Single tilde ~ in expressions compiles, but shouldn't #16135
Description
Using the .NET 7 SDK and above, a single tilde ~ can be used in nearly any expression context and will compile (and can even make otherwise nonsensical syntax compile), but such an expression will fail or behave unintuitively at runtime.
Repro steps
- Put a single tilde
~in nearly any expression context: prefix, infix, postfix, circumfix... - The code compiles, but either:
a. Fails at runtime with aSystem.NullReferenceException.
b. Fails at runtime withFatal error. Internal CLR error. (0x80131506), which crashes the runtime altogether. Exciting!
b. Generates a default value (0,null, etc.)—neat shorthand forUnchecked.defaultof<_>!
// These all compile...
let f x = ~x
let g x y = x ~ y
let h x y = (x ~) y
let p x = x~
let q x = x ~
let x : int = ~
let _ : int = ~ ~ ~ ~
let _ = ~1
let _ = 1~
let _ = ~1~
let _ = (~) 1
let _ = (~) 1 1 1 1
let _ = ~ 1 1 1 1
let _ = 1 1 1 1 ~
let _ = 1 1 ~ 1 1
let _ = ~ DDD ~
let _ = ~ _._. ~
let _ = ~ ``(👉ﾟヮﾟ)👉`` ~
let _ = ~ <int> ~
let () = ignore ~
let zero = ~ + ~Expected behavior
I don't believe that any code using a single ~ should compile (prefix operators involving ~ always have at least one other character).
Actual behavior
The code compiles—including outlandish code that would otherwise never compile—and strange things happen at runtime.
Known workarounds
Don't accidentally type ~ anywhere.
Related information
This behavior happens for me when compiling with the .NET 7 and 8 SDKs, but not with .NET 6, so it seems likely that it was introduced in 7.