fixes #1833

compiler/ccgexprs.nim

@@ -1569,7 +1569,8 @@ proc genRangeChck(p: BProc, n: PNode, d: var TLoc, magic: string) =
         toRope(magic)]))
 
 proc genConv(p: BProc, e: PNode, d: var TLoc) =
-  if compareTypes(e.typ, e.sons[1].typ, dcEqIgnoreDistinct):
+  let destType = e.typ.skipTypes({tyVar, tyGenericInst})
+  if compareTypes(destType, e.sons[1].typ, dcEqIgnoreDistinct):
     expr(p, e.sons[1], d)
   else:
     genSomeCast(p, e, d)

tests/ccgbugs/twrong_tupleconv.nim

@@ -0,0 +1,20 @@
+# bug #1833
+iterator myitems*[T](a: var seq[T]): var T {.inline.} =
+  ## iterates over each item of `a` so that you can modify the yielded value.
+  var i = 0
+  let L = len(a)
+  while i < L:
+    yield a[i]
+    inc(i)
+    assert(len(a) == L, "seq modified while iterating over it")
+
+# Works fine
+var xs = @[1,2,3]
+for x in myitems(xs):
+  inc x
+
+# Tuples don't work
+var ys = @[(1,"a"),(2,"b"),(3,"c")]
+for y in myitems(ys):
+  inc y[0]
+