Fix binary encoding of tag types

Author: rossberg

document/core/binary/modules.rst

@@ -181,7 +181,7 @@ It decodes into a vector of :ref:`imports <syntax-import>` that represent the |M
      \hex{01}~~\X{tt}{:}\Btabletype &\Rightarrow& \IDTABLE~\X{tt} \\ &&|&
      \hex{02}~~\X{mt}{:}\Bmemtype &\Rightarrow& \IDMEM~\X{mt} \\ &&|&
      \hex{03}~~\X{gt}{:}\Bglobaltype &\Rightarrow& \IDGLOBAL~\X{gt} \\ &&|&
-     \hex{04}~~\X{tt}{:}\Btagtype &\Rightarrow& \IDTAG~\X{tt} \\
+     \hex{04}~~x{:}\Btagtype &\Rightarrow& \IDTAG~x \\
    \end{array}
 
 

document/core/binary/types.rst

@@ -321,17 +321,14 @@ Global Types
 Tag Types
 ~~~~~~~~~
 
-:ref:`Tag types <syntax-tagtype>` are encoded by their function type.
+:ref:`Tag types <syntax-tagtype>` are encoded by a :ref:`type index <syntax-typeidx>` denoting a :ref:`function type <syntax-functype>`.
 
 .. math::
    \begin{array}{llclll}
    \production{tag type} & \Btagtype &::=&
-     \hex{00}~~ft{:}\Bfunctype &\Rightarrow& ft \\
+     \hex{00}~~x{:}\Btypeidx &\Rightarrow& x \\
    \end{array}
 
-The |Bfunctype| of a tag is used to characterise exceptions.
-The :math:`\hex{00}` bit signifies an exception and is currently the only allowed value.
-
 .. note::
    In future versions of WebAssembly,
    the preceding zero byte may encode additional flags.

document/core/syntax/modules.rst

@@ -445,7 +445,7 @@ The |MIMPORTS| component of a module defines a set of *imports* that are require
      \IDTABLE~\tabletype \\&&|&
      \IDMEM~\memtype \\&&|&
      \IDGLOBAL~\globaltype \\&&|&
-     \IDTAG~\tagtype \\
+     \IDTAG~\typeidx \\
    \end{array}
 
 Each import is labeled by a two-level :ref:`name <syntax-name>` space, consisting of a |IMODULE| name and a |INAME| for an entity within that module.