[Issue 21562] New: Allow mixin template declarations without parentheses

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21562

          Issue ID: 21562
           Summary: Allow mixin template declarations without parentheses
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: qs.il.paperinik at gmail.com

A mixin template declaration needs to have parentheses following the name.
If the parentheses are empty, it should be valid to omit them entirely.

Grammar changes:

TemplateMixinDeclaration:
+   mixin template Identifier TemplateParameters[opt] Constraint[opt] {
DeclDefs[opt] }
-   mixin template Identifier TemplateParameters Constraint[opt] {
DeclDefs[opt] }

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