checker: don't cast match branch structs to IError, when the return t…

…ype of a function is !SumType (#22444)

vlib/v/checker/match.v

@@ -102,6 +102,8 @@ fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 					if ret_type.idx() != expr_type.idx() {
 						if node.expected_type.has_option_or_result()
 							&& c.table.sym(stmt.typ).kind == .struct_
+							&& (c.table.sym(ret_type).kind != .sum_type
+							|| !c.check_types(expr_type, ret_type))
 							&& c.type_implements(stmt.typ, ast.error_type, node.pos) {
 							stmt.expr = ast.CastExpr{
 								expr:      stmt.expr

vlib/v/tests/return_match_expr_of_sumtype_do_not_cast_to_ierror_test.v

@@ -0,0 +1,73 @@
+type Foo = int | string | Bar
+
+struct Bar {}
+
+fn foo_result_struct_second() !Foo {
+	return match true {
+		true { 1 }
+		else { Bar{} }
+	}
+}
+
+fn foo_option_struct_second() ?Foo {
+	return match true {
+		true { 1 }
+		else { Bar{} }
+	}
+}
+
+fn foo_result_string_second() !Foo {
+	return match true {
+		true { 1 }
+		else { '' }
+	}
+}
+
+fn foo_option_string_second() ?Foo {
+	return match true {
+		true { 1 }
+		else { '' }
+	}
+}
+
+fn foo_result_struct_first() !Foo {
+	return match true {
+		true { Bar{} }
+		else { 7 }
+	}
+}
+
+fn foo_option_struct_first() ?Foo {
+	return match true {
+		true { Bar{} }
+		else { 7 }
+	}
+}
+
+fn test_return_match_expr_of_sumtype_opt_res() {
+	mut ret := Foo{}
+
+	ret = foo_result_struct_second() or { return }
+	println(ret)
+	assert '${ret}' == 'Foo(1)'
+
+	ret = foo_option_struct_second() or { return }
+	println(ret)
+	assert '${ret}' == 'Foo(1)'
+
+	ret = foo_result_string_second() or { return }
+	println(ret)
+	assert '${ret}' == 'Foo(1)'
+
+	ret = foo_option_string_second() or { return }
+	println(ret)
+	assert '${ret}' == 'Foo(1)'
+
+	ret = foo_result_struct_first() or { return }
+	println(ret)
+	assert '${ret}' == 'Foo(Bar{})'
+
+	ret = foo_option_struct_first() or { return }
+	println(ret)
+	assert '${ret}' == 'Foo(Bar{})'
+}