table: remove fn to set idx & add new derive fn

vlib/v/checker/checker.v

@@ -1743,8 +1743,7 @@ fn (mut c Checker) stmts(stmts []ast.Stmt) {
 pub fn (c &Checker) unwrap_generic(typ table.Type) table.Type {
 	if typ.idx() == table.t_type_idx {
 		// return c.cur_generic_type
-		// its more efficient to set the id rather than to copy flags/nr_muls
+		return c.cur_generic_type.derive(typ)
-		return typ.set_idx(c.cur_generic_type)
 	}
 	return typ
 }

vlib/v/gen/fn.v

@@ -331,8 +331,7 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 pub fn (g &Gen) unwrap_generic(typ table.Type) table.Type {
 	if typ.idx() == table.t_type_idx {
 		// return g.cur_generic_type
-		// its more efficient to set the id rather than to copy flags/nr_muls
+		return g.cur_generic_type.derive(typ)
-		return typ.set_idx(g.cur_generic_type)
 	}
 	return typ
 }

vlib/v/table/atypes.v

@@ -70,12 +70,6 @@ pub fn (t Type) is_ptr() bool {
 	return (int(t) >> 16) & 0xff > 0
 }
 
-// set idx on `t` and return it
-[inline]
-pub fn (t Type) set_idx(idx int) Type {
-	return (((int(t) >> 24) & 0xff) << 24) | (((int(t) >> 16) & 0xff) << 16) | (u16(idx) & 0xffff)
-}
-
 // set nr_muls on `t` and return it
 [inline]
 pub fn (t Type) set_nr_muls(nr_muls int) Type {
@@ -129,6 +123,12 @@ pub fn (t Type) has_flag(flag TypeFlag) bool {
 	return (((int(t) >> 24) & 0xff) >> int(flag)) & 1 == 1
 }
 
+// copy flags & nr_muls from `t_from` to `t` and return `t`
+[inline]
+pub fn (t Type) derive(t_from Type) Type {
+	return (((int(t_from) >> 24) & 0xff) << 24) | (((int(t_from) >> 16) & 0xff) << 16) | (u16(t) & 0xffff)
+}
+
 // return new type with TypeSymbol idx set to `idx`
 [inline]
 pub fn new_type(idx int) Type {