cgen: fix optional sum types (#8319)

vlib/v/gen/cgen.v

@@ -4519,7 +4519,7 @@ fn (mut g Gen) return_statement(node ast.Return) {
 				}
 			}
 			for i, expr in node.exprs {
-				g.expr(expr)
+				g.expr_with_cast(expr, node.types[i], g.fn_decl.return_type.clear_flag(.optional))
 				if i < node.exprs.len - 1 {
 					g.write(', ')
 				}
@@ -5299,7 +5299,7 @@ fn (mut g Gen) or_block(var_name string, or_block ast.OrExpr, return_type table.
 					if is_opt_call {
 						g.write('*($mr_styp*) ')
 					}
-					g.expr(expr_stmt.expr)
+					g.expr_with_cast(expr_stmt.expr, expr_stmt.typ, return_type.clear_flag(.optional))
 					if is_opt_call {
 						g.write('.data')
 					}

vlib/v/tests/option_test.v

@@ -323,16 +323,52 @@ fn test_option_void_return_types_of_anon_fn_in_struct() {
 	}
 }
 
-fn get_string(param bool) ?string {
-	if param {
-		return 'Hello World'
-	}
-	return none
+type AA = BB | CC
+
+struct BB {
+	str string
 }
 
-fn test_option_auto_add_return_none() {
-	r := get_string(false) or {
-		'test'
-	}
-	assert r == 'test'
+struct CC {
+	str string
+}
+
+fn optional_sum_type(a int) ?AA {
+	match a {
+		1 {
+			return BB{'Test'}
+		}
+		2 {
+			return CC{'Test'}
+		}
+		else {
+			return error('Wrong number')
+		}
+	}
+}
+
+fn test_optional_sum_type() {
+	res1 := optional_sum_type(1) or {
+		assert false
+		BB{}
+	}
+	res2 := optional_sum_type(2) or {
+		assert false
+		CC{}
+	}
+	if res1 is BB {
+		assert res1.str == 'Test'
+	} else {
+		assert false
+	}
+	if res2 is CC {
+		assert res2.str == 'Test'
+	} else {
+		assert false
+	}
+	optional_sum_type(3) or {
+		assert true
+		return
+	}
+	assert false
 }