Reset comparersInUse to zero in ContextState.reset (#18915)

Fixes #18855 The code in #18855 used to crash the compiler with an out-of-bound access exception. It was because, previously, `ContextState.reset` clears the `comparers`, the cached pool of `TypeComparer`s, while keeping the variable `comparersInUse` intact. However, the `comparer` method of a context assumes `comparers` has a size greater than or equal to `comparersInUse`. So the invariant is clearly violated here and thus triggers the crash. This PR proposes a straightforward fix. Yet, the reason why only under both macros and cc this code path will be triggered remains to be investigated.
scala · Nov 14, 2023 · 0cd94fc · 0cd94fc
2 parents 563fab9 + 44c9674
commit 0cd94fc
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diff --git a/compiler/src/dotty/tools/dotc/core/Contexts.scala b/compiler/src/dotty/tools/dotc/core/Contexts.scala
@@ -1063,6 +1063,7 @@ object Contexts {
       sources.clear()
       files.clear()
       comparers.clear()  // forces re-evaluation of top and bottom classes in TypeComparer
+      comparersInUse = 0
 
     // Test that access is single threaded
 

diff --git a/tests/pos-macros/i18855/invoc.scala b/tests/pos-macros/i18855/invoc.scala
@@ -0,0 +1,3 @@
+import scala.language.experimental.captureChecking
+val x = run()
+
diff --git a/tests/pos-macros/i18855/macro.scala b/tests/pos-macros/i18855/macro.scala
@@ -0,0 +1,7 @@
+import scala.quoted.*
+import scala.language.experimental.captureChecking
+
+def impl()(using Quotes): Expr[Unit] = '{()}
+inline def run(): Unit = ${impl()}
+
+