fix optimization ast parent linkage problem

codeOptimizers/src/prog8/optimizer/ConstantIdentifierReplacer.kt

@@ -77,8 +77,9 @@ class VarConstantValueTypeAdjuster(
                         // variable is never written to, so it can be replaced with a constant, IF the value is a constant
                         errors.info("variable '${decl.name}' is never written to and was replaced by a constant", decl.position)
                         val const = VarDecl(VarDeclType.CONST, decl.origin, decl.datatype, decl.zeropage, decl.arraysize, decl.name, decl.names, declValue, decl.sharedWithAsm, decl.splitArray, decl.position)
+                        decl.value = null
                         return listOf(
-                            IAstModification.ReplaceNode(decl, const, parent),
+                            IAstModification.ReplaceNode(decl, const, parent)
                         )
                     }
                 }

compiler/test/TestOptimization.kt

@@ -793,6 +793,18 @@ main {
         (expr.right as? NumericLiteral)?.number shouldBe 10.0
     }
 
+    test("var to const inside typecast") {
+        val src="""
+main {
+    uword variable  ; will be made a const
+
+    sub start() {
+        cx16.r0 = msb(variable) as uword
+    }
+}"""
+        compileText(VMTarget(), true, src, writeAssembly = false) shouldNotBe null
+    }
+
     test("De Morgan's laws") {
         val src="""
 main {

compilerAst/src/prog8/ast/expressions/AstExpressions.kt

@@ -375,6 +375,7 @@ class TypecastExpression(var expression: Expression, var type: DataType, val imp
     override fun inferType(program: Program) = InferredTypes.knownFor(type)
     override fun constValue(program: Program): NumericLiteral? {
         val cv = expression.constValue(program) ?: return null
+        cv.linkParents(parent)
         val cast = cv.cast(type, implicit)
         return if(cast.isValid) {
             val newval = cast.valueOrZero()