fixes #7816 (#11261)

compiler/renderer.nim

@@ -943,7 +943,7 @@ proc gsub(g: var TSrcGen, n: PNode, c: TContext) =
   of nkCharLit: put(g, tkCharLit, atom(g, n))
   of nkNilLit: put(g, tkNil, atom(g, n))    # complex expressions
   of nkCall, nkConv, nkDotCall, nkPattern, nkObjConstr:
-    if n.len > 0 and isBracket(n[0]):
+    if renderIds notin g.flags and n.len > 0 and isBracket(n[0]):
       gsub(g, n, 1)
       put(g, tkBracketLe, "[")
       gcomma(g, n, 2)

compiler/semtypinst.nim

@@ -201,6 +201,7 @@ proc replaceTypeVarsN(cl: var TReplTypeVars, n: PNode; start=0): PNode =
   case n.kind
   of nkNone..pred(nkSym), succ(nkSym)..nkNilLit:
     discard
+  of nkOpenSymChoice, nkClosedSymChoice: result = n
   of nkSym:
     result.sym = replaceTypeVarsS(cl, n.sym)
     if result.sym.typ.kind == tyVoid:

tests/stdlib/tsugar.nim

@@ -1,5 +1,5 @@
 discard """
-  output: ""
+  output: "@[@[], @[], @[], @[], @[]]"
 """
 import sugar
 import macros
@@ -26,3 +26,12 @@ block distinctBase:
       Uint[bits: static[int]] = distinct uintImpl(bits)
 
     doAssert Uint[128].distinctBase is UintImpl[uint64]
+
+# bug #7816
+import sequtils
+
+proc tester[T](x: T) =
+  let test = toSeq(0..4).map(i => newSeq[int]())
+  echo test
+
+tester(1)