patch 9.0.1265: using an interface method may give a compilation error

Problem:    Using an interface method may give a compilation error.
Solution:   Do not try to compile the body of a method of an interface.
            (closes #11885)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 3f97c00..27b71f7 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3181,6 +3181,16 @@
     }
     ufunc->uf_args_visible = ufunc->uf_args.ga_len;
 
+    // Compiling a function in an interface is done to get the function type.
+    // No code is actually compiled.
+    if (ufunc->uf_class != NULL
+			   && (ufunc->uf_class->class_flags & CLASS_INTERFACE))
+    {
+	ufunc->uf_def_status = UF_NOT_COMPILED;
+	ret = OK;
+	goto erret;
+    }
+
     /*
      * Loop over all the lines of the function and generate instructions.
      */
@@ -3705,7 +3715,7 @@
 	    iemsg("Type stack underflow");
 	    goto erret;
 	}
-    }
+    } // END of the loop over all the function body lines.
 
     if (cctx.ctx_scope != NULL)
     {