Spec: Apply soft-modifiers.

Author: sjrd

docs/_spec/01-lexical-syntax.md

@@ -27,8 +27,6 @@ The principle of optional braces is that any keyword that can be followed by `{`
 
 The lexical analyzer inserts `indent` and `outdent` tokens that represent regions of indented code [at certain points](./other-new-features/indentation.md).
 
-´\color{red}{\text{TODO SCALA3: Port soft-modifier.md and link it here.}}´
-
 In the context-free productions below we use the notation `<<< ts >>>` to indicate a token sequence `ts` that is either enclosed in a pair of braces `{ ts }` or that constitutes an indented region `indent ts outdent`.
 Analogously, the notation `:<<< ts >>>` indicates a token sequence `ts` that is either enclosed in a pair of braces `{ ts }` or that constitutes an indented region `indent ts outdent` that follows a `colon` token.
 
@@ -121,12 +119,35 @@ type      val       var       while     with      yield
 
 Additionally, the following soft keywords are reserved only in some situations.
 
-´\color{red}{\text{TODO SCALA3: Port soft-modifier.md and link it here.}}´
-
 ```
-as  derives  end  extension  infix  inline  opaque  open  transparent  using  |  *  +  -
+as      derives      end       extension infix     inline    opaque
+open    transparent  using
+|       *            +         -
 ```
 
+A soft modifier is one of the identifiers `infix`, `inline`, `opaque`, `open` and `transparent`.
+
+A soft keyword is a soft modifier, or one of `as`, `derives`, `end`, `extension`, `using`, `|`, `+`, `-`, `*`.
+
+A soft modifier is treated as an actual modifier of a definition if it is followed by a hard modifier or a keyword combination starting a definition (`def`, `val`, `var`, `type`, `given`, `class`, `trait`, `object`, `enum`, `case class`, `case object`).
+Between the two words, there may be a sequence of newline tokens and/or other soft modifiers.
+
+Otherwise, soft keywords are treated as actual keywords in the following situations:
+
+ - `as`, if it appears in a renaming import clause.
+ - `derives`, if it appears after an extension clause or after the name and possibly parameters of a class, trait, object, or enum definition.
+ - `end`, if it appears at the start of a line following a statement (i.e. definition or toplevel expression) and is followed on the same line by a single non-comment token that is:
+   - one of the keywords `for`, `given`, `if`, `match`, `new`, `this`, `throw`, `try`, `val`, `while`, or
+   - an identifier.
+ - `extension`, if it appears at the start of a statement and is followed by `(` or `[`.
+ - `inline`, if it is followed by any token that can start an expression.
+ - `using`, if it appears at the start of a parameter or argument list.
+ - `|`, if it separates two patterns in an alternative.
+ - `+`, `-`, if they appear in front of a type parameter.
+ - `*`, if it appears in a wildcard import, or if it follows the type of a parameter, or if it appears in a vararg splice `x*`.
+
+Everywhere else, a soft keyword is treated as a normal identifier.
+
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 > When one needs to access Java identifiers that are reserved words in Scala, use backquote-enclosed strings.