compiler: convert to ByteArray for string variables

Convert to ByteArray when converting variable to `string`, because
underlying byte-slice changes should not affect result string.

pkg/compiler/codegen.go

@@ -773,7 +773,11 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			c.convertBuiltin(n)
 		case name != "":
 			// Function was not found thus is can be only an invocation of func-typed variable or type conversion.
-			if isFunc {
+			// We care only about string conversions because all others are effectively no-op in NeoVM.
+			// E.g. one cannot write `bool(int(a))`, only `int32(int(a))`.
+			if isString(c.typeOf(n.Fun)) {
+				c.emitConvert(stackitem.ByteArrayT)
+			} else if isFunc {
 				c.emitLoadVar(name)
 				emit.Opcode(c.prog.BinWriter, opcode.CALLA)
 			}

pkg/compiler/convert_test.go

@@ -86,3 +86,15 @@ func TestTypeConversion(t *testing.T) {
 
 	eval(t, src, big.NewInt(42))
 }
+
+func TestTypeConversionString(t *testing.T) {
+	src := `package foo
+	type mystr string
+	func Main() mystr {
+		b := []byte{'l', 'a', 'm', 'a', 'o'}
+		s := mystr(b)
+		b[0] = 'u'
+		return s
+	}`
+	eval(t, src, []byte("lamao"))
+}