Has there ever been a full implementation of PL/I?
Well all of the IBM, GE/Honeywell and DEC compilers were certified. Bob Freiburghouse (who was part of the Multics compiler if I understand this right), created a firm in Mass that built a number of commercial compilers for a number of folks, with PL/1 being their prime. In fact when DEC bought the PL/1 front-end from them (which was in PL/1 of course), Culter and team wrote the VAX back-end, they had to cross-compile in Cambridge (I think at MIT) and bring the assembler source back to ZKO in Nashua to assemble and test.
It seems akin to solving the halting problem...
No more than Algol-68 and many modern languages.
Yes, I've used PL/I in my CompSci days, and was told that IBM had trademarked everything from /I to /C :-)
I think that is more like an urban legend and IBM's notorious marketing behavior since Gary Kidall (who was originally a compiler guy) created PL/M for the 8080 and sold it to Intel.
-- Dave, who loved PL/360
Yeah - it might have been Nicklaus Wirth's best language. I still have the Standford manuals, but I can not say I have seen a working compiler since the late 1970s :-)