introduce precise string '[]', '[]=' accessors; fixes #8049 (#8817)

compiler/sigmatch.nim

@@ -2132,6 +2132,10 @@ proc paramTypesMatch*(m: var TCandidate, f, a: PType,
       styleCheckUse(arg.info, arg.sons[best].sym)
       result = paramTypesMatchAux(m, f, arg.sons[best].typ, arg.sons[best],
                                   argOrig)
+  when false:
+    if m.calleeSym != nil and m.calleeSym.name.s == "[]":
+      echo m.c.config $ arg.info, " for ", m.calleeSym.name.s, " ", m.c.config $ m.calleeSym.info
+      writeMatches(m)
 
 proc setSon(father: PNode, at: int, son: PNode) =
   let oldLen = father.len
@@ -2380,6 +2384,11 @@ proc matches*(c: PContext, n, nOrig: PNode, m: var TCandidate) =
   if m.magic in {mArrGet, mArrPut}:
     m.state = csMatch
     m.call = n
+    # Note the following doesn't work as it would produce ambiguities.
+    # Instead we patch system.nim, see bug #8049.
+    when false:
+      inc m.genericMatches
+      inc m.exactMatches
     return
   var marker = initIntSet()
   matchesAux(c, n, nOrig, m, marker)