FormattableString interpolated string mishandles escaped brackets

[chkn]

When creating a FormattableString from an interpolated string, escaped brackets ({{ and }}) are not correctly handled, resulting in an invalid format string. See the comparison with C# below:

Repro steps

$ csi
Microsoft (R) Visual C# Interactive Compiler version 3.9.0-6.21124.20 ()
Copyright (C) Microsoft Corporation. All rights reserved.

Type "#help" for more information.
> FormattableString str = $"{{Hello}} world";
> str.Format
"{{Hello}} world"

$ dotnet fsi

Microsoft (R) F# Interactive version 11.4.2.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> let str = $"{{Hello}} world" : System.FormattableString;;
val str : System.FormattableString =
  <ToString exception: Input string was not in a correct format.>

> str.Format;;
val it : string = "{Hello} world"

Expected behavior

The FormattableString created by F# should match the one created by C# for the same interpolated string.

Actual behavior

Escaped brackets are unescaped into the format string, causing it to be invalid.

Known workarounds

Modify the interpolated string or call FormattableStringFactory.Create directly.

Related information

Provide any related information (optional):

• macOS 10.15.7
• .NET Runtime kind (.NET Core, .NET Framework, Mono): mono 6.12.0.140 (2020-02/51d876a041e, .NET Core 5.0.301
• Editing Tools (e.g. Visual Studio Version, Visual Studio)

[dsyme]

@vzarytovskii You might like to take a look at this. We should fix this to follow the C# spec

[dsyme]

@vzarytovskii I could fix this but we can use this as a first Up for Grabs issue I think?

[vzarytovskii]

@vzarytovskii I could fix this but we can use this as a first Up for Grabs issue I think?

Yeah, let's have it as up for grabs, I will share some of the issues in slack and discord, maybe someone will be interested in fixing them.