SI-8777 Avoid redundant disambiguation in error messages

When printing types in error messages, we attempt to disambiguate
symbol names by qualifying them in various ways.

Type paramters symbols are qualified by adding `(in someMethod)`.

However, the type errors generated by higher kinded subtyping
can contain clones of type parameter symbols, as creater in
`isPolySubType`. The disambiguation tries fruitlessly to distinguish
them but ended up adding the same suffix to both names repeatedly.

```
found   : [F[_]]Foo[[X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]Bar[F,X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]]
required: Foo[[X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]Bar[[X]Or[String,X],X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]]
```

This commit limits the post qualification of type parameter symbols
to a single attempt to limit the damage. An alternative might
be to mark a clone (we could determine its status by checking whether
it is a type parameter of its owner.) But I'm not sure how to present
this information in a comphrenensible way, so for now I'm limiting
my ambitions to stopping the stutter.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala

@@ -309,6 +309,7 @@ trait TypeDiagnostics {
     // save the name because it will be mutated until it has been
     // distinguished from the other types in the same error message
     private val savedName = sym.name
+    private var postQualifiedWith: List[Symbol] = Nil
     def restoreName()     = sym.name = savedName
     def modifyName(f: String => String) = sym setName newTypeName(f(sym.name.toString))
 
@@ -322,7 +323,7 @@ trait TypeDiagnostics {
 
     // functions to manipulate the name
     def preQualify()   = modifyName(trueOwner.fullName + "." + _)
-    def postQualify()  = modifyName(_ + "(in " + trueOwner + ")")
+    def postQualify()  = if (!(postQualifiedWith contains trueOwner)) { postQualifiedWith ::= trueOwner; modifyName(_ + "(in " + trueOwner + ")") }
     def typeQualify()  = if (sym.isTypeParameterOrSkolem) postQualify()
     def nameQualify()  = if (trueOwner.isPackageClass) preQualify() else postQualify()
 

test/files/neg/t6895b.check

@@ -2,8 +2,8 @@ t6895b.scala:20: error: could not find implicit value for parameter e: Foo[[X]Ba
   implicitly[Foo[({type L[X] = Bar[StringOr, X]})#L]]
             ^
 t6895b.scala:23: error: polymorphic expression cannot be instantiated to expected type;
- found   : [F[_]]Foo[[X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]Bar[F,X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]]
- required: Foo[[X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]Bar[[X]Or[String,X],X(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)(in type L)]]
+ found   : [F[_]]Foo[[X(in type L)]Bar[F,X(in type L)]]
+ required: Foo[[X(in type L)]Bar[[X]Or[String,X],X(in type L)]]
   barFoo(null) : Foo[({type L[X] = Bar[StringOr, X]})#L]
         ^
 two errors found

test/files/neg/t8777.check

@@ -0,0 +1,6 @@
+t8777.scala:3: error: type mismatch;
+ found   : Foo.this.TreePrinter(in trait Printers)
+ required: Foo.this.TreePrinter(in trait Printers)
+    super.newCodePrinter(out, tree, printRootPkg)
+                        ^
+one error found

test/files/neg/t8777.scala

@@ -0,0 +1,4 @@
+trait Foo extends scala.tools.nsc.Global {
+  override def newCodePrinter(out: java.io.PrintWriter, tree: Tree, printRootPkg: Boolean): TreePrinter =
+    super.newCodePrinter(out, tree, printRootPkg)
+}