T20485.hs:7:3: warning: [GHC-58008] [-Wgadt-mono-local-binds (in -Wdefault)]
    Pattern matching on GADTs without MonoLocalBinds is fragile.
    Suggested fix:
      Enable any of the following extensions: GADTs, TypeFamilies

T20485.hs:9:19: warning: [GHC-58008] [-Wgadt-mono-local-binds (in -Wdefault)]
    Pattern matching on GADTs without MonoLocalBinds is fragile.
    Suggested fix:
      Enable any of the following extensions: GADTs, TypeFamilies

T20485.hs:12:3: warning: [GHC-58008] [-Wgadt-mono-local-binds (in -Wdefault)]
    Pattern matching on GADTs without MonoLocalBinds is fragile.
    Suggested fix:
      Enable any of the following extensions: GADTs, TypeFamilies