Prohibit variable and identifier patterns from being named when or as, to simplify parsing. (#3033)

Author: stereotype441

accepted/future-releases/0546-patterns/feature-specification.md

@@ -4,7 +4,7 @@ Author: Bob Nystrom
 
 Status: Accepted
 
-Version 2.31 (see [CHANGELOG](#CHANGELOG) at end)
+Version 2.32 (see [CHANGELOG](#CHANGELOG) at end)
 
 Note: This proposal is broken into a couple of separate documents. See also
 [records][] and [exhaustiveness][].
@@ -512,6 +512,10 @@ switch (record) {
 }
 ```
 
+To simplify parsing, the identifier in a variable pattern may not be `when` or
+`as`. *This reduces the amount of lookahead needed to correctly parse a guarded
+pattern such as `foo when !bar`.*
+
 *There are some restrictions on when `var` and `final` can and can't be used.
 They are specified later in the "Pattern context" section.*
 
@@ -543,6 +547,13 @@ The answer is it's both. Depending on the context where it appears, a bare
 identifier pattern may behave like a constant pattern or like a variable
 pattern. The section on pattern context below lays out the precise rules.
 
+For consistency with the corresponding rule for variable patterns, the
+identifier in an identifier pattern may not be `when` or `as`. *This is not
+strictly necessary for parsing, but it should reduce user confusion by making
+identifier patterns consistent with variable patterns. Note however that these
+identifiers are allowed as parts of a qualifiedName that forms a
+constantPattern, e.g. `when.as`.*
+
 #### Wildcards
 
 As with variable patterns, an identifier pattern named `_` is a wildcard that
@@ -3524,6 +3535,11 @@ Here is one way it could be broken down into separate pieces:
 
 ## Changelog
 
+### 2.32
+
+-   Prohibit variable and identifier patterns from being named `when` or `as`,
+    to simplify parsing.
+
 ### 2.31
 
 -   Change the technique for resolving the "function expression in guard