Enhancement: Force all exception classes to be pure

Impure exceptions can't be thrown anyway since `throw` takes a
pure `Throwable` as operand.

The change avoids having to declare explicit pure self types for
exception classes.

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -326,6 +326,11 @@ object CaptureSet:
   /** Used as a recursion brake */
   @sharable private[dotc] val Pending = Const(SimpleIdentitySet.empty)
 
+  /** The empty capture set with a description that says it's the elf type of an
+   *  exception class.
+   */
+  val emptyOfException: CaptureSet.Const = Const(emptySet, "of an exception class")
+
   def apply(elems: CaptureRef*)(using Context): CaptureSet.Const =
     if elems.isEmpty then empty
     else Const(SimpleIdentitySet(elems.map(_.normalizedRef)*))

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -504,6 +504,8 @@ class CheckCaptures extends Recheck, SymTransformer:
         for param <- cls.paramGetters do
           if !param.hasAnnotation(defn.ConstructorOnlyAnnot) then
             checkSubset(param.termRef.captureSet, thisSet, param.srcPos) // (3)
+        if cls.derivesFrom(defn.ThrowableClass) then
+          checkSubset(thisSet, CaptureSet.emptyOfException, tree.srcPos)
         super.recheckClassDef(tree, impl, cls)
       finally
         curEnv = saved

tests/neg-custom-args/captures/exception-definitions.check

@@ -0,0 +1,13 @@
+-- Error: tests/neg-custom-args/captures/exception-definitions.scala:2:6 -----------------------------------------------
+2 |class Err extends Exception: // error
+  |^
+  |reference (scala.caps.* : Any) is not included in allowed capture set {} of an exception class
+3 |  self: {*} Err =>
+-- Error: tests/neg-custom-args/captures/exception-definitions.scala:7:12 ----------------------------------------------
+7 |    val x = c  // error
+  |            ^
+  |   (c : {*} Any) cannot be referenced here; it is not included in the allowed capture set {} of an exception class
+-- Error: tests/neg-custom-args/captures/exception-definitions.scala:8:8 -----------------------------------------------
+8 |  class Err3(c: {*} Any) extends Exception // error
+  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |  reference (Err3.this.c : {*} Any) is not included in allowed capture set {} of an exception class

tests/neg-custom-args/captures/exception-definitions.scala

@@ -0,0 +1,10 @@
+
+class Err extends Exception: // error
+  self: {*} Err =>
+
+def test(c: {*} Any) =
+  class Err2 extends Exception:
+    val x = c  // error
+  class Err3(c: {*} Any) extends Exception // error
+
+