i16601 Fix class name in error message (#16635)

Author: prolativ

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

@@ -325,7 +325,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
                 // Check the constructor type as well; it could be an illegal singleton type
                 // which would not be reflected as `tree.tpe`
                 ctx.typer.checkClassType(nu.tpe, tree.srcPos, traitReq = false, stablePrefixReq = false)
-              Checking.checkInstantiable(tree.tpe, nu.srcPos)
+              Checking.checkInstantiable(tree.tpe, nu.tpe, nu.srcPos)
               withNoCheckNews(nu :: Nil)(app1)
             case _ =>
               app1
@@ -413,7 +413,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
             Checking.checkGoodBounds(tree.symbol)
           super.transform(tree)
         case tree: New if isCheckable(tree) =>
-          Checking.checkInstantiable(tree.tpe, tree.srcPos)
+          Checking.checkInstantiable(tree.tpe, tree.tpe, tree.srcPos)
           super.transform(tree)
         case tree: Closure if !tree.tpt.isEmpty =>
           Checking.checkRealizable(tree.tpt.tpe, tree.srcPos, "SAM type")

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

@@ -185,12 +185,14 @@ object Checking {
   /** Check that `tp` refers to a nonAbstract class
    *  and that the instance conforms to the self type of the created class.
    */
-  def checkInstantiable(tp: Type, pos: SrcPos)(using Context): Unit =
+  def checkInstantiable(tp: Type, srcTp: Type, pos: SrcPos)(using Context): Unit =
     tp.underlyingClassRef(refinementOK = false) match
       case tref: TypeRef =>
         val cls = tref.symbol
-        if (cls.isOneOf(AbstractOrTrait))
-          report.error(CantInstantiateAbstractClassOrTrait(cls, isTrait = cls.is(Trait)), pos)
+        if (cls.isOneOf(AbstractOrTrait)) {
+          val srcCls = srcTp.underlyingClassRef(refinementOK = false).typeSymbol
+          report.error(CantInstantiateAbstractClassOrTrait(srcCls, isTrait = srcCls.is(Trait)), pos)
+        }
         if !cls.is(Module) then
           // Create a synthetic singleton type instance, and check whether
           // it conforms to the self type of the class as seen from that instance.

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -159,6 +159,7 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/i9517.scala", defaultOptions.and("-Xprint-types")),
       compileFile("tests/neg-custom-args/i11637.scala", defaultOptions.and("-explain")),
       compileFile("tests/neg-custom-args/i15575.scala", defaultOptions.and("-explain")),
+      compileFile("tests/neg-custom-args/i16601a.scala", defaultOptions.and("-explain")),
       compileFile("tests/neg-custom-args/interop-polytypes.scala", allowDeepSubtypes.and("-Yexplicit-nulls")),
       compileFile("tests/neg-custom-args/conditionalWarnings.scala", allowDeepSubtypes.and("-deprecation").and("-Xfatal-warnings")),
       compileFilesInDir("tests/neg-custom-args/isInstanceOf", allowDeepSubtypes and "-Xfatal-warnings"),

tests/neg-custom-args/i16601a.check

@@ -0,0 +1,18 @@
+-- [E042] Type Error: tests/neg-custom-args/i16601a.scala:1:27 ---------------------------------------------------------
+1 |@main def Test: Unit = new concurrent.ExecutionContext  // error
+  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                           ExecutionContext is a trait; it cannot be instantiated
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | Abstract classes and traits need to be extended by a concrete class or object
+  | to make their functionality accessible.
+  |
+  | You may want to create an anonymous class extending ExecutionContext with
+  |   class ExecutionContext { }
+  |
+  | or add a companion object with
+  |   object ExecutionContext extends ExecutionContext
+  |
+  | You need to implement any abstract members in both cases.
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg-custom-args/i16601a.scala

@@ -0,0 +1 @@
+@main def Test: Unit = new concurrent.ExecutionContext  // error

tests/neg/i16601.check

@@ -0,0 +1,6 @@
+-- [E042] Type Error: tests/neg/i16601.scala:1:27 ----------------------------------------------------------------------
+1 |@main def Test: Unit = new concurrent.ExecutionContext  // error
+  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |                           ExecutionContext is a trait; it cannot be instantiated
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i16601.scala

@@ -0,0 +1 @@
+@main def Test: Unit = new concurrent.ExecutionContext  // error