As an extension for backwards compatibility with other compilers, GNU Fortran allows the implicit conversion of LOGICAL values to INTEGER values and vice versa. When converting from a LOGICAL to an INTEGER, .FALSE. is interpreted as zero, and .TRUE. is interpreted as one. When converting from INTEGER to LOGICAL, the value zero is interpreted as .FALSE. and any nonzero value is interpreted as .TRUE..
LOGICAL :: l l = 1
INTEGER :: i i = .TRUE.
However, there is no implicit conversion of INTEGER values in if-statements, nor of LOGICAL or INTEGER values in I/O operations.
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https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gfortran/Implicitly-convert-LOGICAL-and-INTEGER-values.html