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>

6 files changed, 139 insertions, 14 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 67d38e2..94015db 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13262,10 +13262,35 @@ joust (struct z_candidate *cand1, struct z_candidate *cand2, bool warn,
   else if (cand2->rewritten ())
     return 1;
 
-  /* F1 is generated from a deduction-guide (13.3.1.8) and F2 is not */
   if (deduction_guide_p (cand1->fn))
     {
       gcc_assert (deduction_guide_p (cand2->fn));
+
+      /* F1 and F2 are generated from class template argument deduction for a
+	 class D, and F2 is generated from inheriting constructors from a base
+	 class of D while F1 is not, and for each explicit function argument,
+	 the corresponding parameters of F1 and F2 are either both ellipses or
+	 have the same type.  */
+      bool inherited1 = inherited_guide_p (cand1->fn);
+      bool inherited2 = inherited_guide_p (cand2->fn);
+      if (int diff = inherited2 - inherited1)
+	{
+	  for (i = 0; i < len; ++i)
+	    {
+	      conversion *t1 = cand1->convs[i + off1];
+	      conversion *t2 = cand2->convs[i + off2];
+	      /* ??? It seems the ellipses part of this tiebreaker isn't
+		 needed since a mismatch should have broken the tie earlier
+		 during ICS comparison.  */
+	      gcc_checking_assert (t1->ellipsis_p == t2->ellipsis_p);
+	      if (!same_type_p (t1->type, t2->type))
+		break;
+	    }
+	  if (i == len)
+	    return diff;
+	}
+
+      /* F1 is generated from a deduction-guide (13.3.1.8) and F2 is not */
       /* We distinguish between candidates from an explicit deduction guide and
 	 candidates built from a constructor based on DECL_ARTIFICIAL.  */
       int art1 = DECL_ARTIFICIAL (cand1->fn);
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 5807f4e..a53fbcb 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -2972,9 +2972,10 @@ struct GTY(()) lang_decl_fn {
      this pointer and result pointer adjusting thunks are
      chained here.  This pointer thunks to return pointer thunks
      will be chained on the return pointer thunk.
-     For a DECL_CONSTUCTOR_P FUNCTION_DECL, this is the base from
-     whence we inherit.  Otherwise, it is the class in which a
-     (namespace-scope) friend is defined (if any).   */
+     For a DECL_CONSTRUCTOR_P or deduction_guide_p FUNCTION_DECL,
+     this is the base from whence we inherit.
+     Otherwise, it is the class in which a (namespace-scope) friend
+     is defined (if any).  */
   tree context;
 
   union lang_decl_u5
@@ -7662,6 +7663,7 @@ extern bool deduction_guide_p			(const_tree);
 extern bool copy_guide_p			(const_tree);
 extern bool template_guide_p			(const_tree);
 extern bool builtin_guide_p			(const_tree);
+extern bool inherited_guide_p			(const_tree);
 extern void store_explicit_specifier		(tree, tree);
 extern tree lookup_explicit_specifier		(tree);
 extern tree lookup_imported_hidden_friend	(tree);
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index c1d4cdc..8725a5e 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -29716,6 +29716,25 @@ builtin_guide_p (const_tree fn)
   return true;
 }
 
+/* True if FN is a C++23 inherited guide.  */
+
+bool
+inherited_guide_p (const_tree fn)
+{
+  gcc_assert (deduction_guide_p (fn));
+  return LANG_DECL_FN_CHECK (fn)->context != NULL_TREE;
+}
+
+/* Set the base class BASE from which the transformed guide FN
+   was inherited as part of C++23 inherited CTAD.  */
+
+static void
+set_inherited_guide_context (const_tree fn, tree base)
+{
+  gcc_assert (deduction_guide_p (fn));
+  LANG_DECL_FN_CHECK (fn)->context = base;
+}
+
 /* OLDDECL is a _DECL for a template parameter.  Return a similar parameter at
    LEVEL:INDEX, using tsubst_args and complain for substitution into non-type
    template parameter types.  Note that the handling of template template
@@ -30486,6 +30505,7 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
 	  TREE_TYPE (fprime) = fntype;
 	  if (TREE_CODE (fprime) == TEMPLATE_DECL)
 	    TREE_TYPE (DECL_TEMPLATE_RESULT (fprime)) = fntype;
+	  set_inherited_guide_context (fprime, utype);
 	}
 
       aguides = lookup_add (fprime, aguides);
@@ -30517,11 +30537,14 @@ inherited_ctad_tweaks (tree tmpl, tree ctor, tsubst_flags_t complain)
      template specialization with the template argument list of A but with C as
      the template.  */
 
-  /* FIXME: Also recognize inherited constructors of the form 'using C::B::B',
-     which seem to be represented with TYPENAME_TYPE C::B as USING_DECL_SCOPE?
-     And recognize constructors inherited from a non-dependent base class, which
-     seem to be missing from the overload set entirely?  */
   tree scope = USING_DECL_SCOPE (ctor);
+  if (TREE_CODE (scope) == TYPENAME_TYPE
+      && (TYPE_IDENTIFIER (TYPE_CONTEXT (scope))
+	  == TYPENAME_TYPE_FULLNAME (scope)))
+    /* Recognize using B<T>::B::B as an inherited constructor.  */
+    /* FIXME: Also recognize using C::B::B?  We might have to call
+       resolve_typename_type for that.  */
+    scope = TYPE_CONTEXT (scope);
   if (!CLASS_TYPE_P (scope)
       || !CLASSTYPE_TEMPLATE_INFO (scope)
       || !PRIMARY_TEMPLATE_P (CLASSTYPE_TI_TEMPLATE (scope)))
@@ -30655,10 +30678,14 @@ ctor_deduction_guides_for (tree tmpl, tsubst_flags_t complain)
     }
 
   if (cxx_dialect >= cxx23)
-    for (tree ctor : ovl_range (CLASSTYPE_CONSTRUCTORS (type)))
-      if (TREE_CODE (ctor) == USING_DECL)
-	{
-	  tree uguides = inherited_ctad_tweaks (tmpl, ctor, complain);
+    /* FIXME: CLASSTYPE_CONSTRUCTORS doesn't contain inherited constructors if
+       e.g. the class also has a user-defined constructor.  So instead iterate
+       over TYPE_FIELDS manually to robustly find all relevant using-decls.  */
+    for (tree field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
+      if (TREE_CODE (field) == USING_DECL
+	  && DECL_NAME (field) == ctor_identifier)
+	{
+	  tree uguides = inherited_ctad_tweaks (tmpl, field, complain);
 	  if (uguides)
 	    cands = lookup_add (uguides, cands);
 	}
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>;