Change enum Desugarings

Change the condition when type parameters of an enum are added to a class case.

Quoting my comment on scala#6095:

We have the following possibilities:

 1. type parameters are added only of there is no extends clause,
 2. (all) type parameters are added if one of them is referenced in the case,
 3. type parameters are always added if the case has value parameters (i.e. is not translated to an object).

(1) is what the current rules say. (2) is what Adriaan proposed. (3) is what is currently implemented.

In addition we have to clarify what should happen if a type parameter that is not added is nevertheless referred to in the case. Since enum expansion is done during desugaring, this could silently rebind to some other type. We should come up with a solution that avoids this, if possible.

This PR changes the rules to implement (2).

docs/docs/reference/enums/desugarEnums.md

@@ -25,9 +25,9 @@ some terminology and notational conventions:
 
 The desugaring rules imply that class cases are mapped to case classes, and singleton cases are mapped to `val` definitions.
 
-There are eight desugaring rules. Rule (1) desugar enum definitions. Rules
+There are nine desugaring rules. Rule (1) desugar enum definitions. Rules
 (2) and (3) desugar simple cases. Rules (4) to (6) define extends clauses for cases that
-are missing them. Rules (7) and (8) define how such cases with extends clauses
+are missing them. Rules (7) to (9) define how such cases with extends clauses
 map into case classes or vals.
 
 1.  An `enum` definition
@@ -80,7 +80,7 @@ map into case classes or vals.
        case C extends E[B1, ..., Bn]
 
    where `Bi` is `Li` if `Vi = '+'` and `Ui` if `Vi = '-'`. This result is then further
-   rewritten with rule (7). Simple cases of enums with non-variant type
+   rewritten with rule (8). Simple cases of enums with non-variant type
    parameters are not permitted.
 
 5. A class case without an extends clause
@@ -91,7 +91,7 @@ map into case classes or vals.
 
         case C <type-params> <value-params> extends E
 
-   This result is then further rewritten with rule (8).
+   This result is then further rewritten with rule (9).
 
 6. If `E` is an enum with type parameters `Ts`, a class case with neither type parameters nor an extends clause
 
@@ -101,9 +101,20 @@ map into case classes or vals.
 
         case C[Ts] <value-params> extends E[Ts]
 
-   This result is then further rewritten with rule (8). For class cases that have type parameters themselves, an extends clause needs to be given explicitly.
+   This result is then further rewritten with rule (9). For class cases that have type parameters themselves, an extends clause needs to be given explicitly.
 
-7. A value case
+7. If `E` is an enum with type parameters `Ts`, a class case without type parameters but with an extends clause
+
+       case C <value-params> extends <parents>
+
+   expands to
+
+       case C[Ts] <value-params> extends <parents>
+
+   provided at least one of the parameters `Ts` is mentioned in a parameter type in
+   `<value-params>` or in a type argument in `<parents>`.
+
+8. A value case
 
        case C extends <parents>
 
@@ -116,7 +127,7 @@ map into case classes or vals.
    as one of the `enumValues` of the enumeration (see below). `$values` is a
    compiler-defined private value in the companion object.
 
-8. A class case
+9. A class case
 
        case C <params> extends <parents>