c: C2x enums wider than int [PR36113]

c: C2x enums wider than int [PR36113]

C2x has two enhancements to enumerations: allowing enumerations whose
values do not all fit in int (similar to an existing extension), and
allowing an underlying type to be specified (as in C++).  This patch
implements the first of those enhancements.

Apart from adjusting diagnostics to reflect this being a standard
feature, there are some semantics differences from the previous
extension:

* The standard feature gives all the values of such an enum the
  enumerated type (while keeping type int if that can represent all
  values of the enumeration), where previously GCC only gave those
  values outside the range of int the enumerated type.  This change
  was previously requested in PR 36113; it seems unlikely that people
  are depending on the detail of the previous extension that some
  enumerators had different types to others depending on whether their
  values could be represented as int, and this patch makes the change
  to types of enumerators unconditionally (if that causes problems in
  practice we could always make it conditional on C2x mode instead).

* The types *while the enumeration is being defined*, for enumerators
  that can't be represented as int, are now based more directly on the
  types of the expressions used, rather than a possibly different type
  with the right precision constructed using c_common_type_for_size.
  Again, this change is made unconditionally.

* Where overflow (or wraparound to 0, in the case of an unsigned type)
  when 1 is implicitly added to determine the value of the next
  enumerator was previously an error, it now results in a wider type
  with the same signedness (as the while-being-defined type of the
  previous enumerator) being chosen, if available.

When a type is chosen in such an overflow case, or when a type is
chosen for the underlying integer type of the enumeration, it's
possible that (unsigned) __int128 is chosen.  Although C2x allows for
such types wider than intmax_t to be considered extended integer
types, we don't have various features required to do so (integer
constant suffixes; sufficient library support would also be needed to
define the associated macros for printf/scanf conversions, and
<stdint.h> typedef names would need to be defined).  Thus, there are
also pedwarns for exceeding the range of intmax_t / uintmax_t, as
while in principle exceeding that range is OK, it's only OK in a
context where the relevant types meet the requirements for extended
integer types, which does not currently apply here.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.  Also
manually checked diagnostics for c2x-enum-3.c with -m32 to confirm the
diagnostics in that { target { ! int128 } } test are as expected.

	PR c/36113

gcc/c-family/
	* c-common.cc (c_common_type_for_size): Add fallback to
	widest_unsigned_literal_type_node or
	widest_integer_literal_type_node for precision that may not
	exactly match the precision of those types.

gcc/c/
	* c-decl.cc (finish_enum): If any enumerators do not fit in int,
	convert all to the type of the enumeration.  pedwarn if no integer
	type fits all enumerators and default to
	widest_integer_literal_type_node in that case.  Otherwise pedwarn
	for type wider than intmax_t.
	(build_enumerator): pedwarn for enumerators outside the range of
	uintmax_t or intmax_t, and otherwise use pedwarn_c11 for
	enumerators outside the range of int.  On overflow, attempt to
	find a wider type that can hold the value of the next enumerator.
	Do not convert value to type determined with
	c_common_type_for_size.

gcc/testsuite/
	* gcc.dg/c11-enum-1.c, gcc.dg/c11-enum-2.c, gcc.dg/c11-enum-3.c,
	gcc.dg/c2x-enum-1.c, gcc.dg/c2x-enum-2.c, gcc.dg/c2x-enum-3.c,
	gcc.dg/c2x-enum-4.c, gcc.dg/c2x-enum-5.c: New tests.
	* gcc.dg/pr30260.c: Explicitly use -std=gnu11.  Update expected
	diagnostics.
	* gcc.dg/torture/pr25183.c: Update expected diagnostics.