• user1234 (35/35) Nov 09 For now the grammar does not allow
• Serg Gini (18/22) Nov 09 what the difference with this:
• Atila Neves (2/26) Nov 11 I was going to ask the same thing.

user1234 <user1234 12.de> writes:

For now the grammar does not allow

```d
void v(int a, deprecated int b = 0);
```

I propose to modify [the 
grammar](https://dlang.org/spec/function.html#ParameterAttributes) to allow
this code:

```diff
ParameterAttributes:
     ParameterStorageClass
     UserDefinedAttribute
     ParameterAttributes ParameterStorageClass
     ParameterAttributes UserDefinedAttribute
+    deprecated?
```

Deprecated parameters will be checked during the semantics 
verification of the CallExp, passed the stage of overload 
resolution, following two simple rules

1. It is an error to use the attribute if a default argument is 
not specified (alternatively can be implemented during 
dsymbolsem).
2. A new message is output if deprecations are enabled and if a 
custom argument is passed to the attributed parameter. The new 
message can be either a warning or an error, according to the 
matching CLI option (-dw or -de). The message points to the 
argument, not the parameter.

```d
void v0(int a, deprecated int b);     // rule 1, error
void v1(int a, deprecated int b = 0); // rule 1, no error

v1(0);     // rule 2, no message
v1(0,123); // rule 2, new message for `123`
```

The goal of this enhancement is to propose a system so that users 
of a library can prepare to a future API change.

Is that worth ? Not sure 🤔 but at least the idea is documented 
now.

Serg Gini <kornburn yandex.ru> writes:

On Monday, 10 November 2025 at 07:13:18 UTC, user1234 wrote:
 For now the grammar does not allow

 ```d
 void v(int a, deprecated int b = 0);
 ```

what the difference with this:
```d
import std;

void v1(int a)
{
     writeln("new");
}

deprecated("use v1 with only 1 parameter a") void v1(int a, int b)
{
     writeln("old");
}

void main()
{
     v1(0);
     v1(0, 123);
}
```

Atila Neves <atila.neves gmail.com> writes:

On Monday, 10 November 2025 at 07:27:55 UTC, Serg Gini wrote:
 On Monday, 10 November 2025 at 07:13:18 UTC, user1234 wrote:
 For now the grammar does not allow

 ```d
 void v(int a, deprecated int b = 0);
 ```

 what the difference with this:
 ```d
 import std;

 void v1(int a)
 {
     writeln("new");
 }

 deprecated("use v1 with only 1 parameter a") void v1(int a, int 
 b)
 {
     writeln("old");
 }

 void main()
 {
     v1(0);
     v1(0, 123);
 }
 ```

I was going to ask the same thing.