checker: disallow sum type holding alias ptrs

[Delta456]

Fixes #20782

[MCausc78]

vlib/v/checker/tests/sum_type_holding_alias_ptr_err.vv:3:12: error: sum type cannot hold a reference type 
     1 | type Alias1 = &int 
     2 | type Alias2 = &string 
     3 | type Qwe = Alias1 | Alias2 
       |            ~~~~~~ 
     4 |  
     5 | fn main() { 
 Details: declare the sum type with non-reference types: `Qwe = Alias1 | ...` 
 and use a reference to the sum type instead: `var := &Qwe(Alias1(val))`

Can we get other Details message?

[Delta456]

vlib/v/checker/tests/sum_type_holding_alias_ptr_err.vv:3:12: error: sum type cannot hold a reference type 
     1 | type Alias1 = &int 
     2 | type Alias2 = &string 
     3 | type Qwe = Alias1 | Alias2 
       |            ~~~~~~ 
     4 |  
     5 | fn main() { 
 Details: declare the sum type with non-reference types: `Qwe = Alias1 | ...` 
 and use a reference to the sum type instead: `var := &Qwe(Alias1(val))`

Can we get other Details message?

Like what?

[MCausc78]

vlib/v/checker/tests/sum_type_holding_alias_ptr_err.vv:3:12: error: sum type cannot hold a reference type 
     1 | type Alias1 = &int 
     2 | type Alias2 = &string 
     3 | type Qwe = Alias1 | Alias2 
       |            ~~~~~~ 
     4 |  
     5 | fn main() { 
 Details: declare the sum type with non-reference types: `Qwe = Alias1 | ...` 
 and use a reference to the sum type instead: `var := &Qwe(Alias1(val))`

Can we get other Details message?

Like what?

Current one is confusing, since Qwe = Alias1 | ... declares sumtype with reference, something like declare alias as non-reference type, then declare sumtype as Qwe = Alias1

[Delta456]

vlib/v/checker/tests/sum_type_holding_alias_ptr_err.vv:3:12: error: sum type cannot hold a reference type 
     1 | type Alias1 = &int 
     2 | type Alias2 = &string 
     3 | type Qwe = Alias1 | Alias2 
       |            ~~~~~~ 
     4 |  
     5 | fn main() { 
 Details: declare the sum type with non-reference types: `Qwe = Alias1 | ...` 
 and use a reference to the sum type instead: `var := &Qwe(Alias1(val))`

Can we get other Details message?

Like what?

Current one is confusing, since Qwe = Alias1 | ... declares sumtype with reference, something like declare alias as non-reference type, then declare sumtype as Qwe = Alias1

It makes it clear, thanks!

[MCausc78]

Forbid type Foo = ∫ type Bar = Foo; type Baz = Bar | int | string too

[Delta456]

type Foo = ∫ type Bar = Foo; type Baz = Bar | int | string

This already works

file.v:1:29: error: type `Foo` is an alias, use the original alias type `int` instead
    1 | type Foo = ∫ type Bar = Foo; type Baz = Bar | int | string
      |                             ~~~

[JalonSolov]

Hmm... that last error message is incorrect/confusing. Should be fixed in another PR.

[spytheman]

Good work.