patch 9.0.2085: Vim9: abstract can be used in interface

Problem:  Vim9: abstract can be used in interface
Solution: Disallow the use of abstract in an interface

fixes: #13456
closes: #13464

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/vim9class.c b/src/vim9class.c
index ede310b..08c3713 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -1557,26 +1557,27 @@
 		break;
 	    }
 
-	    if (!is_class)
-		// ignore "abstract" in an interface (as all the methods in an
-		// interface are abstract.
-		p = skipwhite(pa + 8);
-	    else
+	    p = skipwhite(pa + 8);
+	    if (STRNCMP(p, "def", 3) != 0 && STRNCMP(p, "static", 6) != 0)
 	    {
-		if (!is_abstract)
-		{
-		    semsg(_(e_abstract_method_in_concrete_class), pa);
-		    break;
-		}
-
-		abstract_method = TRUE;
-		p = skipwhite(pa + 8);
-		if (STRNCMP(p, "def", 3) != 0)
-		{
-		    emsg(_(e_abstract_must_be_followed_by_def));
-		    break;
-		}
+		emsg(_(e_abstract_must_be_followed_by_def_or_static));
+		break;
 	    }
+
+	    if (!is_class)
+	    {
+		// "abstract" not supported in an interface
+		emsg(_(e_abstract_cannot_be_used_in_interface));
+		break;
+	    }
+
+	    if (!is_abstract)
+	    {
+		semsg(_(e_abstract_method_in_concrete_class), pa);
+		break;
+	    }
+
+	    abstract_method = TRUE;
 	}
 
 	int has_static = FALSE;