c++: Fix SEGV with malformed constructor decl.

In the testcase, since there's no declaration of T, ref_view(T) declares a
non-static data member T of type ref_view, the same type as its enclosing
class.  Then when we try to do C++20 aggregate class template argument
deduction we recursively try to adjust the braced-init-list to match the
template class definition until we run out of stack.

Fixed by rejecting the template data member.

	PR c++/92593
	* decl.c (grokdeclarator): Reject field of current class type even
	in a template.

gcc/cp/ChangeLog

+2020-02-05  Jason Merrill  <jason@redhat.com>
+
+	PR c++/92593
+	* decl.c (grokdeclarator): Reject field of current class type even
+	in a template.
+
 2020-02-05  Bin Cheng  <bin.cheng@linux.alibaba.com>
 
 	* coroutines.cc (maybe_promote_captured_temps): Increase the index

gcc/cp/decl.c

@@ -13259,10 +13259,13 @@ grokdeclarator (const cp_declarator *declarator,
 	    if (declspecs->explicit_specifier)
 	      store_explicit_specifier (decl, declspecs->explicit_specifier);
 	  }
-	else if (!staticp && !dependent_type_p (type)
-		 && !COMPLETE_TYPE_P (complete_type (type))
-		 && (!complete_or_array_type_p (type)
-		     || initialized == 0))
+	else if (!staticp
+		 && ((current_class_type
+		      && same_type_p (type, current_class_type))
+		     || (!dependent_type_p (type)
+			 && !COMPLETE_TYPE_P (complete_type (type))
+			 && (!complete_or_array_type_p (type)
+			     || initialized == 0))))
 	  {
 	    if (TREE_CODE (type) != ARRAY_TYPE
 		|| !COMPLETE_TYPE_P (TREE_TYPE (type)))

gcc/testsuite/g++.dg/cpp1z/class-deduction68.C

+// PR c++/92593
+// { dg-do compile { target c++17 } }
+
+template<int I>
+struct ref_view
+{
+  ref_view(T) { };		// { dg-error "incomplete" }
+};
+
+ref_view r{1};			// { dg-error "no match|deduction failed" }

gcc/testsuite/g++.dg/parse/undefined3.C

@@ -2,5 +2,5 @@
 // Origin: Volker Reichelt <reichelt@igpm.rwth-aachen.de>
 // { dg-do compile }
 
-template<typename T> struct A { A(B); };
+template<typename T> struct A { A(B); }; // { dg-error "incomplete" }
 template<typename T> A<T>::A(B) {} // { dg-error "" }

gcc/testsuite/g++.dg/template/pr71710.C

@@ -3,8 +3,8 @@
 
 template < typename > struct A
 {
-  A a;
+  A a;			       // { dg-error "incomplete" }
   template < int > using B = decltype (a);
-  B < 0 > b;
+  B < 0 > b;		       // { dg-prune-output "B. does not name a type" }
   template < int C > B < C > foo ();
 };