Disallow throws clauses over RuntimeExceptions

Fixes scala#13846

Author: odersky

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

@@ -1942,14 +1942,20 @@ class Typer extends Namer
       }
       var checkedArgs = preCheckKinds(args1, paramBounds)
         // check that arguments conform to bounds is done in phase PostTyper
-      if (tpt1.symbol == defn.andType)
+      val tycon = tpt1.symbol
+      if (tycon == defn.andType)
         checkedArgs = checkedArgs.mapconserve(arg =>
           checkSimpleKinded(checkNoWildcard(arg)))
-      else if (tpt1.symbol == defn.orType)
+      else if (tycon == defn.orType)
         checkedArgs = checkedArgs.mapconserve(arg =>
           checkSimpleKinded(checkNoWildcard(arg)))
+      else if tycon == defn.throwsAlias
+          && checkedArgs.length == 2
+          && checkedArgs(1).tpe.derivesFrom(defn.RuntimeExceptionClass)
+      then
+        report.error(em"throws clause cannot be defined for RuntimeException", checkedArgs(1).srcPos)
       else if (ctx.isJava)
-        if (tpt1.symbol eq defn.ArrayClass) then
+        if tycon eq defn.ArrayClass then
           checkedArgs match {
             case List(arg) =>
               val elemtp = arg.tpe.translateJavaArrayElementType

tests/neg/i13846.check

@@ -0,0 +1,17 @@
+-- Error: tests/neg/i13846.scala:3:22 ----------------------------------------------------------------------------------
+3 |def foo(): Int throws ArithmeticException = 1 / 0 // error
+  |                      ^^^^^^^^^^^^^^^^^^^
+  |                      throws clause cannot be defined for RuntimeException
+-- Error: tests/neg/i13846.scala:7:9 -----------------------------------------------------------------------------------
+7 |    foo() // error
+  |         ^
+  |         The capability to throw exception ArithmeticException is missing.
+  |         The capability can be provided by one of the following:
+  |          - A using clause `(using CanThrow[ArithmeticException])`
+  |          - A `throws` clause in a result type such as `X throws ArithmeticException`
+  |          - an enclosing `try` that catches ArithmeticException
+  |
+  |         The following import might fix the problem:
+  |
+  |           import unsafeExceptions.canThrowAny
+  |

tests/neg/i13846.scala

@@ -0,0 +1,9 @@
+import language.experimental.saferExceptions
+
+def foo(): Int throws ArithmeticException = 1 / 0 // error
+
+def test(): Unit =
+  try
+    foo() // error
+  catch
+    case _: ArithmeticException => println("Caught")