Editorial: Introduce IdentifierStartChar + IdentifierPartChar (tc39#2392)

Extract `IdentifierStartChar` from `IdentifierStart` and `RegExpIdentifierStart`.
Extract `IdentifierPartChar` from `IdentifierPart` and `RegExpIdentifierPart`.

This has 3 benefits:

- We eliminate some repetition between the productions for
  Identifiers and RegExpIdentifiers.

- We can simplify 4 Early Error rules involving escape sequences,
  because the constraint can now be expressed in terms of a single nonterminal,
  rather than a nonterminal plus some terminals.

- We can eliminate the Early Error rule for `RegularExpressionFlags`
  by instead expressing its constraint in the grammar:
  in the production for `RegularExpressionFlags`,
  replace `IdentifierPart` with `IdentifierPartChar`.

(As a consequence of the last point, this commit undefines the following id:
sec-literals-regular-expression-literals-static-semantics-early-errors
There didn't seem to be a sensible place to relocate it as an oldid.)

Author: jmdyck

spec.html

@@ -16175,15 +16175,21 @@ <h2>Syntax</h2>
         IdentifierName IdentifierPart
 
       IdentifierStart ::
+        IdentifierStartChar
+        `\` UnicodeEscapeSequence
+
+      IdentifierPart ::
+        IdentifierPartChar
+        `\` UnicodeEscapeSequence
+
+      IdentifierStartChar ::
         UnicodeIDStart
         `$`
         `_`
-        `\` UnicodeEscapeSequence
 
-      IdentifierPart ::
+      IdentifierPartChar ::
         UnicodeIDContinue
         `$`
-        `\` UnicodeEscapeSequence
         &lt;ZWNJ&gt;
         &lt;ZWJ&gt;
 
@@ -16209,13 +16215,13 @@ <h1>Static Semantics: Early Errors</h1>
         <emu-grammar>IdentifierStart :: `\` UnicodeEscapeSequence</emu-grammar>
         <ul>
           <li>
-            It is a Syntax Error if the SV of |UnicodeEscapeSequence| is none of *"$"*, or *"_"*, or ! UTF16EncodeCodePoint(_cp_) for some Unicode code point _cp_ matched by the |UnicodeIDStart| lexical grammar production.
+            It is a Syntax Error if the SV of |UnicodeEscapeSequence| is not ! UTF16EncodeCodePoint(_cp_) for some Unicode code point _cp_ matched by the |IdentifierStartChar| lexical grammar production.
           </li>
         </ul>
         <emu-grammar>IdentifierPart :: `\` UnicodeEscapeSequence</emu-grammar>
         <ul>
           <li>
-            It is a Syntax Error if the SV of |UnicodeEscapeSequence| is none of *"$"*, *"_"*, ! UTF16EncodeCodePoint(&lt;ZWNJ&gt;), ! UTF16EncodeCodePoint(&lt;ZWJ&gt;), or ! UTF16EncodeCodePoint(_cp_) for some Unicode code point _cp_ that would be matched by the |UnicodeIDContinue| lexical grammar production.
+            It is a Syntax Error if the SV of |UnicodeEscapeSequence| is not ! UTF16EncodeCodePoint(_cp_) for some Unicode code point _cp_ matched by the |IdentifierPartChar| lexical grammar production.
           </li>
         </ul>
       </emu-clause>
@@ -17057,22 +17063,12 @@ <h2>Syntax</h2>
 
         RegularExpressionFlags ::
           [empty]
-          RegularExpressionFlags IdentifierPart
+          RegularExpressionFlags IdentifierPartChar
       </emu-grammar>
       <emu-note>
         <p>Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence `//` starts a single-line comment. To specify an empty regular expression, use: `/(?:)/`.</p>
       </emu-note>
 
-      <emu-clause id="sec-literals-regular-expression-literals-static-semantics-early-errors">
-        <h1>Static Semantics: Early Errors</h1>
-        <emu-grammar>RegularExpressionFlags :: RegularExpressionFlags IdentifierPart</emu-grammar>
-        <ul>
-          <li>
-            It is a Syntax Error if |IdentifierPart| contains a Unicode escape sequence.
-          </li>
-        </ul>
-      </emu-clause>
-
       <emu-clause id="sec-static-semantics-bodytext" type="sdo">
         <h1>Static Semantics: BodyText</h1>
         <dl class="header">
@@ -34244,19 +34240,14 @@ <h2>Syntax</h2>
           RegExpIdentifierName[?UnicodeMode] RegExpIdentifierPart[?UnicodeMode]
 
         RegExpIdentifierStart[UnicodeMode] ::
-          UnicodeIDStart
-          `$`
-          `_`
+          IdentifierStartChar
           `\` RegExpUnicodeEscapeSequence[+UnicodeMode]
           [~UnicodeMode] UnicodeLeadSurrogate UnicodeTrailSurrogate
 
         RegExpIdentifierPart[UnicodeMode] ::
-          UnicodeIDContinue
-          `$`
+          IdentifierPartChar
           `\` RegExpUnicodeEscapeSequence[+UnicodeMode]
           [~UnicodeMode] UnicodeLeadSurrogate UnicodeTrailSurrogate
-          &lt;ZWNJ&gt;
-          &lt;ZWJ&gt;
 
         RegExpUnicodeEscapeSequence[UnicodeMode] ::
           [+UnicodeMode] `u` HexLeadSurrogate `\u` HexTrailSurrogate
@@ -34418,7 +34409,7 @@ <h1>Static Semantics: Early Errors</h1>
         <emu-grammar>RegExpIdentifierStart :: `\` RegExpUnicodeEscapeSequence</emu-grammar>
         <ul>
           <li>
-            It is a Syntax Error if the CharacterValue of |RegExpUnicodeEscapeSequence| is not the code point value of *"$"*, *"_"*, or some code point matched by the |UnicodeIDStart| lexical grammar production.
+            It is a Syntax Error if the CharacterValue of |RegExpUnicodeEscapeSequence| is not the code point value of some code point matched by the |IdentifierStartChar| lexical grammar production.
           </li>
         </ul>
         <emu-grammar>RegExpIdentifierStart :: UnicodeLeadSurrogate UnicodeTrailSurrogate</emu-grammar>
@@ -34430,7 +34421,7 @@ <h1>Static Semantics: Early Errors</h1>
         <emu-grammar>RegExpIdentifierPart :: `\` RegExpUnicodeEscapeSequence</emu-grammar>
         <ul>
           <li>
-            It is a Syntax Error if the CharacterValue of |RegExpUnicodeEscapeSequence| is not the code point value of *"$"*, *"_"*, &lt;ZWNJ&gt;, &lt;ZWJ&gt;, or some code point matched by the |UnicodeIDContinue| lexical grammar production.
+            It is a Syntax Error if the CharacterValue of |RegExpUnicodeEscapeSequence| is not the code point value of some code point matched by the |IdentifierPartChar| lexical grammar production.
           </li>
         </ul>
         <emu-grammar>RegExpIdentifierPart :: UnicodeLeadSurrogate UnicodeTrailSurrogate</emu-grammar>
@@ -46022,6 +46013,8 @@ <h1>Lexical Grammar</h1>
     <emu-prodref name="IdentifierName"></emu-prodref>
     <emu-prodref name="IdentifierStart"></emu-prodref>
     <emu-prodref name="IdentifierPart"></emu-prodref>
+    <emu-prodref name="IdentifierStartChar"></emu-prodref>
+    <emu-prodref name="IdentifierPartChar"></emu-prodref>
     <emu-prodref name="UnicodeIDStart"></emu-prodref>
     <emu-prodref name="UnicodeIDContinue"></emu-prodref>
     <emu-prodref name="ReservedWord"></emu-prodref>