Possible verbatim string literal regression

[SteveSandersonMS]

Repro code in any .razor file:

@for(var i = 0; i < 3; i++)
{
    var filename = @$"ticket-{i}.png";
    <h1>@filename</h1>
}

This is legal code as far as I know and used to work without errors or warnings. However in Version 17.11.0 Preview 2.0 this shows an error:

Severity	Code	Description	Project	File	Line	Suppression State	Details
Error (active)	RZ1009	The "@" character must be followed by a ":", "(", or a C# identifier.  If you intended to switch to markup, use an HTML start tag, for example: ...

On my machine this appears in the VS Error UI but does not occur during command-line compilation (probably a slightly older SDK).

[jjonescz]

Looks like this is caused by #10232. cc @333fred

[333fred]

This will be solved by the lexer changes.

This is legal code as far as I know

Not really. Interpolated strings of any kind have never been actually supported by the razor parser. I agree that they should be, and the lexer changes will actually support them, but any amount of interpolated strings working has simply been a happy accident, not intentional implementation.

[333fred]

Oh, I almost forgot to mention: a workaround is just switching from @$ to $@ here.

[AMUCOMO]

I also noticed this bug, but only after updating my VS Version to 17.11.2 (current latest), seemed to work before in 17.10.*
in any .razor file:
@{ string foo = @$"""a"; }

[ssg]

any amount of interpolated strings working has simply been a happy accident, not intentional implementation.

Accident or not, it was still a behavior relied by applications. This impacts existing applications in the wild as it blocks their migration path to newer .NET versions and might cause unnoticed regressions of varying severity especially when apps use runtime compilation. The workaround isn't obvious either.

Solely updating to a newer .NET version ideally should never break a running app. But, in case it does, the breaking changes should also be documented clearly.

Was this breaking change known when this patch was approved?

What's the course of action to reduce the impact?

"We do not break the userspace" --Linus Torvalds.