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.

1 files changed, 98 insertions, 39 deletions

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index a7571cc..bcb4d7b 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -9385,6 +9385,10 @@ finish_enum (tree enumtype, tree values, tree attributes)
   precision = MAX (tree_int_cst_min_precision (minnode, sign),
 		   tree_int_cst_min_precision (maxnode, sign));
 
+  bool wider_than_int =
+    (tree_int_cst_lt (minnode, TYPE_MIN_VALUE (integer_type_node))
+     || tree_int_cst_lt (TYPE_MAX_VALUE (integer_type_node), maxnode));
+
   /* If the precision of the type was specified with an attribute and it
      was too small, give an error.  Otherwise, use it.  */
   if (TYPE_PRECISION (enumtype) && lookup_attribute ("mode", attributes))
@@ -9407,9 +9411,20 @@ finish_enum (tree enumtype, tree values, tree attributes)
       tem = c_common_type_for_size (precision, sign == UNSIGNED ? 1 : 0);
       if (tem == NULL)
 	{
-	  warning (0, "enumeration values exceed range of largest integer");
-	  tem = long_long_integer_type_node;
-	}
+	  /* This should only occur when both signed and unsigned
+	     values of maximum precision occur among the
+	     enumerators.  */
+	  pedwarn (input_location, 0,
+		   "enumeration values exceed range of largest integer");
+	  tem = widest_integer_literal_type_node;
+	}
+      else if (precision > TYPE_PRECISION (intmax_type_node)
+	       && !tree_int_cst_lt (minnode, TYPE_MIN_VALUE (intmax_type_node))
+	       && !tree_int_cst_lt (TYPE_MAX_VALUE (uintmax_type_node),
+				    maxnode))
+	pedwarn (input_location, OPT_Wpedantic,
+		 "enumeration values exceed range of %qs",
+		 sign == UNSIGNED ? "uintmax_t" : "intmax_t");
     }
   else
     tem = sign == UNSIGNED ? unsigned_type_node : integer_type_node;
@@ -9439,17 +9454,17 @@ finish_enum (tree enumtype, tree values, tree attributes)
 
 	  TREE_TYPE (enu) = enumtype;
 
-	  /* The ISO C Standard mandates enumerators to have type int,
-	     even though the underlying type of an enum type is
-	     unspecified.  However, GCC allows enumerators of any
-	     integer type as an extensions.  build_enumerator()
-	     converts any enumerators that fit in an int to type int,
-	     to avoid promotions to unsigned types when comparing
-	     integers with enumerators that fit in the int range.
-	     When -pedantic is given, build_enumerator() would have
-	     already warned about those that don't fit. Here we
-	     convert the rest to the enumerator type. */
-	  if (TREE_TYPE (ini) != integer_type_node)
+	  /* Before C2X, the ISO C Standard mandates enumerators to
+	     have type int, even though the underlying type of an enum
+	     type is unspecified.  However, C2X allows enumerators of
+	     any integer type, and if an enumeration has any
+	     enumerators wider than int, all enumerators have the
+	     enumerated type after it is parsed.  Any enumerators that
+	     fit in int are given type int in build_enumerator (which
+	     is the correct type while the enumeration is being
+	     parsed), so no conversions are needed here if all
+	     enumerators fit in int.  */
+	  if (wider_than_int)
 	    ini = convert (enumtype, ini);
 
 	  DECL_INITIAL (enu) = ini;
@@ -9517,7 +9532,7 @@ tree
 build_enumerator (location_t decl_loc, location_t loc,
 		  struct c_enum_contents *the_enum, tree name, tree value)
 {
-  tree decl, type;
+  tree decl;
 
   /* Validate and default VALUE.  */
 
@@ -9568,21 +9583,45 @@ build_enumerator (location_t decl_loc, location_t loc,
     }
   /* Even though the underlying type of an enum is unspecified, the
      type of enumeration constants is explicitly defined as int
-     (6.4.4.3/2 in the C99 Standard).  GCC allows any integer type as
-     an extension.  */
-  else if (!int_fits_type_p (value, integer_type_node))
-    pedwarn (loc, OPT_Wpedantic,
-	     "ISO C restricts enumerator values to range of %<int%>");
-
-  /* The ISO C Standard mandates enumerators to have type int, even
-     though the underlying type of an enum type is unspecified.
-     However, GCC allows enumerators of any integer type as an
-     extensions.  Here we convert any enumerators that fit in an int
-     to type int, to avoid promotions to unsigned types when comparing
-     integers with enumerators that fit in the int range.  When
-     -pedantic is given, we would have already warned about those that
-     don't fit. We have to do this here rather than in finish_enum
-     because this value may be used to define more enumerators.  */
+     (6.4.4.3/2 in the C99 Standard).  C2X allows any integer type, and
+     GCC allows such types for older standards as an extension.  */
+  bool warned_range = false;
+  if (!int_fits_type_p (value,
+			(TYPE_UNSIGNED (TREE_TYPE (value))
+			 ? uintmax_type_node
+			 : intmax_type_node)))
+    /* GCC does not consider its types larger than intmax_t to be
+       extended integer types (although C2X would permit such types to
+       be considered extended integer types if all the features
+       required by <stdint.h> and <inttypes.h> macros, such as support
+       for integer constants and I/O, were present), so diagnose if
+       such a wider type is used.  (If the wider type arose from a
+       constant of such a type, that will also have been diagnosed,
+       but this is the only diagnostic in the case where it arises
+       from choosing a wider type automatically when adding 1
+       overflows.)  */
+    warned_range = pedwarn (loc, OPT_Wpedantic,
+			    "enumerator value outside the range of %qs",
+			    (TYPE_UNSIGNED (TREE_TYPE (value))
+			     ? "uintmax_t"
+			     : "intmax_t"));
+  if (!warned_range && !int_fits_type_p (value, integer_type_node))
+    pedwarn_c11 (loc, OPT_Wpedantic,
+		 "ISO C restricts enumerator values to range of %<int%> "
+		 "before C2X");
+
+  /* The ISO C Standard mandates enumerators to have type int before
+     C2X, even though the underlying type of an enum type is
+     unspecified.  C2X allows enumerators of any integer type.  During
+     the parsing of the enumeration, C2X specifies that constants
+     representable in int have type int, constants not representable
+     in int have the type of the given expression if any, and
+     constants not representable in int and derived by adding 1 to the
+     previous constant have the type of that constant unless the
+     addition would overflow or wraparound, in which case a wider type
+     of the same signedness is chosen automatically; after the
+     enumeration is parsed, all the constants have the type of the
+     enumeration if any do not fit in int.  */
   if (int_fits_type_p (value, integer_type_node))
     value = convert (integer_type_node, value);
 
@@ -9591,18 +9630,38 @@ build_enumerator (location_t decl_loc, location_t loc,
     = build_binary_op (EXPR_LOC_OR_LOC (value, input_location),
 		       PLUS_EXPR, value, integer_one_node, false);
   the_enum->enum_overflow = tree_int_cst_lt (the_enum->enum_next_value, value);
+  if (the_enum->enum_overflow)
+    {
+      /* Choose a wider type with the same signedness if
+	 available.  */
+      int prec = TYPE_PRECISION (TREE_TYPE (value)) + 1;
+      bool unsignedp = TYPE_UNSIGNED (TREE_TYPE (value));
+      tree new_type = (unsignedp
+		       ? long_unsigned_type_node
+		       : long_integer_type_node);
+      if (prec > TYPE_PRECISION (new_type))
+	new_type = (unsignedp
+		    ? long_long_unsigned_type_node
+		    : long_long_integer_type_node);
+      if (prec > TYPE_PRECISION (new_type))
+	new_type = (unsignedp
+		    ? widest_unsigned_literal_type_node
+		    : widest_integer_literal_type_node);
+      if (prec <= TYPE_PRECISION (new_type))
+	{
+	  the_enum->enum_overflow = false;
+	  the_enum->enum_next_value
+	    = build_binary_op (EXPR_LOC_OR_LOC (value, input_location),
+			       PLUS_EXPR, convert (new_type, value),
+			       integer_one_node, false);
+	  gcc_assert (!tree_int_cst_lt (the_enum->enum_next_value, value));
+	}
+    }
 
   /* Now create a declaration for the enum value name.  */
 
-  type = TREE_TYPE (value);
-  type = c_common_type_for_size (MAX (TYPE_PRECISION (type),
-				      TYPE_PRECISION (integer_type_node)),
-				 (TYPE_PRECISION (type)
-				  >= TYPE_PRECISION (integer_type_node)
-				  && TYPE_UNSIGNED (type)));
-
-  decl = build_decl (decl_loc, CONST_DECL, name, type);
-  DECL_INITIAL (decl) = convert (type, value);
+  decl = build_decl (decl_loc, CONST_DECL, name, TREE_TYPE (value));
+  DECL_INITIAL (decl) = value;
   pushdecl (decl);
 
   return tree_cons (decl, value, NULL_TREE);