ok, here is the bug - there may be more than one actually (unless my understanding of what happens here is incorrect).
The code (as is) will not build (a LNK2019 error is issued).
1. If MODULE S on line 1 is changed to something else, say, MODULE S1 (and the rest of the code is changed to accommodate this new name) then the code compiles, builds and runs.
2. If MODULE S stays the same, but the name of the variable S on line 4 is changed to S1, then the code compiles, builds and runs.
3. If the (empty) submodule SI is entirely eliminated (and line 19 is changed accordingly), then the code compiles, builds and runs.
So, there is definitely some strangeness occurring here.
MODULE S
IMPLICIT NONE
INTERFACE
MODULE FUNCTION F(S)
IMPLICIT NONE
CHARACTER(LEN=*),INTENT(IN) :: S
CHARACTER(LEN=:),ALLOCATABLE :: F
END FUNCTION F
END INTERFACE
END MODULE S
SUBMODULE (S) SI
IMPLICIT NONE
INTEGER :: I
END SUBMODULE SI
SUBMODULE (S:SI) SF
IMPLICIT NONE
CONTAINS
MODULE FUNCTION F(S)
IMPLICIT NONE
CHARACTER(LEN=*),INTENT(IN) :: S
CHARACTER(LEN=:),ALLOCATABLE :: F
F = S
END FUNCTION F
END SUBMODULE SF
MODULE M
IMPLICIT NONE
TYPE T
CHARACTER(LEN=:),ALLOCATABLE :: S
CONTAINS
PROCEDURE,PASS :: P
END TYPE T
PRIVATE :: P
INTERFACE
MODULE SUBROUTINE P(V,ST)
IMPLICIT NONE
CHARACTER(LEN=*),INTENT(IN) :: ST
CLASS(T),INTENT(INOUT) :: V
END SUBROUTINE P
END INTERFACE
END MODULE M
SUBMODULE (M) SM
IMPLICIT NONE
CONTAINS
MODULE SUBROUTINE P(V,ST)
USE S
IMPLICIT NONE
CHARACTER(LEN=*),INTENT(IN) :: ST
CLASS(T),INTENT(INOUT) :: V
V%S = F(ST)
END SUBROUTINE P
END SUBMODULE SM
PROGRAM MAIN
USE M
IMPLICIT NONE
TYPE(T) :: TT
CALL TT%P('test')
WRITE(*,*) TT%S
END PROGRAM MAIN
