Reproduction steps

2.13.16
With scalacOptions including

    "-Wtostring-interpolated",

and optionally (to make it an error)

    "-Wconf:cat=w-flag-tostring-interpolated:e",

compile this code:

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

seehttps://scastie.scala-lang.org/MQWIvMVXSrCwU9u9D8NTCA

Output

there is a warning/error 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 3.7.1-RC1
Feature introduced inscala/scala#10776