https://issues.dlang.org/show_bug.cgi?id=23615

          Issue ID: 23615
           Summary: Trying to alias a certain template expression fails
                    with "Error: unknown, please file report on
                    issues.dlang.org"
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: ice-on-invalid-code
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: schveiguy@gmail.com

alias comb = to!string.map!q{a - '0'};

This is the only error message that occurs:

Error: unknown, please file report on issues.dlang.org

Found by a user here: https://forum.dlang.org/post/wvedxnofxdwgtrhrjzak@forum.dlang.org

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