Return a type error when an enum has more than 256 members

docs/080-breaking-changes.rst

@@ -54,3 +54,7 @@ New Restrictions
 * The global functions ``log0``, ``log1``, ``log2``, ``log3`` and ``log4`` have been removed.
 
   These are low-level functions that were largely unused. Their behaviour can be accessed from inline assembly.
+
+* ``enum`` definitions cannot contain more than 256 members.
+
+  This will make it safe to assume that the underlying type in the ABI is always ``uint8``.

docs/abi-spec.rst

@@ -109,14 +109,15 @@ them.
 +-------------------------------+-----------------------------------------------------------------------------+
 |:ref:`contract<contracts>`     |``address``                                                                  |
 +-------------------------------+-----------------------------------------------------------------------------+
-|:ref:`enum<enums>`             |smallest ``uint`` type that is large enough to hold all values               |
-|                               |                                                                             |
-|                               |For example, an ``enum`` of 256 values or less is mapped to ``uint8`` and    |
-|                               |an ``enum`` of 256 values is mapped to ``uint16``.                           |
+|:ref:`enum<enums>`             |``uint8``                                                                    |
 +-------------------------------+-----------------------------------------------------------------------------+
 |:ref:`struct<structs>`         |``tuple``                                                                    |
 +-------------------------------+-----------------------------------------------------------------------------+
 
+.. warning::
+    Before version ``0.8.0`` enums could have more than 256 members and were represented by the
+    smallest integer type just big enough to hold the value of any member.
+
 Design Criteria for the Encoding
 ================================
 

docs/types/value-types.rst

@@ -565,6 +565,7 @@ to and from all integer types but implicit conversion is not allowed.  The expli
 from integer checks at runtime that the value lies inside the range of the enum and causes a
 :ref:`Panic error<assert-and-require>` otherwise.
 Enums require at least one member, and its default value when declared is the first member.
+Enums cannot have more than 256 members.
 
 The data representation is the same as for enums in C: The options are represented by
 subsequent unsigned integer values starting from ``0``.
@@ -586,9 +587,7 @@ subsequent unsigned integer values starting from ``0``.
 
         // Since enum types are not part of the ABI, the signature of "getChoice"
         // will automatically be changed to "getChoice() returns (uint8)"
-        // for all matters external to Solidity. The integer type used is just
-        // large enough to hold all enum values, i.e. if you have more than 256 values,
-        // `uint16` will be used and so on.
+        // for all matters external to Solidity.
         function getChoice() public view returns (ActionChoices) {
             return choice;
         }