Tracking suggestion: Active patterns disables the detection of simple duplicates cases.

[dsyme]

Tracking this suggestion: dotnet/fsharp#2754

Presence of active patterns disables the detection of simple duplicates cases.

type Foo = A | B | C

let good = function
    | A -> ()
    | B -> ()
    | B -> () // Squiggly with warning "This rule will never be matched"
    | _ -> ()
    | _ -> () // Squiggly with warning "This rule will never be matched"

let (|IsA|_|) x = if x = A then Some () else None

let bad = function            
    | IsA -> ()
    | B -> ()
    | B -> () // no warning
    | _ -> ()
    | _ -> () // no warning

When active patterns are present checking whether a match is exhaustive clearly becomes difficult/impossible but duplicate cases can still be detected.