checker,cgen: fix cast_expr() for alias (fix #26076) (#26079)

Author: kbkpbot

vlib/v/ast/types.v

@@ -397,6 +397,12 @@ pub fn (t Type) deref() Type {
 	return t & 0xff00ffff | (nr_muls - 1) << 16
 }
 
+// flags returns type's flags
+@[inline]
+pub fn (t Type) flags() int {
+	return t >> 16
+}
+
 // has_flag returns whether the given named `flag` is set
 @[inline]
 pub fn (t Type) has_flag(flag TypeFlag) bool {

vlib/v/checker/checker.v

@@ -3539,6 +3539,13 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	} else {
 		to_type
 	}
+
+	if final_to_sym == final_from_sym && final_to_type.flags() == from_type.flags()
+		&& to_type.flags() == from_type.flags() {
+		// type alias, and flags are same, e.g. option, result, nr_muls...
+		return node.typ
+	}
+
 	final_to_is_ptr := to_type.is_ptr() || final_to_type.is_ptr()
 	c.markused_castexpr(mut node, to_type, mut final_to_sym)
 	if to_type.has_flag(.result) {

vlib/v/gen/c/cgen.v

@@ -5663,6 +5663,8 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 	if g.comptime.is_comptime(node.expr) {
 		expr_type = g.unwrap_generic(g.type_resolver.get_type(node.expr))
 	}
+	expr_sym := g.table.sym(expr_type)
+	final_expr_sym := g.table.final_sym(expr_type)
 	node_typ_is_option := node.typ.has_flag(.option)
 	if sym.kind in [.sum_type, .interface] {
 		if g.table.unaliased_type(expr_type) == node_typ {
@@ -5679,8 +5681,10 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 		} else {
 			g.expr_with_cast(node.expr, expr_type, node_typ)
 		}
-	} else if !node_typ_is_option && !node_typ.is_ptr() && sym.info is ast.Struct
-		&& !sym.info.is_typedef {
+	} else if !node_typ_is_option && !node_typ.is_ptr() && !expr_type.is_ptr()
+		&& ((sym.info is ast.Struct && !sym.info.is_typedef)
+		|| (expr_sym.kind == .alias && final_expr_sym.info is ast.Struct
+		&& !final_expr_sym.info.is_typedef)) {
 		// deprecated, replaced by Struct{...exr}
 		styp := g.styp(node_typ)
 		g.write('*((${styp} *)(&')

vlib/v/tests/casts/cast_to_alias_test.v

@@ -49,3 +49,29 @@ fn test_cast_to_alias_of_ref_struct() {
 	println(typeof(bar).name)
 	assert typeof(bar).name == '&Alias'
 }
+
+type Bar1 = Foo
+type Bar2 = Foo
+type Bar3 = Foo
+
+fn test_cast_to_from_alias() {
+	bar2 := Bar2{
+		x: 100
+	}
+	foo := Foo(bar2)
+	bar1 := Bar1(bar2)
+	bar3 := Bar3(bar2)
+	assert typeof(foo).name == 'Foo'
+	assert typeof(bar1).name == 'Bar1'
+	assert typeof(bar2).name == 'Bar2'
+	assert typeof(bar3).name == 'Bar3'
+
+	foo1_1 := Foo(foo)
+	bar1_1 := Bar1(foo)
+	bar2_1 := Bar2(foo)
+	bar3_1 := Bar3(foo)
+	assert typeof(foo1_1).name == 'Foo'
+	assert typeof(bar1_1).name == 'Bar1'
+	assert typeof(bar2_1).name == 'Bar2'
+	assert typeof(bar3_1).name == 'Bar3'
+}