Recap

Current syntax (from lang ref):

UserDefinedAttribute:
    @ ( TemplateArgumentList )
    @ TemplateSingleArgument
    @ Identifier ( NamedArgumentList[opt] )
    @ TemplateInstance
    @ TemplateInstance ( NamedArgumentList[opt] )

TemplateSingleArgument:
    Identifier
    FundamentalType
    CharacterLiteral
    StringLiteral
    InterpolationExpressionSequence
    IntegerLiteral
    FloatLiteral
    true
    false
    null
    this
    SpecialKeyword

After expanding TemplateSingleArgument, we have this:

UserDefinedAttribute:
    @ ( TemplateArgumentList )
    @ CharacterLiteral
    @ StringLiteral
    @ InterpolationExpressionSequence
    @ IntegerLiteral
    @ FloatLiteral
    @ true
    @ false
    @ null
    @ this
    @ SpecialKeyword
    @ FundamentalType
    @ Identifier
    @ Identifier ( NamedArgumentList[opt] )
    @ TemplateInstance
    @ TemplateInstance ( NamedArgumentList[opt] )

Proposal #1

Replace comma-separated list of expressions

    @ ( TemplateArgumentList )

with single expression

    @ ( TemplateArgument )

Example: instead of @(A,B,C), one should write @(A) @(B) @(C).

I believe this is much clearer because it forces single attribute per single @-expression. This will also reduce cognitive load on weird things like @(_=>(2,4),2,4,_=>2,4).

Proposal #2

Replace

    @ FundamentalType
    @ Identifier
    @ Identifier ( NamedArgumentList[opt] )
    @ TemplateInstance
    @ TemplateInstance ( NamedArgumentList[opt] )

with parts of PostfixExpression and PrimaryExpression:

UserDefinedAttribute:
    ...
    @ UserDefinedAttributeExpression

UserDefinedAttributeExpression:
    FundamentalType
    Identifier
    . Identifier
    TemplateInstance
    . TemplateInstance
    UserDefinedAttributeExpression . Identifier
    UserDefinedAttributeExpression . TemplateInstance
    UserDefinedAttributeExpression ( NamedArgumentList[opt] )

This will allow writing !/.-chained expressions without parentheses:
@A!B.C(D) instead of @(A!B.C(D)) currently.

Proposal #3

Remove undocumented behavior from compiler which allows using lambda in UDA without parentheses:

@_ => 2
int a;

In this case, one should be forced to use parentheses:

@(_ => 2)
int a;

On Tuesday, 19 November 2024 at 04:26:17 UTC, Andrey Zherikov wrote:

UserDefinedAttribute:
    @ ( TemplateArgumentList )
    @ TemplateSingleArgument
    @ Identifier ( NamedArgumentList[opt] )
    @ TemplateInstance
    @ TemplateInstance ( NamedArgumentList[opt] )

[...]

Yes, I noticed a long time ago that you can declare

alias F = float;
@F int i;

but not

@float int i;

which proposal #2 would address.