[libgfortran] Replace implicit conversions between enums in io/transfer.c by explicit casts.

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

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Hi,
I am working on patch for PR78736
(https://gcc.gnu.org/ml/gcc-patches/2017-09/msg00011.html),
which adds a new warning -Wenum-conversion to C front-end to warn for
implicit conversion between different enums.
The warning in that patch triggered on io/transfer.c for following
implicit conversions:
i) Implicit conversion from unit_mode to file_mode
ii) Implicit conversion from unit_sign_s to unit_sign.

I was wondering if the warning for above implicit conversions would be
correct since unit_mode
and file_mode are different enums and similarly unit_sign_s and
unit_sign are different enums ?
Or are these warnings false positives ?

The attached patch makes the conversion explicit to silence the warnings.
Bootstrap+tested on x86_64-unknown-linux-gnu.
Does the patch look OK ?

Thanks,
Prathamesh

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2017-09-12  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>

libgfortran/
	* io/transfer.c (current_mode): Cast FORM_UNSPECIFIED to file_mode.
	(formatted_transfer_scalar_read): Cast SIGN_S, SIGN_SS, SIGN_SP to
	unit_sign.
	(formatted_transfer_scalar_write): Likewise.

diff --git a/libgfortran/io/transfer.c b/libgfortran/io/transfer.c
index 529637061b1..3307d213bb7 100644
--- a/libgfortran/io/transfer.c
+++ b/libgfortran/io/transfer.c
@@ -196,7 +196,7 @@ current_mode (st_parameter_dt *dtp)
 {
   file_mode m;
 
-  m = FORM_UNSPECIFIED;
+  m = (file_mode) FORM_UNSPECIFIED;
 
   if (dtp->u.p.current_unit->flags.access == ACCESS_DIRECT)
     {
@@ -1671,17 +1671,17 @@ formatted_transfer_scalar_read (st_parameter_dt *dtp, bt type, void *p, int kind
 
 	case FMT_S:
 	  consume_data_flag = 0;
-	  dtp->u.p.sign_status = SIGN_S;
+	  dtp->u.p.sign_status = (unit_sign) SIGN_S;
 	  break;
 
 	case FMT_SS:
 	  consume_data_flag = 0;
-	  dtp->u.p.sign_status = SIGN_SS;
+	  dtp->u.p.sign_status = (unit_sign) SIGN_SS;
 	  break;
 
 	case FMT_SP:
 	  consume_data_flag = 0;
-	  dtp->u.p.sign_status = SIGN_SP;
+	  dtp->u.p.sign_status = (unit_sign) SIGN_SP;
 	  break;
 
 	case FMT_BN:
@@ -2130,17 +2130,17 @@ formatted_transfer_scalar_write (st_parameter_dt *dtp, bt type, void *p, int kin
 
 	case FMT_S:
 	  consume_data_flag = 0;
-	  dtp->u.p.sign_status = SIGN_S;
+	  dtp->u.p.sign_status = (unit_sign) SIGN_S;
 	  break;
 
 	case FMT_SS:
 	  consume_data_flag = 0;
-	  dtp->u.p.sign_status = SIGN_SS;
+	  dtp->u.p.sign_status = (unit_sign) SIGN_SS;
 	  break;
 
 	case FMT_SP:
 	  consume_data_flag = 0;
-	  dtp->u.p.sign_status = SIGN_SP;
+	  dtp->u.p.sign_status = (unit_sign) SIGN_SP;
 	  break;
 
 	case FMT_BN: