[no-release-notes] tighten up enum behavior (#2755)

* [no-release-notes] tighten up enum behavior

* delete comment

Author: max-hoffman

enginetest/queries/script_queries.go

@@ -132,7 +132,7 @@ var ScriptTests = []ScriptTest{
 			},
 			{
 				Query:       "alter table xy modify y enum('a')",
-				ExpectedErr: sql.ErrEnumTypeTruncated,
+				ExpectedErr: types.ErrConvertingToEnum,
 			},
 		},
 	},
@@ -7511,7 +7511,7 @@ where
 			},
 			{
 				Query:       "alter table t modify column e enum('abc');",
-				ExpectedErr: sql.ErrEnumTypeTruncated,
+				ExpectedErr: types.ErrConvertingToEnum,
 			},
 		},
 	},

sql/analyzer/validate_create_table.go

@@ -470,15 +470,6 @@ func validateModifyColumn(ctx *sql.Context, initialSch sql.Schema, schema sql.Sc
 		return nil, err
 	}
 
-	if e1, ok := newCol.Type.(sql.EnumType); ok {
-		oldCol := initialSch[initialSch.IndexOfColName(oldColName)]
-		if e2, ok := oldCol.Type.(sql.EnumType); ok {
-			if len(e1.Values()) < len(e2.Values()) {
-				return nil, sql.ErrEnumTypeTruncated.New()
-			}
-		}
-	}
-
 	// TODO: When a column is being modified, we should ideally check that any existing table check constraints
 	//       are still valid (e.g. if the column type changed) and throw an error if they are invalidated.
 	//       That would be consistent with MySQL behavior.