Inconsistent parsing of symbols wrapped in backticks

[nrinaudo]

Compiler version

3.1.2

Minimized code

def `=`: String = "="
def `+`: String = "+"

val withEq: String   = `=` + "3"
val withPlus: String = `+` + "3"

Output

Compile error on the last line:

; expected but string constant found

See https://scastie.scala-lang.org/WCnbXR7RRzOuc8zYkrlxUQ for a "live" reproduction

Expectation

Either a compile error for both withEq and withPlus, or none, but this seems inconsistent.

[prolativ]

Not sure of the source of the discrepancy but I'm getting slightly different errors with the same compiler version when trying to compile this locally

[error] Bug.scala:5:30: end of toplevel definition expected but string literal found
[error] val withPlus: String = `+` + "3"
[error]                              ^
[error] Bug.scala:5:24: value unary_+ is not a member of String
[error] val withPlus: String = `+` + "3"
[error]                        ^^^^^   

The point is that + is not really treated as a keyword by the parser while = is. This means that + and + are equivalent so you could write e.g.

def + : String = "+"

instead of

def `+`: String = "+"

while

def = : String = "="

would be syntactically incorrect.
Going further all of

+ + "3"
`+` + "3"
+ `+` "3"
`+` `+` "3"

mean the same and the first + is being interpreted as a prefix operator here.
This seems consistent with the behaviour in Scala 2.13 and I'm not sure if this could be changed. @odersky WDYT?

[odersky]

I can't reproduce this on latest main. Everything compiles. I remember we had a PR recently that changed the status of backticked unary operators. Maybe that fixed it.

[prolativ]

Indeed seems to be fixed by #15198

[som-snytt]

was

PackageDef(Ident(<empty>), List(DefDef(f, List(), TypeTree(), PrefixOp(Ident(+), Ident(+)))))

now

PackageDef(Ident(<empty>), List(DefDef(f, List(), TypeTree(), InfixOp(Ident(+), Ident(+), Literal("*")))))

Noting that -Vprint:parser -Ystop-after:parser does not work as one might expect. (Result is empty.)