Illegal syntax in an active pattern produces an internal compiler error

[misterspeedy]

Description: A specific kind of incorrect syntax in an active pattern definition causes an internal compiler error rather than a message specific to the error.

Repro Steps: Add the following code:

open System.Text.RegularExpressions

let (|USZipPlus4Code|_|) s =
    let m = Regex.Match(s, @"^(\d{5})\-(\d{4})$")
    if m.Success then
        USZipPlus4Code(x=m.Groups.[1].Value, 
                       y=m.Groups.[2].Value)
        |> Some
    else
        None

Expected: An error along the lines of:

'Active pattern bindings do not support field names.'

Actual:

'error FS0073: internal error: impossible (Failure)'

Severity: minor.

Version: F# 4.5

Workaround: Use correct active pattern syntax. In this case, omit the x= and y=.

[cartermp]

From F# 4.0:

exit status 1

/home/runner/main.fs(6,9): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

/home/runner/main.fs(6,9): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

/home/runner/main.fs(5,5): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

/home/runner/main.fs(4,5): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

/home/runner/main.fs(4,5): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

/home/runner/main.fs(3,1): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

/home/runner/main.fs(1,1): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

/home/runner/main.fs(1,1): error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

error FS0193: internal error: The index was outside the range of elements in the list.
Parameter name: index

error FS0073: internal error: The index was outside the range of elements in the list.
Parameter name: index (ArgumentException)

error FS0193: internal error: The index was outside the range of elements in the list.
Parameter name: index

Unhandled Exception:
System.ArgumentException: The index was outside the range of elements in the list.
Parameter name: index
  at Microsoft.FSharp.Compiler.ErrorLogger+ErrorLoggerExtensions.ReraiseIfWatsonable (System.Exception exn) <0x40ad6fe0 + 0x00173> in <filename unknown>:0 
  at Microsoft.FSharp.Compiler.ErrorLogger+ErrorLoggerExtensions.ErrorLogger.ErrorRecovery (Microsoft.FSharp.Compiler.ErrorLogger x, System.Exception exn, range m) <0x40ac22e0 + 0x002e7> in <filename unknown>:0 
[ERROR] FATAL UNHANDLED EXCEPTION: System.ArgumentException: The index was outside the range of elements in the list.
Parameter name: index
  at Microsoft.FSharp.Compiler.ErrorLogger+ErrorLoggerExtensions.ReraiseIfWatsonable (System.Exception exn) <0x40ad6fe0 + 0x00173> in <filename unknown>:0 
  at Microsoft.FSharp.Compiler.ErrorLogger+ErrorLoggerExtensions.ErrorLogger.ErrorRecovery (Microsoft.FSharp.Compiler.ErrorLogger x, System.Exception exn, range m) <0x40ac22e0 + 0x002e7> in <filename unknown>:0 

So the error message is different, but it's still an internal error.

[forki]

would this make more sense? it says UnionCase USZipPlus4Code doesn't have field y.

[forki]

I assume we should replace the "UnionCase" with "ActivePattern". But would this be helpful?