c++: dependent generic lambda template-id [PR106024]

We were wrongly looking up the generic lambda op() in a dependent scope, and
then trying to look up its instantiation at substitution time, but lambdas
aren't instantiated, so we crashed.  The fix is to not look into dependent
class scopes.

But this created trouble with wrongly trying to use a template from the
enclosing scope when we aren't actually looking at a template-argument-list,
in template/lookup18.C, so let's avoid that.

	PR c++/106024

gcc/cp/ChangeLog:

	* parser.cc (missing_template_diag): Factor out...
	(cp_parser_id_expression): ...from here.
	(cp_parser_lookup_name): Don't look in dependent object_type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-generic10.C: New test.

gcc/cp/parser.cc

@@ -30561,9 +30561,11 @@ cp_parser_lookup_name (cp_parser *parser, tree name,
     }
   else if (object_type)
     {
+      bool dep = dependent_scope_p (object_type);
+
       /* Look up the name in the scope of the OBJECT_TYPE, unless the
 	 OBJECT_TYPE is not a class.  */
-      if (CLASS_TYPE_P (object_type))
+      if (!dep && CLASS_TYPE_P (object_type))
 	/* If the OBJECT_TYPE is a template specialization, it may
 	   be instantiated during name lookup.  In that case, errors
 	   may be issued.  Even if we rollback the current tentative
@@ -30587,6 +30589,25 @@ cp_parser_lookup_name (cp_parser *parser, tree name,
 			    : is_template ? LOOK_want::TYPE
 			    : prefer_type_arg (tag_type));
 
+      /* If we did unqualified lookup of a dependent member-qualified name and
+	 found something, do we want to use it?  P1787 clarified that we need
+	 to look in the object scope first even if it's dependent, but for now
+	 let's still use it in some cases.
+	 FIXME remember unqualified lookup result to use if member lookup fails
+	 at instantiation time.	 */
+      if (decl && dep && is_template)
+	{
+	  saved_token_sentinel toks (parser->lexer, STS_ROLLBACK);
+	  /* Only use the unqualified class template lookup if we're actually
+	     looking at a template arg list.  */
+	  if (!cp_parser_skip_entire_template_parameter_list (parser))
+	    decl = NULL_TREE;
+	  /* And only use the unqualified lookup if we're looking at ::.  */
+	  if (decl
+	      && !cp_lexer_next_token_is (parser->lexer, CPP_SCOPE))
+	    decl = NULL_TREE;
+	}
+
       /* If we know we're looking for a type (e.g. A in p->A::x),
 	 mock up a typename.  */
       if (!decl && object_type && tag_type != none_type

gcc/testsuite/g++.dg/cpp2a/lambda-generic10.C

@@ -0,0 +1,14 @@
+// PR c++/106024
+// { dg-do compile { target c++20 } }
+
+void sink(...);
+template <int... args> void f()
+{
+  sink ([] <int T> (int...) { return 1; }
+        .operator()<args>(args...)...); // { dg-warning "-Wmissing-template-keyword" }
+} // { dg-prune-output {expected '\)'} }
+
+int main()
+{
+  f<1,2,3>();
+}