A manifest-scoped compile only sees the libraries the project declares, so
it dies on the first undeclared transitive dependency — a header the user
had no reason to name. The scan-all retry exists to rescue exactly that,
and when it succeeds nothing is lost. When it fails as well, we returned
the SCOPED failure, which describes a library only our own scoping made
missing.
Today a hand-written SSD1306 sketch was reported as
Adafruit_GFX.h:12:10: fatal error: Adafruit_I2CDevice.h: No such file
while the retry had already resolved BusIO and reached the real bug, an
`#include <Adafruit_SSD1306>` with no .h. The user went looking for a
missing library that was installed all along.
The retry runs in the superset environment, so its error is the sketch's
own in both compilers. Prefer it whenever it carries text; the scoped
result still stands in when it does not.
David Montero