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As the 2015 business model of the
AdaCore is to
sell the GNAT Ada compiler licenses, the GNAT
imposes
GPL to all binaries that are compiled with the GNAT.
The mechanism is that the binaries are linked with GPL licensed stdlib.
They even say it out explicitly
in their FAQ(
archive).
As of 2015 alternative technologies might be:
mbeddr,
Frama-C with its
Jessie plugin,
ATS that does not impose GPL by having an exception at its stdlib.
May be the 2015-not-production-ready
Irdis might also be of some interest.
Ada 2012 Wikibook.