With invalid code, down to at least gfortran 4.8 :


$ cat z1.f90
program p
   character(2) :: c = 'a' // [character :: [1]]
end

$ cat z2.f90
program p
   real :: c = 'a' // [character :: [1]]
end

$ cat z3.f90
program p
   character(2) :: c = 'a' // [character :: 1.0]
end

# ...


$ gfortran-9-20180610 -c z1.f90
f951: internal compiler error: in gfc_arith_concat, at fortran/arith.c:985
0x629922 gfc_arith_concat
        ../../gcc/fortran/arith.c:985
0x6294b8 reduce_binary_ca
        ../../gcc/fortran/arith.c:1333
0x629482 reduce_binary_ca
        ../../gcc/fortran/arith.c:1335
0x6296c4 reduce_binary
        ../../gcc/fortran/arith.c:1418
0x629ffb eval_intrinsic
        ../../gcc/fortran/arith.c:1596
0x62a25e eval_intrinsic_f3
        ../../gcc/fortran/arith.c:1733
0x69204a match_level_3
        ../../gcc/fortran/matchexp.c:571
0x692134 match_level_4
        ../../gcc/fortran/matchexp.c:599
0x692134 match_and_operand
        ../../gcc/fortran/matchexp.c:693
0x6922f2 match_or_operand
        ../../gcc/fortran/matchexp.c:722
0x6923e2 match_equiv_operand
        ../../gcc/fortran/matchexp.c:765
0x6924d2 match_level_5
        ../../gcc/fortran/matchexp.c:811
0x691841 gfc_match_expr(gfc_expr**)
        ../../gcc/fortran/matchexp.c:870
0x662912 gfc_match_init_expr(gfc_expr**)
        ../../gcc/fortran/expr.c:2807
0x6504e9 variable_decl
        ../../gcc/fortran/decl.c:2681
0x6504e9 gfc_match_data_decl()
        ../../gcc/fortran/decl.c:5893
0x6ab509 match_word_omp_simd
        ../../gcc/fortran/parse.c:93
0x6aebfe match_word
        ../../gcc/fortran/parse.c:376
0x6aebfe decode_statement
        ../../gcc/fortran/parse.c:376
0x6b0b24 next_free
        ../../gcc/fortran/parse.c:1230