interpret tuple as a class and tuple[] as the empty tuple

When the indentation syntax is allowed it is always interpreted as a
tuple:

type
  Unit = tuple

compiler/ast.nim

@@ -196,6 +196,7 @@ type
     nkTypeOfExpr,         # type(1+2)
     nkObjectTy,           # object body
     nkTupleTy,            # tuple body
+    nkTupleClassTy,       # tuple type class
     nkTypeClassTy,        # user-defined type class
     nkStaticTy,           # ``static[T]``
     nkRecList,            # list of object parts

compiler/parser.nim

@@ -865,6 +865,7 @@ proc parseTuple(p: var TParser, indentAllowed = false): PNode =
   #|     [' optInd  (identColonEquals (comma/semicolon)?)*  optPar ']'
   #| extTupleDecl = 'tuple'
   #|     COMMENT? (IND{>} identColonEquals (IND{=} identColonEquals)*)?
+  #| tupleClass = 'tuple'
   result = newNodeP(nkTupleTy, p)
   getTok(p)
   if p.tok.tokType == tkBracketLe:
@@ -894,6 +895,8 @@ proc parseTuple(p: var TParser, indentAllowed = false): PNode =
             parMessage(p, errIdentifierExpected, p.tok)
             break
           if not sameInd(p): break
+  else:
+    result = newNodeP(nkTupleClassTy, p)
 
 proc parseParamList(p: var TParser, retColon = true): PNode =
   #| paramList = '(' declColonEquals ^* (comma/semicolon) ')'

compiler/renderer.nim

@@ -395,6 +395,7 @@ proc lsub(n: PNode): int =
   of nkClosedSymChoice, nkOpenSymChoice: 
     result = lsons(n) + len("()") + sonsLen(n) - 1
   of nkTupleTy: result = lcomma(n) + len("tuple[]")
+  of nkTupleClassTy: result = len("tuple")
   of nkDotExpr: result = lsons(n) + 1
   of nkBind: result = lsons(n) + len("bind_")
   of nkBindStmt: result = lcomma(n) + len("bind_")
@@ -1292,10 +1293,11 @@ proc gsub(g: var TSrcGen, n: PNode, c: TContext) =
       gsub(g, n.sons[0])
   of nkTupleTy: 
     put(g, tkTuple, "tuple")
-    if sonsLen(n) > 0:
-      put(g, tkBracketLe, "[")
-      gcomma(g, n)
-      put(g, tkBracketRi, "]")
+    put(g, tkBracketLe, "[")
+    gcomma(g, n)
+    put(g, tkBracketRi, "]")
+  of nkTupleClassTy:
+    put(g, tkTuple, "tuple")
   of nkMetaNode_Obsolete:
     put(g, tkParLe, "(META|")
     gsub(g, n.sons[0])

compiler/semexprs.nim

@@ -2055,7 +2055,7 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
   of nkBind:
     message(n.info, warnDeprecated, "bind")
     result = semExpr(c, n.sons[0], flags)
-  of nkTypeOfExpr, nkTupleTy, nkRefTy..nkEnumTy, nkStaticTy:
+  of nkTypeOfExpr, nkTupleTy, nkTupleClassTy, nkRefTy..nkEnumTy, nkStaticTy:
     var typ = semTypeNode(c, n, nil).skipTypes({tyTypeDesc, tyIter})
     result.typ = makeTypeDesc(c, typ)
     #result = symNodeFromType(c, typ, n.info)

compiler/semgnrc.nim

@@ -356,7 +356,7 @@ proc semGenericStmt(c: PContext, n: PNode,
         of nkIdent: a = n.sons[i]
         else: illFormedAst(n)
         addDecl(c, newSymS(skUnknown, getIdentNode(a.sons[i]), c))
-  of nkObjectTy, nkTupleTy:
+  of nkObjectTy, nkTupleTy, nkTupleClassTy:
     discard
   of nkFormalParams:
     checkMinSonsLen(n, 1)

compiler/semtypes.nim

@@ -344,9 +344,8 @@ proc semTypeIdent(c: PContext, n: PNode): PSym =
     else:
       localError(n.info, errIdentifierExpected)
       result = errorSym(c, n)
-
-proc semTuple(c: PContext, n: PNode, prev: PType): PType =
-  if n.sonsLen == 0: return newConstraint(c, tyTuple)
+  
+proc semTuple(c: PContext, n: PNode, prev: PType): PType = 
   var typ: PType
   result = newOrPrevType(tyTuple, prev, c)
   result.n = newNodeI(nkRecList, n.info)
@@ -1227,6 +1226,7 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
       result = newOrPrevType(tyError, prev, c)
   of nkObjectTy: result = semObjectNode(c, n, prev)
   of nkTupleTy: result = semTuple(c, n, prev)
+  of nkTupleClassTy: result = newConstraint(c, tyTuple)
   of nkTypeClassTy: result = semTypeClass(c, n, prev)
   of nkRefTy: result = semAnyRef(c, n, tyRef, prev)
   of nkPtrTy: result = semAnyRef(c, n, tyPtr, prev)

compiler/typesrenderer.nim

@@ -68,7 +68,6 @@ proc renderType(n: PNode): string =
       assert n[i].kind == nkIdent
       result.add(',' & typeStr)
   of nkTupleTy:
-    assert len(n) > 0
     result = "tuple["
     for i in 0 .. <len(n): result.add(renderType(n[i]) & ',')
     result[<len(result)] = ']'

lib/core/macros.nim

@@ -60,7 +60,7 @@ type
     nnkStmtListType, nnkBlockType,
     nnkWith, nnkWithout,
     nnkTypeOfExpr, nnkObjectTy,
-    nnkTupleTy, nnkTypeClassTy, nnkStaticTy,
+    nnkTupleTy, nnkTupleClassTy, nnkTypeClassTy, nnkStaticTy,
     nnkRecList, nnkRecCase, nnkRecWhen,
     nnkRefTy, nnkPtrTy, nnkVarTy,
     nnkConstTy, nnkMutableTy,