Define.
	(set_inherited_guide_context): Define.
	(alias_ctad_tweaks): Use set_inherited_guide_context.
	(inherited_ctad_tweaks): Recognize some inherited constructors
	whose scope is a TYPENAME_TYPE.
	(ctor_deduction_guides_for): For C++23 inherited CTAD, iterate
	over TYPE_FIELDS instead of CLASSTYPE_CONSTRUCTORS to recognize
	all inherited constructors.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/class-deduction-inherited4.C: Remove an xfail.
	* g++.dg/cpp23/class-deduction-inherited5.C: New test.
	* g++.dg/cpp23/class-deduction-inherited6.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

3 files changed, 73 insertions, 2 deletions

diff --git a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited4.C b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited4.C
index 5e3a7f4..806f016 100644
--- a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited4.C
+++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited4.C
@@ -13,10 +13,10 @@ using ty1 = B<int>;
 
 template<class T=void>
 struct C : A<int> {
-  using A<int>::A; // FIXME: we don't notice this one either
+  using A<int>::A;
 };
 
-using ty2 = decltype(C(0)); // { dg-bogus "" "" { xfail *-*-* } }
+using ty2 = decltype(C(0));
 using ty2 = C<void>;
 
 template<class T>
diff --git a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited5.C b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited5.C
new file mode 100644
index 0000000..d835acb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited5.C
@@ -0,0 +1,25 @@
+// PR c++/116276
+// { dg-do compile { target c++20 } }
+
+template<class T>
+struct Base1 { };
+
+template<class T>
+struct Base2 { };
+
+template<class T = int>
+struct Derived : public Base1<T>, Base2<T> {
+  using Base1<T>::Base1;
+  using Base2<T>::Base2;
+};
+
+Derived d;
+
+template<class T = int>
+struct Derived2 : public Base1<T>, Base2<T> {
+  using Base1<T>::Base1::Base1;
+  using Base2<T>::Base2::Base2;
+  Derived2();
+};
+
+Derived2 d2;
diff --git a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited6.C b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited6.C
new file mode 100644
index 0000000..df8199c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited6.C
@@ -0,0 +1,46 @@
+// PR c++/116276
+// { dg-do compile { target c++23 } }
+
+template<class T>
+struct Base1 {
+  Base1();
+  Base1(T);
+};
+
+template<class T>
+struct Base2 {
+  Base2();
+  Base2(T*);
+};
+
+template<class T = int>
+struct Derived : public Base1<T>, Base2<T> {
+  using Base1<T>::Base1;
+  using Base2<T>::Base2;
+};
+
+using ty1 = decltype(Derived{});
+using ty1 = Derived<int>;
+
+using ty2 = decltype(Derived{true});
+using ty2 = Derived<bool>;
+
+using ty3 = decltype(Derived{(char*)nullptr});
+using ty3 = Derived<char>;
+
+template<class T = int>
+struct Derived2 : public Base1<T>, Base2<T> {
+  using Base1<T>::Base1;
+  using Base2<T>::Base2;
+  Derived2();
+  Derived2(T);
+};
+
+using ty4 = decltype(Derived2{});
+using ty4 = Derived2<int>;
+
+using ty5 = decltype(Derived2{true});
+using ty5 = Derived2<bool>;
+
+using ty6 = decltype(Derived2{(char*)nullptr});
+using ty6 = Derived2<char>;