I have noticed some very strange behaviour related to line directives, which in some cases lead to compiler errors.

Repro steps
This exact piece of code shows the behaviour:

open System

# 10
FSharp.Core.ExtraTopLevelOperators.printfn "%s" "ab"

# 15



// end

Expected behavior

The code should compile fine and print ab when executed.

Actual behavior

FS0193:10: internal error: startIndex cannot be larger than length of string. (Parameter 'startIndex')

Also, very strangely, if I remove FSharp.Core.ExtraTopLevelOperators., I get another strange error:

FS0001:10: This expression was expected to have type
''a -> 'b'
but here has type
'unit'

And, if I instead replace it with do , I get:

FS0193:10: internal error: Index and length must refer to a location within the string. (Parameter 'length')

I don't think the particular sequences I replace it with matter, it seems to be more a line position thing.

Known workarounds

I did not manage to find a workaround that preserves the line information. I would certainly appreciate a solution that works for older compilers.

Related information

Tested on .NET SDK 9.0.201.
Examples:

• https://sharplab.io/#v2:DYLgZgzgPg9gDgUwHYAIDKBPCAXBBbAWAChiBiFARgAZiAxNACwEMAnOAOgGEYWF2BRAB7YWTACrwAMggBuCYAHlEo7Dwjs4LAJZJsYVACIApBAMoDTAEYHiZSgFZbJZwHoXKZABMgA=
• https://sharplab.io/#v2:DYLgZgzgPg9gDgUwHYAIDKBPCAXBBbAWAChiBiFARgAZi4AnASyWzFQCIBSCNlNgQwBGbYmUoBWESSnEA9DJTIAJkA==
• https://sharplab.io/#v2:DYLgZgzgPg9gDgUwHYAIDKBPCAXBBbAWAChiBiFARgAZi4AnASyWzFQCIBSCNlNgQwBGbYmUoBWESSmTJAelkpkAEyA=