c++: permit deduction guides at class scope [PR79501]

This adds support for declaring (class-scope) deduction guides for a
member class template.  Fortunately it seems only a couple of changes
are needed in order for the existing CTAD machinery to handle them
properly: we need to make sure to give them a FUNCTION_TYPE instead of a
METHOD_TYPE, and we need to avoid using a BASELINK when looking them up.

	PR c++/79501
	PR c++/100983

gcc/cp/ChangeLog:

	* decl.c (grokfndecl): Don't require that deduction guides are
	declared at namespace scope.  Check that class-scope deduction
	guides have the same access as the member class template.
	(grokdeclarator): Pretend class-scope deduction guides are static.
	* search.c (lookup_member): Don't use a BASELINK for (class-scope)
	deduction guides.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction92.C: New test.
	* g++.dg/cpp1z/class-deduction93.C: New test.
	* g++.dg/cpp1z/class-deduction94.C: New test.
	* g++.dg/cpp1z/class-deduction95.C: New test.

6 files changed, 87 insertions, 7 deletions

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 0df689b..01d64a1 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -10042,12 +10042,6 @@ grokfndecl (tree ctype,
 
   if (deduction_guide_p (decl))
     {
-      if (!DECL_NAMESPACE_SCOPE_P (decl))
-	{
-	  error_at (location, "deduction guide %qD must be declared at "
-		    "namespace scope", decl);
-	  return NULL_TREE;
-	}
       tree type = TREE_TYPE (DECL_NAME (decl));
       if (in_namespace == NULL_TREE
 	  && CP_DECL_CONTEXT (decl) != CP_TYPE_CONTEXT (type))
@@ -10057,6 +10051,13 @@ grokfndecl (tree ctype,
 	  inform (location_of (type), "  declared here");
 	  return NULL_TREE;
 	}
+      if (DECL_CLASS_SCOPE_P (decl)
+	  && current_access_specifier != declared_access (TYPE_NAME (type)))
+	{
+	  error_at (location, "deduction guide %qD must have the same access "
+			      "as %qT", decl, type);
+	  inform (location_of (type), "  declared here");
+	}
       if (funcdef_flag)
 	error_at (location,
 		  "deduction guide %qD must not have a function body", decl);
@@ -12037,6 +12038,10 @@ grokdeclarator (const cp_declarator *declarator,
   storage_class = declspecs->storage_class;
   if (storage_class == sc_static)
     staticp = 1 + (decl_context == FIELD);
+  else if (decl_context == FIELD && sfk == sfk_deduction_guide)
+    /* Treat class-scope deduction guides as static member functions
+       so that they get a FUNCTION_TYPE instead of a METHOD_TYPE.  */
+    staticp = 2;
 
   if (virtualp)
     {
diff --git a/gcc/cp/search.c b/gcc/cp/search.c
index 7b18368..af41bfe 100644
--- a/gcc/cp/search.c
+++ b/gcc/cp/search.c
@@ -1226,7 +1226,10 @@ lookup_member (tree xbasetype, tree name, int protect, bool want_type,
       rval = error_mark_node;
     }
 
-  if (rval && is_overloaded_fn (rval))
+  if (rval && is_overloaded_fn (rval)
+      /* Don't use a BASELINK for class-scope deduction guides since
+	 they're not actually member functions.  */
+      && !dguide_name_p (name))
     rval = build_baselink (rval_binfo, basetype_path, rval,
 			   (IDENTIFIER_CONV_OP_P (name)
 			   ? TREE_TYPE (name): NULL_TREE));
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
new file mode 100644
index 0000000..4920ca4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction92.C
@@ -0,0 +1,17 @@
+// PR c++/79501
+// { dg-do compile { target c++17 } }
+
+template<auto V>
+struct X {
+  template<class T, auto>
+  struct B { T t; };
+
+  template<class T> B(T, decltype(V)=V) -> B<const T, V>;
+
+  auto foo() { return B{V}; }
+};
+
+X<42> x;
+using type = decltype(x.foo());
+using type = decltype(decltype(x)::B{42});
+using type = X<42>::B<const int, 42>;
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
new file mode 100644
index 0000000..9d2db7a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction93.C
@@ -0,0 +1,25 @@
+// PR c++/79501
+// { dg-do compile { target c++17 } }
+// A variant of class-deduction78.C where List and its deduction guides are
+// defined at class scope.
+
+using size_t = decltype(sizeof(42));
+
+struct A {
+  template<typename T, size_t N = 0>
+  struct List {
+    T head;
+    List<T, N-1> tail;
+  };
+
+  template<typename T>
+  struct List<T, 0> {};
+
+  template<typename T> List(T) -> List<T, 1>;
+  template<typename T, size_t N> List(T, List<T, N>) -> List<T, N+1>;
+};
+
+int main() {
+  using type = decltype(A::List{0, A::List{1, A::List{2}}});
+  using type = A::List<int, 3>;
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction94.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction94.C
new file mode 100644
index 0000000..f29ebd2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction94.C
@@ -0,0 +1,19 @@
+// PR c++/79501
+// { dg-do compile { target c++17 } }
+
+struct X {
+protected:
+  template<class T>
+  struct B { T t; };
+
+  template<class T> B(T) -> B<T>;
+};
+
+struct Y {
+protected:
+  template<class T>
+  struct B { T t; };
+
+private:
+  template<class T> B(T) -> B<T>; // { dg-error "access" }
+};
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction95.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction95.C
new file mode 100644
index 0000000..05cbb2b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction95.C
@@ -0,0 +1,11 @@
+// PR c++/100983
+// { dg-do compile { target c++17 } }
+
+struct X {
+  template<int N>
+  struct Y { template<class... Ts> Y(Ts...); };
+
+  template<class... Ts> Y(Ts...) -> Y<sizeof...(Ts)>;
+};
+
+X::Y y{1,2,3};