checker: check sumtype as mismatched type (#12743)

vlib/v/ast/table.v

@@ -1121,7 +1121,7 @@ pub fn (mut t Table) register_fn_concrete_types(fn_name string, types []Type) bo
 
 // TODO: there is a bug when casting sumtype the other way if its pointer
 // so until fixed at least show v (not C) error `x(variant) =  y(SumType*)`
-pub fn (t &Table) sumtype_has_variant(parent Type, variant Type) bool {
+pub fn (t &Table) sumtype_has_variant(parent Type, variant Type, is_as bool) bool {
 	parent_sym := t.get_type_symbol(parent)
 	if parent_sym.kind == .sum_type {
 		parent_info := parent_sym.info as SumType
@@ -1129,14 +1129,14 @@ pub fn (t &Table) sumtype_has_variant(parent Type, variant Type) bool {
 		if var_sym.kind == .aggregate {
 			var_info := var_sym.info as Aggregate
 			for var_type in var_info.types {
-				if !t.sumtype_has_variant(parent, var_type) {
+				if !t.sumtype_has_variant(parent, var_type, is_as) {
 					return false
 				}
 			}
 			return true
 		} else {
 			for v in parent_info.variants {
-				if v.idx() == variant.idx() {
+				if v.idx() == variant.idx() && (!is_as || v.nr_muls() == variant.nr_muls()) {
 					return true
 				}
 			}

vlib/v/checker/check_types.v

@@ -117,7 +117,7 @@ pub fn (mut c Checker) check_basic(got ast.Type, expected ast.Type) bool {
 		return true
 	}
 	// sum type
-	if c.table.sumtype_has_variant(expected, c.table.mktyp(got)) {
+	if c.table.sumtype_has_variant(expected, c.table.mktyp(got), false) {
 		return true
 	}
 	// type alias

vlib/v/checker/checker.v

@@ -5421,8 +5421,10 @@ pub fn (mut c Checker) expr(node ast.Expr) ast.Type {
 			type_sym := c.table.get_type_symbol(node.typ)
 			if expr_type_sym.kind == .sum_type {
 				c.ensure_type_exists(node.typ, node.pos) or {}
-				if !c.table.sumtype_has_variant(node.expr_type, node.typ) {
-					c.error('cannot cast `$expr_type_sym.name` to `$type_sym.name`', node.pos)
+				if !c.table.sumtype_has_variant(node.expr_type, node.typ, true) {
+					addr := '&'.repeat(node.typ.nr_muls())
+					c.error('cannot cast `$expr_type_sym.name` to `$addr$type_sym.name`',
+						node.pos)
 				}
 			} else if expr_type_sym.kind == .interface_ && type_sym.kind == .interface_ {
 				c.ensure_type_exists(node.typ, node.pos) or {}
@@ -5734,7 +5736,7 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			node.expr_type = c.promote_num(node.expr_type, xx)
 			from_type = node.expr_type
 		}
-		if !c.table.sumtype_has_variant(to_type, from_type) && !to_type.has_flag(.optional) {
+		if !c.table.sumtype_has_variant(to_type, from_type, false) && !to_type.has_flag(.optional) {
 			c.error('cannot cast `$from_type_sym.name` to `$to_type_sym.name`', node.pos)
 		}
 	} else if mut to_type_sym.info is ast.Alias {

vlib/v/checker/tests/sumtype_as_mismatched_type.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/sumtype_as_mismatched_type.vv:5:9: error: cannot cast `Type` to `&int`
+    3 | fn main() {
+    4 |     mut t := Type(123)
+    5 |     v := t as &int
+      |            ~~
+    6 |     t = 456
+    7 |     println(v)

vlib/v/checker/tests/sumtype_as_mismatched_type.vv

@@ -0,0 +1,8 @@
+type Type = bool | int | string
+
+fn main() {
+	mut t := Type(123)
+	v := t as &int
+	t = 456
+	println(v)
+}

vlib/v/gen/c/cgen.v

@@ -2364,7 +2364,7 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 			unwrapped_expected_type
 		}
 		got_deref_type := if got_is_ptr { unwrapped_got_type.deref() } else { unwrapped_got_type }
-		if g.table.sumtype_has_variant(expected_deref_type, got_deref_type) {
+		if g.table.sumtype_has_variant(expected_deref_type, got_deref_type, false) {
 			mut is_already_sum_type := false
 			scope := g.file.scope.innermost(expr.position().pos)
 			if expr is ast.Ident {