Can't prefix format specifier with '%' in interpolated strings

[b-hodge]

Interpolated strings in F# support the format specifiers described in the F# Plain Text Formatting documentation. This includes producing a literal % character by prefixing it with another %.

This no longer works when the sequence of characters is immediately followed by a format specifier.

Repro steps

Microsoft (R) F# Interactive version 12.9.100.0 for F# 9.0

> let x = "abc";;
val x: string = "abc"

> $"%%%s{x}%%";;
val it: string = "abc%"

Note that this works as expected in F# 8.0:

Microsoft (R) F# Interactive version 12.8.400.0 for F# 8.0

> let x = "abc";;
val x: string = "abc"

> $"%%%s{x}%%";;
val it: string = "%abc%"

Related information

Tested with sdk-9.0.101-macos-arm64.

[brianrourkeboll]

Hmm. I think there might have been a bug in #16556, specifically here:

fsharp/src/Compiler/Checking/Expressions/CheckExpressions.fs

Lines 171 to 181 in f2fe832

	// Removes trailing "%s" unless it was escaped by another '%' (checks for odd sequence of '%' before final "%s")
	let (|WithTrailingStringSpecifierRemoved|) (s: string) =
	if s.EndsWith "%s" then
	let i = s.AsSpan(0, s.Length - 2).LastIndexOfAnyExcept '%'
	let diff = s.Length - 2 - i
	if diff &&& 1 <> 0 then
	s[..i]
	else
	s
	else
	s

Should this be something like

-s[..i]
+s[..s.Length - 3]

?

[abonie]

Actually I've just built #16556 and it doesn't repro there, so must be something else

Edit: nevermind, I forgot it was behind langversion:preview at that point, so that's why it didn't repro. It is most likely that commit

[edgarfgp]

This is a regression right ?