Implement boxing for singleton type arguments #23418
                
     Merged
            
            
          
      
        
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          −40
        
        
          
        
      
    
  
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A type
A[b.type]with a singleton type argument is converted toA[box b.type^{b}]so that the box status is recorded. Since box status only comes with CapturingTypes we have to convert the singleton type to a capturing type.Fixes #23207