fix #6277

compiler/ccgexprs.nim

@@ -849,7 +849,7 @@ proc genArrayElem(p: BProc, n, x, y: PNode, d: var TLoc) =
   var a, b: TLoc
   initLocExpr(p, x, a)
   initLocExpr(p, y, b)
-  var ty = skipTypes(skipTypes(a.t, abstractVarRange), abstractPtrs)
+  var ty = skipTypes(a.t, abstractVarRange + abstractPtrs + tyUserTypeClasses)
   var first = intLiteral(firstOrd(ty))
   # emit range check:
   if optBoundsCheck in p.options and tfUncheckedArray notin ty.flags:

tests/concepts/titerable.nim

@@ -0,0 +1,20 @@
+discard """
+  nimout: "int\nint"
+  output: 15
+"""
+
+import typetraits
+
+type
+  Iterable[T] = concept x
+    for value in x:
+      type(value) is T
+
+proc sum*[T](iter: Iterable[T]): T =
+  static: echo T.name
+  for element in iter:
+    static: echo element.type.name
+    result += element
+
+echo sum([1, 2, 3, 4, 5])
+