[Issue 18834] New: Use ConditionalExpression instead of AssignExpression in TemplateArgument

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18834

          Issue ID: 18834
           Summary: Use ConditionalExpression instead of AssignExpression
                    in TemplateArgument
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: trivial
          Priority: P1
         Component: dlang.org
          Assignee: nobody at puremagic.com
          Reporter: alphaglosined at gmail.com

Current grammar suggests that assignments are valid as part of template
initiations.

I.e.

```
func!(x=6);
```

This is because of the usage of AssignExpression inside of TemplateArgument.

AssignExpression refers to ConditionalExpression for both sides. We should
consider and probably use it instead.

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