[Bug fortran/69368] [6 Regression] spec2006 test case 416.gamess fails with the g++ 6.0 compiler starting with r232508

["dominiq at lps dot ens.fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69368

--- Comment #59 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> We already warn about mismatches sizes at LTO link time

Confirmed

[Book15] f90/bug% gfc -c -O2 pr69368_a.f90 -flto
[Book15] f90/bug% gfc -O2 pr69368_a.o pr69368_b.f90 -flto
pr69368_a.f90:3:0: warning: type of 'blk' does not match original declaration
[-Wlto-type-mismatch]
       COMMON /BLK/ K(1)

pr69368_b.f90:3:0: note: 'blk' was previously declared here
       COMMON /BLK/ K(64)

pr69368_b.f90:3:0: note: code may be misoptimized unless -fno-strict-aliasing
is used

and the executable gives the expected output. IMO the second note is bogus
(pr68717).

If I replace

       COMMON /BLK/ K(1)

with

       COMMON /BLK/ K(2)

the executable gives the expected output also for all the compiling options I
have tried.

AFAICT the code (invalid for any value of I and J)

      FUNCTION FOO(I, J)
      COMMON /BLK/ K(1)
      FOO = K(I) + K(J) + K(2*I) + K(2*J)
      END FUNCTION

is optimized as

      FOO = K(I) + K(I) + K(I) + K(I)

Although I know that a compiler can do whatever it deems suitable with invalid
code, I don't understand the rationale of the above "optimization": it is as
invalid as the original code.

Another thing I don't understand is that the following code is not "optimized"
while as invalid as the one above:

      INTEGER FUNCTION FOO(I, J, K)
      INTEGER K(1)
      FOO = K(I) + K(J) + K(2*I) + K(2*J)
      END FUNCTION

      INTEGER FOO
      INTEGER K(64)
      DO I=1,64
      K(I)=I
      END DO
      IF (FOO(5,12,K).NE.51) CALL ABORT
      END