concepts: fixes the stack overflow that happens for #6691

compiler/sigmatch.nim

@@ -1057,9 +1057,10 @@ proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
            else: isNone
 
   of tyUserTypeClass, tyUserTypeClassInst:
-    if c.c.matchedConcept != nil:
+    if c.c.matchedConcept != nil and c.c.matchedConcept.depth <= 4:
       # consider this: 'var g: Node' *within* a concept where 'Node'
       # is a concept too (tgraph)
+      inc c.c.matchedConcept.depth
       let x = typeRel(c, a, f, flags + {trDontBind})
       if x >= isGeneric:
         return isGeneric

tests/concepts/tinfrecursion.nim

@@ -0,0 +1,13 @@
+
+# bug #6691
+type
+  ConceptA = concept c
+
+  ConceptB = concept c
+      c.myProc(ConceptA)
+
+  Obj = object
+
+proc myProc(obj: Obj, x: ConceptA) = discard
+
+echo Obj is ConceptB