> t <- select_topics("x + ", topics)
Warning: In '_pkgdown.yml', topic must be valid R code
x Not 'x + '

> t <- select_topics("y", topics)
Warning: In '_pkgdown.yml', topic must be a known topic name or alias
x Not 'y'

> t <- select_topics("paste(1)", topics)
Warning: In '_pkgdown.yml', topic must be a known selector function
x Not 'paste(1)'

> t <- select_topics("starts_with", topics)
Warning: In '_pkgdown.yml', topic must be a known topic name or alias
x Not 'starts_with'

> t <- select_topics("1", topics)
Warning: In '_pkgdown.yml', topic must be a string or function call
x Not '1'

> t <- select_topics("starts_with('y')", topics, check = TRUE)
Warning: No topics matched in '_pkgdown.yml'. No topics selected.