Stop catching Throwable/IIOBE/NPE in the compiler, outside of stack overflow handling by SolalPirelli · Pull Request #25832 · scala/scala3

SolalPirelli · 2026-04-16T15:39:09Z

How much have you relied on LLM-based tools in this contribution?

Not at all

How was the solution tested?

Covered by existing tests (this is a refactoring)

SolalPirelli · 2026-04-17T09:49:14Z

          case ex: ClosedByInterruptException =>
            try Files.deleteIfExists(path) // don't leave a empty of half-written classfile around after an interrupt
-            catch { case _: Throwable => () }
+            catch { case _: java.io.IOException => () }


for the IO-related ones I followed the Javadoc, and in one case removed a catch because it was calling one of our own util methods that already catches and ignores internally

SolalPirelli · 2026-04-17T09:49:37Z

        case ex: CompilationUnit.SuspendException => throw ex
-        case ex: Throwable =>
-          if !ex.isInstanceOf[TypeError] then ex.printStackTrace()
+        case ex: TypeError =>


given that we were willing to print a stack trace for non-typeerrors, I think letting it bubble up to "compiler bug" is better

TODO: double check that if something bad happens in the backend we get the usual compiler "plz file a bug" message (says Seb)

SolalPirelli · 2026-04-17T09:50:10Z

      catch {
-        case ex: Throwable =>
-          handleRecursive("member names", i"of $this", ex)
+        case ex: Throwable => handleRecursive("member names", i"of $this", ex)


I changed the formatting for the handleRecursives where it was easy to do so they're easier to look at in search results, since I'll deal with those in the next step

lrytz

For the standard library this looks like a very substantial breaking change that I think we cannot "just" do?

mbovel

As discussed orally, I was wondering if NonFatal is really the right tool (it seems ad hoc, will need to be updated, is potentially slow with its 5 type tests, etc.) or if we could more radically only catch Exception instead of Throwable. I believe it was the original idea behind the separation between Error and Exception, but I don't know if it would be practical.

SolalPirelli · 2026-04-28T13:25:14Z

            Interpreter.suspendOnMissing(sym, origin, annot.tree)
+          // We're dealing with user-thrown things here, so we must use NonFatal to catch anything realistic,
+          // including `Error`s like `NotImplementedError`
          case NonFatal(ex) =>


the only remaining use of NonFatal, can't really get rid of it :/

fun fact: ??? is an Error not an Exception so if someone uses it in a macro we have to be able to catch that... there is indeed a test that does so

SolalPirelli · 2026-04-29T08:34:07Z

+                  val instantiated = dealiased.instantiate(args)
+                  if (followAlias) instantiated.normalized else instantiated
+                catch
+                  case ex: Throwable => handleRecursive("try to instantiate", i"$dealiased[$args%, %]", ex)


this is the only non-mechanical change, I had to add a specific guard for what the catch case IndexOutOfBoundsException was trying to prevent

sjrd

Is the title still accurate? It suggests that we still catch IOOBE and NPE, but the PR content suggests that those are not caught anymore.

sjrd · 2026-04-29T08:46:50Z

+      case e: Exception =>
        println(s"Compile $path exception:")
        e.printStackTrace()
+        throw e


I don't think we should drop exceptions on the floor during tests. I've already had to fix one such case in another PR where I was seeing crashes due to stack overflows while running tests yet the tests "passed"

…verflow handling (scala#25832) Part of scala#25799 ## How much have you relied on LLM-based tools in this contribution? Not at all ## How was the solution tested? Covered by existing tests (this is a refactoring)

SolalPirelli requested a review from a team as a code owner April 16, 2026 15:39

SolalPirelli added the needs-squashing PR whose commits should be squashed by the author or via the "Squash and Merge" button label Apr 16, 2026

SolalPirelli requested a review from mbovel April 17, 2026 09:48

SolalPirelli assigned tgodzik Apr 17, 2026

SolalPirelli commented Apr 17, 2026

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lrytz reviewed Apr 20, 2026

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Comment thread library/src/scala/concurrent/impl/Promise.scala Outdated

SolalPirelli force-pushed the solal/no-catch-throwable branch from e40c10d to a9d11dc Compare April 21, 2026 13:26

SolalPirelli requested a review from sjrd April 24, 2026 13:03

SolalPirelli force-pushed the solal/no-catch-throwable branch 3 times, most recently from 3381d22 to 11742a6 Compare April 24, 2026 13:34

SolalPirelli unassigned tgodzik Apr 24, 2026

SolalPirelli mentioned this pull request Apr 27, 2026

Replace catching StackOverflowError with fuel #25937

Draft

mbovel reviewed Apr 28, 2026

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SolalPirelli force-pushed the solal/no-catch-throwable branch 2 times, most recently from 2c3a9b3 to 3a196e0 Compare April 28, 2026 13:10

SolalPirelli commented Apr 28, 2026

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SolalPirelli changed the title ~~Stop catching Throwable, outside of stack overflow handling~~ Stop catching Throwable outside of stack overflow handling + IOOBE + NPE Apr 28, 2026

SolalPirelli requested a review from mbovel April 29, 2026 08:32

SolalPirelli commented Apr 29, 2026

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sjrd requested changes Apr 29, 2026

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SolalPirelli changed the title ~~Stop catching Throwable outside of stack overflow handling + IOOBE + NPE~~ Stop catching Throwable/IIOBE/NPE in the compiler, outside of stack overflow handling Apr 29, 2026

SolalPirelli added 6 commits April 29, 2026 11:05

Stop catching Throwable

cc82a01

Done

ee2a584

PR feedback

c0a2621

Replace a custom version of handleRecursive by handleRecursive

b5d523e

Remove dummy catch blocks

e0b60d0

Stop using NonFatal inside the compiler, catch Exception instead

b40ebeb

SolalPirelli added 5 commits April 29, 2026 11:05

update scaladoc

82ba6b4

fixes

f681edf

Stop catching IndexOutOfBoundsException

86d5ef0

Stop catching NullPointerException

6a78b72

PR feedback

fb22933

SolalPirelli force-pushed the solal/no-catch-throwable branch from 72ad800 to fb22933 Compare April 29, 2026 09:10

SolalPirelli requested a review from sjrd April 29, 2026 09:11

sjrd approved these changes Apr 29, 2026

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SolalPirelli merged commit a06284b into scala:main Apr 29, 2026
53 of 54 checks passed

SolalPirelli deleted the solal/no-catch-throwable branch April 29, 2026 11:10

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SolalPirelli commented Apr 16, 2026 •

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