It's possible to use a type which implements fmt.Formatter without
importing fmt directly, if the type is imported from another package
such as math/big.
On top of that, it's possible to use printf-like functions without
importing fmt directly, such as using testing.T.Logf.
These two scenarios combined can lead to the printf check not finding
the fmt.Formatter type, since it's not a direct dependency of the root
package.
fmt must still be in the import graph somewhere, so we could search for
it via types.Package.Imports. However, at that point it's simpler to
just look for the Format method manually via go/types.
Fixes#30399.
Change-Id: Id78454bb6a51b3c5e1bcb1984a7fbfb4a29a5be0
Reviewed-on: https://go-review.googlesource.com/c/163817
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
589c23e65e