Fix for #5695 make subscript operator overloadable for tuples (#5749)

compiler/semexprs.nim

@@ -1266,7 +1266,7 @@ proc semSubscript(c: PContext, n: PNode, flags: TExprFlags): PNode =
     result.typ = makeTypeDesc(c, semTypeNode(c, n, nil))
     #result = symNodeFromType(c, semTypeNode(c, n, nil), n.info)
   of tyTuple:
-    checkSonsLen(n, 2)
+    if n.len != 2: return nil
     n.sons[0] = makeDeref(n.sons[0])
     c.p.bracketExpr = n.sons[0]
     # [] operator for tuples requires constant expression:
@@ -1276,9 +1276,9 @@ proc semSubscript(c: PContext, n: PNode, flags: TExprFlags): PNode =
       var idx = getOrdValue(n.sons[1])
       if idx >= 0 and idx < sonsLen(arr): n.typ = arr.sons[int(idx)]
       else: localError(n.info, errInvalidIndexValueForTuple)
-    else:
-      localError(n.info, errIndexTypesDoNotMatch)
       result = n
+    else:
+      result = nil
   else:
     let s = if n.sons[0].kind == nkSym: n.sons[0].sym
             elif n[0].kind in nkSymChoices: n.sons[0][0].sym

tests/tuples/tuple_subscript.nim

@@ -0,0 +1,40 @@
+discard """
+  output: '''5
+5
+str2
+str2
+4'''
+"""
+
+proc`[]` (t: tuple, key: string): string =
+  for name, field in fieldPairs(t):
+    if name == key: 
+      return $field
+  return ""
+
+
+proc`[]` [A,B](t: tuple, key: string, op: (proc(x: A): B)): B =
+  for name, field in fieldPairs(t):
+    when field is A:
+      if name == key: 
+        return op(field)
+
+proc`[]=`[T](t: var tuple, key: string, val: T) =
+  for name, field in fieldPairs(t):
+    when field is T:
+      if name == key: 
+        field = val
+
+var tt = (a: 1, b: "str1")
+
+# test built in operator
+tt[0] = 5
+echo tt[0] 
+echo `[]`(tt, 0)
+
+
+# test overloaded operator
+tt["b"] = "str2"
+echo tt["b"] 
+echo `[]`(tt, "b")
+echo tt["b", proc(s: string) : int = s.len]