2020-04-06  Fritz Reese  <foreese@gcc.gnu.org>

This patch reorganizes I/O checking code. Checks which were done in the
matching phase which do not affect the match result are moved to the
resolution phase. Checks which were duplicated in both the matching phase
and resolution phase have been reduced to one check in the resolution phase.

Another section of code which used a global async_io_dt flag to check for
and assign the asynchronous attribute to variables used in asynchronous I/O
has been simplified.

Furthermore, this patch improves error reporting and expands test coverage
of I/O tags:

 - "TAG must be an initialization expression" reported by io.c
   (check_io_constraints) is replaced with an error from expr.c
   (gfc_reduce_init_expr) indicating _why_ the expression is not a valid
   initialization expression.

 - Several distinct error messages regarding the check for scalar
   + character + default kind have been unified to one message reported by
   resolve_tag or check_*_constraints.

gcc/fortran/ChangeLog:

2020-04-09  Fritz Reese  <foreese@gcc.gnu.org>

	PR fortran/87923
	* gfortran.h (gfc_resolve_open, gfc_resolve_close): Add
	locus parameter.
	(gfc_resolve_dt): Add code parameter.
	* io.c (async_io_dt, check_char_variable, is_char_type): Removed.
	(resolve_tag_format): Add locus to error message regarding
	zero-sized array in FORMAT tag.
	(check_open_constraints, check_close_constraints): New functions
	called at resolution time.
	(gfc_match_open, gfc_match_close, match_io): Move checks which don't
	affect the match result to new functions check_open_constraints,
	check_close_constraints, check_io_constraints.
	(gfc_resolve_open, gfc_resolve_close): Call new functions
	check_open_constraints, check_close_constraints after all tags have
	been independently resolved.  Remove duplicate constraints which are
	already verified by resolve_tag. Explicitly pass locus to all error
	reports.
	(compare_to_allowed_values): Add locus parameter and provide
	explicit locus all error reports.
	(match_open_element, match_close_element, match_file_element,
	match_dt_element, match_inquire_element): Remove redundant special
	cases for ASYNCHRONOUS and IOMSG tags.
	(gfc_resolve_dt): Remove redundant special case for format
	expression.  Call check_io_constraints, forwarding an I/O list as
	the io_code parameter if present.
	(check_io_constraints): Change return type to bool. Pass explicit
	locus to error reports. Move generic checks after tag-specific
	checks, since errors are no longer buffered.  Move simplification of
	format string to match_io.  Remove redundant checks which are
	verified by resolve_tag.  Remove usage of async_io_dt flag and
	explicitly mark symbols used in asynchronous I/O with the
	asynchronous attribute.
	* resolve.c (resolve_transfer, resolve_fl_namelist): Remove checks
	for async_io_dt flag. This is now done in io.c
	(check_io_constraints).
	(gfc_resolve_code): Pass code locus to gfc_resolve_open,
	gfc_resolve_close, gfc_resolve_dt.

gcc/testsuite/ChangeLog:

2020-04-09  Fritz Reese  <foreese@gcc.gnu.org>

	PR fortran/87923
	* gfortran.dg/f2003_io_8.f03: Fix expected error messages.
	* gfortran.dg/io_constraints_8.f90: Likewise.
	* gfortran.dg/iomsg_2.f90: Likewise.
	* gfortran.dg/pr66725.f90: Likewise.
	* gfortran.dg/pr88205.f90: Likewise.
	* gfortran.dg/write_check4.f90: Likewise.
	* gfortran.dg/asynchronous_5.f03: New test.
	* gfortran.dg/io_constraints_15.f90: Likewise.
	* gfortran.dg/io_constraints_16.f90: Likewise.
	* gfortran.dg/io_constraints_17.f90: Likewise.
	* gfortran.dg/io_constraints_18.f90: Likewise.
	* gfortran.dg/io_tags_1.f90: Likewise.
	* gfortran.dg/io_tags_10.f90: Likewise.
	* gfortran.dg/io_tags_2.f90: Likewise.
	* gfortran.dg/io_tags_3.f90: Likewise.
	* gfortran.dg/io_tags_4.f90: Likewise.
	* gfortran.dg/io_tags_5.f90: Likewise.
	* gfortran.dg/io_tags_6.f90: Likewise.
	* gfortran.dg/io_tags_7.f90: Likewise.
	* gfortran.dg/io_tags_8.f90: Likewise.
	* gfortran.dg/io_tags_9.f90: Likewise.
	* gfortran.dg/write_check5.f90: Likewise.