Backport "Add addendum to private val parameter variance error message" to 3.7.4 (#23997)

Backports #23876 to the 3.7.4.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

compiler/src/dotty/tools/dotc/typer/VarianceChecker.scala

@@ -178,7 +178,20 @@ class VarianceChecker(using Context) {
               i"\n${hl("enum case")} ${towner.name} requires explicit declaration of $tvar to resolve this issue.\n$example"
             else
               ""
-          em"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym$enumAddendum"
+          val privateParamAddendum =
+            if sym.flags.is(ParamAccessor) && sym.flags.is(Private) then
+              val varOrVal = if sym.is(Mutable) then "var" else "val"
+              val varFieldInstead = if sym.is(Mutable) then " and add\na field inside the class instead" else ""
+              s"""
+                 |
+                 |Implementation limitation: ${hl(f"private $varOrVal")} parameters cannot be inferred to be local
+                 |and therefore are always variance-checked.
+                 |
+                 |Potential fix: remove the ${hl(f"private $varOrVal")} modifiers on the parameter ${sym.name}$varFieldInstead.
+               """.stripMargin
+            else
+              ""
+          em"${varianceLabel(tvar.flags)} $tvar occurs in ${varianceLabel(required)} position in type ${sym.info} of $sym$enumAddendum$privateParamAddendum"
         if (migrateTo3 &&
             (sym.owner.isConstructor || sym.ownersIterator.exists(_.isAllOf(ProtectedLocal))))
           report.migrationWarning(

tests/neg/i22620.check

@@ -0,0 +1,21 @@
+-- Error: tests/neg/i22620.scala:4:34 ----------------------------------------------------------------------------------
+4 |class PrivateTest[-M](private val v: ArrayBuffer[M]) // error
+  |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |      contravariant type M occurs in invariant position in type scala.collection.mutable.ArrayBuffer[M] of value v
+  |
+  |      Implementation limitation: private val parameters cannot be inferred to be local
+  |      and therefore are always variance-checked.
+  |
+  |      Potential fix: remove the private val modifiers on the parameter v.
+  |                     
+-- Error: tests/neg/i22620.scala:6:37 ----------------------------------------------------------------------------------
+6 |class PrivateTestMut[-M](private var v: ArrayBuffer[M]) // error
+  |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |   contravariant type M occurs in invariant position in type scala.collection.mutable.ArrayBuffer[M] of variable v
+  |
+  |   Implementation limitation: private var parameters cannot be inferred to be local
+  |   and therefore are always variance-checked.
+  |
+  |   Potential fix: remove the private var modifiers on the parameter v and add
+  |   a field inside the class instead.
+  |                  

tests/neg/i22620.scala

@@ -2,3 +2,7 @@
 import scala.collection.mutable.ArrayBuffer
 
 class PrivateTest[-M](private val v: ArrayBuffer[M]) // error
+
+class PrivateTestMut[-M](private var v: ArrayBuffer[M]) // error
+
+class PrivateTestParamOnly[-M](v: ArrayBuffer[M]) // no error