Do not allow Module#include to insert modules before the origin in the lookup chain

Module#include should only be able to insert modules after the origin,
otherwise it ends up working like Module#prepend.

This fixes the case where one of the modules in the included module
chain is included in a module that is already prepended to the receiver.

Fixes [Bug #7844]

Backport of https://github.com/ruby/ruby/pull/3796 to 2.7

1 files changed, 6 insertions, 1 deletions

diff --git a/class.c b/class.c
index 2e4b31d7c6..f17e7f053a 100644
--- a/class.c
+++ b/class.c
@@ -911,17 +911,22 @@ include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
     }
 
     while (module) {
+        int origin_seen = FALSE;
 	int superclass_seen = FALSE;
 	struct rb_id_table *tbl;
 
+        if (klass == c)
+            origin_seen = TRUE;
 	if (klass_m_tbl && klass_m_tbl == RCLASS_M_TBL(module))
 	    return -1;
 	/* ignore if the module included already in superclasses */
 	for (p = RCLASS_SUPER(klass); p; p = RCLASS_SUPER(p)) {
 	    int type = BUILTIN_TYPE(p);
+            if (c == p)
+                origin_seen = TRUE;
 	    if (type == T_ICLASS) {
 		if (RCLASS_M_TBL(p) == RCLASS_M_TBL(module)) {
-		    if (!superclass_seen) {
+		    if (!superclass_seen && origin_seen) {
 			c = p;  /* move insertion point */
 		    }
 		    goto skip;