bugfix: compiler errors for explicit initialization of thread local variables; os.nim does not rely on it anymore

compiler/msgs.nim

@@ -92,7 +92,8 @@ type
     errCannotInterpretNodeX, errFieldXNotFound, errInvalidConversionFromTypeX, 
     errAssertionFailed, errCannotGenerateCodeForX, errXRequiresOneArgument, 
     errUnhandledExceptionX, errCyclicTree, errXisNoMacroOrTemplate, 
-    errXhasSideEffects, errIteratorExpected, errLetNeedsInit, errWrongSymbolX,
+    errXhasSideEffects, errIteratorExpected, errLetNeedsInit,
+    errThreadvarCannotInit, errWrongSymbolX,
     errUser,
     warnCannotOpenFile, 
     warnOctalEscape, warnXIsNeverRead, warnXmightNotBeenInit, 
@@ -321,6 +322,7 @@ const
     errXhasSideEffects: "\'$1\' can have side effects", 
     errIteratorExpected: "iterator within for loop context expected",
     errLetNeedsInit: "'let' symbol requires an initialization",
+    errThreadvarCannotInit: "a thread var cannot be initialized explicitly",
     errWrongSymbolX: "usage of \'$1\' is a user-defined error", 
     errUser: "$1", 
     warnCannotOpenFile: "cannot open \'$1\' [CannotOpenFile]",

compiler/semstmts.nim

@@ -269,7 +269,8 @@ proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
       if def != nil and def.kind != nkEmpty:
         # this is only needed for the evaluation pass:
         v.ast = def
-      if a.kind != nkVarTuple: 
+        if sfThread in v.flags: LocalError(def.info, errThreadvarCannotInit)
+      if a.kind != nkVarTuple:
         v.typ = typ
         b = newNodeI(nkIdentDefs, a.info)
         addSon(b, newSymNode(v))