Improve "...is not stable" error message

Give some context why an immutable path is required, and explain
what an immutable path is.

Author: odersky

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.scala

@@ -91,7 +91,7 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     ExpectedTopLevelDefID,
     AnonymousFunctionMissingParamTypeID,
     SuperCallsNotAllowedInlineableID,
-    UNUSED1,  // not used anymore, but left so that error numbers stay the same
+    NotAPathID,
     WildcardOnTypeArgumentNotAllowedOnNewID,
     FunctionTypeNeedsNonEmptyParameterListID,
     WrongNumberOfParametersID,

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1701,10 +1701,19 @@ object messages {
   case class SuperCallsNotAllowedInlineable(symbol: Symbol)(implicit ctx: Context)
     extends Message(SuperCallsNotAllowedInlineableID) {
     val kind: String = "Syntax"
-    val msg: String = s"Super call not allowed in inlineable $symbol"
+    val msg: String = em"Super call not allowed in inlineable $symbol"
     val explanation: String = "Method inlining prohibits calling superclass methods, as it may lead to confusion about which super is being called."
   }
 
+  case class NotAPath(tp: Type, usage: String)(using Context) extends Message(NotAPathID):
+    val kind: String = "Type"
+    val msg: String = em"$tp is not a valid $usage, since it is not an immutable path"
+    val explanation: String =
+      i"""An immutable path is
+         | - a reference to an immutable value, or
+         | - a reference to `this`, or
+         | - a selection of an immutable path with an immutable value."""
+
   case class WrongNumberOfParameters(expected: Int)(implicit ctx: Context)
     extends Message(WrongNumberOfParametersID) {
     val kind: String = "Syntax"

compiler/src/dotty/tools/dotc/transform/PCPCheckAndHeal.scala

@@ -239,7 +239,7 @@ class PCPCheckAndHeal(@constructorOnly ictx: Context) extends TreeMapWithStages(
 
     tag.tpe match
       case tp: TermRef =>
-        checkStable(tp, pos)
+        checkStable(tp, pos, "type witness")
         Some(ref(getQuoteTypeTags.getTagRef(tp)))
       case _: SearchFailureType =>
         levelError(sym, tp, pos,

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

@@ -652,8 +652,8 @@ trait Checking {
     Checking.checkNonCyclicInherited(joint, parents, decls, posd)
 
   /** Check that type `tp` is stable. */
-  def checkStable(tp: Type, pos: SourcePosition)(implicit ctx: Context): Unit =
-    if (!tp.isStable) ctx.error(ex"$tp is not stable", pos)
+  def checkStable(tp: Type, pos: SourcePosition, kind: String)(implicit ctx: Context): Unit =
+    if !tp.isStable then ctx.error(NotAPath(tp, kind), pos)
 
   /** Check that all type members of `tp` have realizable bounds */
   def checkRealizableBounds(cls: Symbol, pos: SourcePosition)(implicit ctx: Context): Unit = {
@@ -712,7 +712,7 @@ trait Checking {
 
   /** Check that `path` is a legal prefix for an import or export clause */
   def checkLegalImportPath(path: Tree)(implicit ctx: Context): Unit = {
-    checkStable(path.tpe, path.sourcePos)
+    checkStable(path.tpe, path.sourcePos, "import prefix")
     if (!ctx.isAfterTyper) Checking.checkRealizable(path.tpe, path.posd)
   }
 
@@ -726,7 +726,7 @@ trait Checking {
     tp.underlyingClassRef(refinementOK = false) match {
       case tref: TypeRef =>
         if (traitReq && !tref.symbol.is(Trait)) ctx.error(TraitIsExpected(tref.symbol), pos)
-        if (stablePrefixReq && ctx.phase <= ctx.refchecksPhase) checkStable(tref.prefix, pos)
+        if (stablePrefixReq && ctx.phase <= ctx.refchecksPhase) checkStable(tref.prefix, pos, "class prefix")
         tp
       case _ =>
         ctx.error(ex"$tp is not a class type", pos)
@@ -1194,7 +1194,7 @@ trait NoChecking extends ReChecking {
   import tpd._
   override def checkNonCyclic(sym: Symbol, info: TypeBounds, reportErrors: Boolean)(implicit ctx: Context): Type = info
   override def checkNonCyclicInherited(joint: Type, parents: List[Type], decls: Scope, posd: Positioned)(implicit ctx: Context): Unit = ()
-  override def checkStable(tp: Type, pos: SourcePosition)(implicit ctx: Context): Unit = ()
+  override def checkStable(tp: Type, pos: SourcePosition, kind: String)(implicit ctx: Context): Unit = ()
   override def checkClassType(tp: Type, pos: SourcePosition, traitReq: Boolean, stablePrefixReq: Boolean)(implicit ctx: Context): Type = tp
   override def checkImplicitConversionDefOK(sym: Symbol)(implicit ctx: Context): Unit = ()
   override def checkImplicitConversionUseOK(sym: Symbol, posd: Positioned)(implicit ctx: Context): Unit = ()

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

@@ -491,7 +491,7 @@ class Typer extends Namer
         case _ => app
       }
     case qual =>
-      if (tree.name.isTypeName) checkStable(qual.tpe, qual.sourcePos)
+      if (tree.name.isTypeName) checkStable(qual.tpe, qual.sourcePos, "type prefix")
       val select = assignType(cpy.Select(tree)(qual, tree.name), qual)
 
       val select1 = toNotNullTermRef(select, pt)
@@ -1429,7 +1429,7 @@ class Typer extends Namer
 
   def typedSingletonTypeTree(tree: untpd.SingletonTypeTree)(implicit ctx: Context): SingletonTypeTree = {
     val ref1 = typedExpr(tree.ref)
-    checkStable(ref1.tpe, tree.sourcePos)
+    checkStable(ref1.tpe, tree.sourcePos, "singleton type")
     assignType(cpy.SingletonTypeTree(tree)(ref1), ref1)
   }
 

tests/neg/i8569.check

@@ -1,8 +1,12 @@
--- Error: tests/neg/i8569.scala:8:2 ------------------------------------------------------------------------------------
+-- [E083] Type Error: tests/neg/i8569.scala:8:2 ------------------------------------------------------------------------
 8 |  outer.Inner(2) // error
   |  ^^^^^
-  |  (Test.outer : => Outer) is not a valid path
--- Error: tests/neg/i8569.scala:9:6 ------------------------------------------------------------------------------------
+  |  (Test.outer : => Outer) is not a valid type prefix, since it is not an immutable path
+
+longer explanation available when compiling with `-explain`
+-- [E083] Type Error: tests/neg/i8569.scala:9:6 ------------------------------------------------------------------------
 9 |  new outer.Inner(2) // error
   |      ^^^^^
-  |      (Test.outer : => Outer) is not a valid path
+  |      (Test.outer : => Outer) is not a valid type prefix, since it is not an immutable path
+
+longer explanation available when compiling with `-explain`