support dynamic if-cast with multiple is

[ttytm]

Describe the feature

Support dynamic if-cast with multiple is type checks.

Use Case

The following would cause: error: field 'commonfield' does not exist or have the same type in all sumtype variants, despite it does exist on the two if is-checked variants.

type MySumType = S1 | S2 | S3

struct S1 {
	commonfield string
	field1      string
}

struct S2 {
	commonfield string
	field2      string
}

struct S3 {
	field3 string
}

s := MySumType(S1{
	commonfield: 'foo'
})

b := true

if b && (s is S1 || s is S2) {
	println(s.commonfield)
}

This is also the case without additional conditions, i.e. if s is S1 || s is S2. b should just illustrate anything additiona that would need to be taken care of in a solution.

Proposed Solution

If there are multiple is in an if condition (i.e. split by a logical or) they could resemble multiple if conditions under the hood.

E.g. this:

if b && (s is S1 || s is S2) {
	println(s.commonfield)
}

Would become this:

if b && s is S1 {
	println(s.commonfield)
}
if b && s is S2 {
	println(s.commonfield)
}

or

if b {
	if s is S1 {
		println(s.commonfield)
	}
	if s is S2 {
		println(s.commonfield)
	}
}

I'm not having looked into how if-is cast are handled so there might be even better approaches.

Other Information

Currently it would be possible to use a coma separated match arm for this which would generate a similar code.

match s {
	S1, S2 { 
		if b {
			println(s.commonfield) 
		}
	}
	else {}
}

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

Version used

0.3.4

Environment details (OS name and version, etc.)

linux,arch;amd64

---

edit(2023.06.09): fix typos
edit(2023.06.14): update example for match equivalent, improve problem description