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Add syntax grammar for allows constraint to ref-struct-interfaces.md #7977

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Feb 29, 2024
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Add syntax grammar for allows constraint to ref-struct-interfaces.md #7977

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52 changes: 52 additions & 0 deletions proposals/ref-struct-interfaces.md
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Original file line number Diff line number Diff line change
Expand Up @@ -146,6 +146,58 @@ Detailed Notes:

### ref struct Generic Parameters

```ANTLR
type_parameter_constraints_clause
: 'where' type_parameter ':' type_parameter_constraints
;

type_parameter_constraints
: restrictive_type_parameter_constraints
| allows_type_parameter_constraints_clause
| restrictive_type_parameter_constraints ',' allows_type_parameter_constraints_clause

restrictive_type_parameter_constraints
: primary_constraint
| secondary_constraints
| constructor_constraint
| primary_constraint ',' secondary_constraints
| primary_constraint ',' constructor_constraint
| secondary_constraints ',' constructor_constraint
| primary_constraint ',' secondary_constraints ',' constructor_constraint
;

primary_constraint
: class_type
| 'class'
| 'struct'
| 'unmanaged'
;

secondary_constraints
: interface_type
| type_parameter
| secondary_constraints ',' interface_type
| secondary_constraints ',' type_parameter
;

constructor_constraint
: 'new' '(' ')'
;

allows_type_parameter_constraints_clause
: 'allows' allows_type_parameter_constraints

allows_type_parameter_constraints
: allows_type_parameter_constraint
| allows_type_parameter_constraints ',' allows_type_parameter_constraint
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Why allow multiple if we have just single option now?

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Why allow multiple if we have just single option now?

Future proofing, we might want to allow more things in the future.


allows_type_parameter_constraint
: ref_struct_clause

ref_struct_clause
: 'ref' 'struct'
```

The language will allow for generic parameters to opt into supporting `ref struct` as arguments by using the `allows ref struct` syntax inside a `where` clause:

```csharp
Expand Down