Compiler version

3.7.1-RC1

Minimized code

With scalacOptions including

    "-Wtostring-interpolated",

compile this code:

val shouldCaps = true
val greeting = s"Hello ${if (shouldCaps) "WORLD" else "world"}"
println(greeting)

see https://scastie.scala-lang.org/Y9lVvpVjRUeL7P08bs8Heg

Output

there is a warning raised on line 2:
"interpolation uses toString"

Expectation

compiles without warnings, as the type of the expression is a String

Additional info

Also fails on 2.13.16
Feature introduced in #20578