cgen: fix alias to sumtype cast initialization (fix #25086) (#25091)

vlib/v/gen/c/cgen.v

@@ -5587,7 +5587,17 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 		} else if sym.info is ast.Alias && sym.info.parent_type.has_flag(.option) {
 			g.expr_with_opt(node.expr, expr_type, sym.info.parent_type)
 		} else {
-			g.write('(${cast_label}(')
+			g.write('(${cast_label}')
+			expr_typ := ast.mktyp(node.expr_type)
+			alias_to_sumtype := sym.info is ast.Alias
+				&& g.table.sumtype_has_variant(sym.info.parent_type, expr_typ, false)
+			if alias_to_sumtype {
+				expr_styp := g.styp(expr_typ)
+				g.write('{._${g.table.sym(expr_typ).cname}=memdup(ADDR(${expr_styp}, ')
+				g.expr(node.expr)
+				g.write('), sizeof(${expr_styp})),._typ=${int(expr_typ)}})')
+			} else {
+				g.write('(')
 				if node.expr is ast.Ident {
 					if !node.typ.is_ptr() && node.expr_type.is_ptr() && node.expr.obj is ast.Var
 						&& node.expr.obj.smartcasts.len > 0 {
@@ -5611,6 +5621,7 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 				g.write('))')
 			}
 		}
+	}
 }
 
 fn (mut g Gen) concat_expr(node ast.ConcatExpr) {

vlib/v/tests/sumtypes/sumtype_init_literal_test.v

@@ -0,0 +1,19 @@
+struct Foo {}
+
+type Sum = int | string | Foo
+type SumAlias = Sum
+
+fn test_struct() {
+	a := SumAlias(Foo{})
+	assert '${a}' == 'SumAlias(Sum(Foo{}))'
+}
+
+fn test_int() {
+	a := SumAlias(0)
+	assert '${a}' == 'SumAlias(Sum(0))'
+}
+
+fn test_string() {
+	a := SumAlias('foo')
+	assert '${a}' == "SumAlias(Sum('foo'))"
+}