close #10735 add testcase (#16690)

Author: ringabout

tests/errmsgs/t10735.nim

@@ -0,0 +1,41 @@
+discard """
+  cmd: "nim check $file"
+  errormsg: "selector must be of an ordinal type, float or string"
+  nimout: '''
+t10735.nim(38, 5) Error: 'let' symbol requires an initialization
+t10735.nim(39, 10) Error: undeclared identifier: 'pos'
+t10735.nim(39, 9) Error: type mismatch: got <cstring, >
+but expected one of:
+proc `[]`(s: string; i: BackwardsIndex): char
+  first type mismatch at position: 0
+proc `[]`(s: var string; i: BackwardsIndex): var char
+  first type mismatch at position: 0
+proc `[]`[I: Ordinal; T](a: T; i: I): T
+  first type mismatch at position: 0
+proc `[]`[Idx, T, U, V](a: array[Idx, T]; x: HSlice[U, V]): seq[T]
+  first type mismatch at position: 0
+proc `[]`[Idx, T](a: array[Idx, T]; i: BackwardsIndex): T
+  first type mismatch at position: 0
+proc `[]`[Idx, T](a: var array[Idx, T]; i: BackwardsIndex): var T
+  first type mismatch at position: 0
+proc `[]`[T, U, V](s: openArray[T]; x: HSlice[U, V]): seq[T]
+  first type mismatch at position: 0
+proc `[]`[T, U](s: string; x: HSlice[T, U]): string
+  first type mismatch at position: 0
+proc `[]`[T](s: openArray[T]; i: BackwardsIndex): T
+  first type mismatch at position: 0
+proc `[]`[T](s: var openArray[T]; i: BackwardsIndex): var T
+  first type mismatch at position: 0
+template `[]`(s: string; i: int): char
+  first type mismatch at position: 0
+
+expression: `[]`(buf, pos)
+t10735.nim(39, 9) Error: selector must be of an ordinal type, float or string
+'''
+  joinable: false
+"""
+
+let buf: cstring
+case buf[pos]
+else:
+  case buf[pos]