Add error reporting for mirror synthesis

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

@@ -327,7 +327,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       val reason = if !cls.isGenericProduct then
         i"because ${cls.whyNotGenericProduct}"
       else if !canAccessCtor(cls) then
-        i"because constructor of $cls is unnaccessible from the calling scope."
+        i"because the constructor of $cls is innaccessible from the calling scope."
       else 
         ""
       i"$cls is not a generic product $reason"
@@ -618,7 +618,7 @@ object Synthesizer:
   private def orElse(treeWithErrors1: TreeWithErrors, treeWithErrors2: => TreeWithErrors): TreeWithErrors = treeWithErrors1 match
     case (tree, errors) if tree eq genericEmptyTree => 
       val (tree2, errors2) = treeWithErrors2
-      (tree2, errors2 ::: errors)
+      (tree2, errors ::: errors2)
     case _ => treeWithErrors1
 
   private def clearErrorsIfNotEmpty(treeWithErrors: TreeWithErrors) = treeWithErrors match 

tests/neg/i14432.check

@@ -2,5 +2,5 @@
 13 |val mFoo = summon[Mirror.Of[Foo]] // error: no mirror found
    |                                 ^
    |No given instance of type deriving.Mirror.Of[example.Foo] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[example.Foo]: 
+   |	* class Foo is not a generic product because the constructor of class Foo is innaccessible from the calling scope.
    |	* class Foo is not a generic sum because it is not a sealed class
-   |	* class Foo is not a generic product because constructor of class Foo is unnaccessible from the calling scope.

tests/neg/i14432a.check

@@ -2,5 +2,5 @@
 14 |  val mFoo = summon[Mirror.Of[example.Foo]] // error: no mirror found
    |                                           ^
    |No given instance of type deriving.Mirror.Of[example.Foo] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[example.Foo]: 
+   |	* class Foo is not a generic product because the constructor of class Foo is innaccessible from the calling scope.
    |	* class Foo is not a generic sum because it is not a sealed class
-   |	* class Foo is not a generic product because constructor of class Foo is unnaccessible from the calling scope.

tests/neg/i14432b.check

@@ -2,5 +2,5 @@
 15 |  val mFoo = summon[Mirror.Of[example.Foo]] // error: no mirror found
    |                                           ^
    |No given instance of type deriving.Mirror.Of[example.Foo] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[example.Foo]: 
+   |	* class Foo is not a generic product because the constructor of class Foo is innaccessible from the calling scope.
    |	* class Foo is not a generic sum because it is not a sealed class
-   |	* class Foo is not a generic product because constructor of class Foo is unnaccessible from the calling scope.

tests/neg/i14432c.check

@@ -10,5 +10,5 @@
 16 |  val mFoo = summon[Mirror.Of[example.Foo]] // error: no mirror
    |                                           ^
    |No given instance of type deriving.Mirror.Of[example.Foo] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[example.Foo]: 
+   |	* class Foo is not a generic product because the constructor of class Foo is innaccessible from the calling scope.
    |	* class Foo is not a generic sum because it is not a sealed class
-   |	* class Foo is not a generic product because constructor of class Foo is unnaccessible from the calling scope.

tests/neg/i14432d.check

@@ -2,5 +2,5 @@
 17 |    val mFoo = summon[Mirror.Of[example.Foo]] // error
    |                                             ^
    |No given instance of type deriving.Mirror.Of[example.Foo] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[example.Foo]: 
+   |	* class Foo is not a generic product because the constructor of class Foo is innaccessible from the calling scope.
    |	* class Foo is not a generic sum because it is not a sealed class
-   |	* class Foo is not a generic product because constructor of class Foo is unnaccessible from the calling scope.

tests/neg/i14823.check

@@ -2,5 +2,5 @@
 8 |val baz = summon[Mirror.Of[SubA[Int] | SubB[Int]]] // error
   |                                                  ^
   |No given instance of type deriving.Mirror.Of[SubA[Int] | SubB[Int]] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[SubA[Int] | SubB[Int]]: 
-  |	* class Cov is not a generic sum because it is not a sealed class
   |	* class Cov is not a generic product 
+  |	* class Cov is not a generic sum because it is not a sealed class

tests/neg/mirror-synthesis-errors.check

@@ -2,41 +2,41 @@
 21 |val testA = summon[Mirror.Of[A]] // error: Not a sealed trait
    |                                ^
    |No given instance of type deriving.Mirror.Of[A] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[A]: 
-   |	* trait A is not a generic sum because it is not a sealed trait
    |	* trait A is not a generic product because it is not a case class
+   |	* trait A is not a generic sum because it is not a sealed trait
 -- Error: tests/neg/mirror-synthesis-errors.scala:22:32 ----------------------------------------------------------------
 22 |val testC = summon[Mirror.Of[C]] // error: Does not have subclasses
    |                                ^
    |No given instance of type deriving.Mirror.Of[C] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[C]: 
-   |	* trait C is not a generic sum because it does not have subclasses
    |	* trait C is not a generic product because it is not a case class
+   |	* trait C is not a generic sum because it does not have subclasses
 -- Error: tests/neg/mirror-synthesis-errors.scala:23:32 ----------------------------------------------------------------
 23 |val testD = summon[Mirror.Of[D]] // error: child SubD takes more than one parameter list
    |                                ^
    |No given instance of type deriving.Mirror.Of[D] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[D]: 
-   |	* class D is not a generic sum because its child class SubD is not a generic product because it takes more than one parameter list
    |	* class D is not a generic product because it is not a case class
+   |	* class D is not a generic sum because its child class SubD is not a generic product because it takes more than one parameter list
 -- Error: tests/neg/mirror-synthesis-errors.scala:24:38 ----------------------------------------------------------------
 24 |val testSubD = summon[Mirror.Of[SubD]] // error: takes more than one parameter list
    |                                      ^
    |No given instance of type deriving.Mirror.Of[SubD] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[SubD]: 
-   |	* class SubD is not a generic sum because it is not a sealed class
    |	* class SubD is not a generic product because it takes more than one parameter list
+   |	* class SubD is not a generic sum because it is not a sealed class
 -- Error: tests/neg/mirror-synthesis-errors.scala:25:32 ----------------------------------------------------------------
 25 |val testE = summon[Mirror.Of[E]] // error: Not an abstract class
    |                                ^
    |No given instance of type deriving.Mirror.Of[E] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[E]: 
-   |	* class E is not a generic sum because it is not an abstract class
    |	* class E is not a generic product because it is not a case class
+   |	* class E is not a generic sum because it is not an abstract class
 -- Error: tests/neg/mirror-synthesis-errors.scala:26:32 ----------------------------------------------------------------
 26 |val testF = summon[Mirror.Of[F]] // error: No children
    |                                ^
    |No given instance of type deriving.Mirror.Of[F] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[F]: 
-   |	* trait F is not a generic sum because it does not have subclasses
    |	* trait F is not a generic product because it is not a case class
+   |	* trait F is not a generic sum because it does not have subclasses
 -- Error: tests/neg/mirror-synthesis-errors.scala:27:36 ----------------------------------------------------------------
 27 |val testG = summon[Mirror.Of[Foo.G]] // error: Has anonymous subclasses
    |                                    ^
    |No given instance of type deriving.Mirror.Of[Foo.G] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.Of[Foo.G]: 
-   |	* trait G is not a generic sum because it has anonymous or inaccessible subclasses
    |	* trait G is not a generic product because it is not a case class
+   |	* trait G is not a generic sum because it has anonymous or inaccessible subclasses