warnings for invalid #nowarn arguments

[Martin521]

Working on "scoped nowarn", I came across the following items.

1. With SDK 8.0, a directive #nowarn "FS0020" creates a warning "Invalid warning number 'FS0020'".
   With the current compiler and "--langversion:8.0", there is no warning. (It got lost in Allow ParsedHashDirectives to take non string arguments #17206.)
   The warning should be brought back for "--langversion:8.0".

2. I also propose to introduces (under the F# 9.0 language flag ParsedHashDirectiveArgumentNonQuotes) warnings for invalid nowarn arguments like in #nowarn "xyz". Specifically, for arguments that do not match the format (FS)?\d+ (for directives) or [A-Z]*\d+ (for compiler options).

3. Independent of the language version, the range of the argument (rather than the whole directive) should be used for the warning, making it more precise.

Draft PR exists (#17871)

[KevinRansom]

@Martin521

I believe the compiler does the right thing:

c:\temp>type C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs
//#nowarn "FS0020"

let something =
  2+2               // => FS0020: This expression should have type 'unit'
  "hello"

produces this output:

c:\temp>fsc C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs --langversion:9.0
Microsoft (R) F# Compiler version 12.9.100.0 for F# 9.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs(4,3): warning FS0020: The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.

and

c:\temp>fsc C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs --langversion:8.0
Microsoft (R) F# Compiler version 12.9.100.0 for F# 9.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs(4,3): warning FS0020: The result of this expression has type 'int' and is implicitly ignored. Consider using 'ignore' to discard this value explicitly, e.g. 'expr |> ignore', or 'let' to bind the result to a name, e.g. 'let result = expr'.

and this file:
c:\temp>type C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs
#nowarn "FS0020"

let something =
2+2 // => FS0020: This expression should have type 'unit'
"hello"

produces

c:\temp>fsc C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs --langversion:8.0
Microsoft (R) F# Compiler version 12.9.100.0 for F# 9.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

c:\temp>fsc C:\Users\kevinr\source\repos\ConsoleApp6\ConsoleApp6\Program.fs --langversion:9.0
Microsoft (R) F# Compiler version 12.9.100.0 for F# 9.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

c:\temp>

I note that the IDE does not seem to produce the correct out: