Fix stack overflow when using Probe with FlatIO

core/src/main/scala/chisel3/Data.scala

@@ -482,7 +482,17 @@ abstract class Data extends HasId with NamedComponent with DataIntf {
     // Trace views to give better error messages
     // Reifying involves checking against ViewParent which requires being in a Builder context
     // Since we're just printing a String, suppress such errors and use this object
-    val thiz = Try(reifySingleTarget(this)).toOption.flatten.getOrElse(this)
+    // Use a guard to prevent infinite recursion when reifySingleTarget triggers toString
+    val thiz = if (Data.avoidReifyingViews.get()) {
+      this
+    } else {
+      Data.avoidReifyingViews.set(true)
+      try {
+        Try(reifySingleTarget(this)).toOption.flatten.getOrElse(this)
+      } finally {
+        Data.avoidReifyingViews.set(false)
+      }
+    }
     thiz.topBindingOpt match {
       case None => chiselTypeWithModifier
       // Handle DontCares specially as they are "literal-like" but not actually literals
@@ -896,6 +906,9 @@ object Data {
   // Needed for the `implicit def toConnectableDefault`
   import scala.language.implicitConversions
 
+  // ThreadLocal to prevent infinite recursion in stringAccessor when reifySingleTarget triggers toString
+  private[chisel3] val avoidReifyingViews: ThreadLocal[Boolean] = ThreadLocal.withInitial(() => false)
+
   private[chisel3] case class ProbeInfo(val writable: Boolean, color: Option[layer.Layer])
 
   /** Provides :<=, :>=, :<>=, and :#= between consumer and producer of the same T <: Data */

src/test/scala-2/chiselTests/ProbeSpec.scala

@@ -771,4 +771,30 @@ class ProbeSpec extends AnyFlatSpec with Matchers with FileCheck with ChiselSim
     }
     ChiselStage.emitCHIRRTL(new TestMod)
   }
+
+  "FlatIO probe" should "not stack overflow" in {
+    class ProbeIO extends Bundle {
+      val a = UInt(32.W)
+    }
+    class WithProbeIO extends Bundle {
+      val probe = Output(Probe(new ProbeIO))
+    }
+    class FlatIOModule extends RawModule {
+      val io = FlatIO(new WithProbeIO)
+      val w = Wire(new ProbeIO)
+      define(io.probe, ProbeValue(w))
+      w.a := 0.U
+      dontTouch(io)
+      dontTouch(w)
+    }
+    class TestTop extends RawModule {
+      val testMod = Module(new FlatIOModule)
+      val r: Bool = testMod.io.probe.a === 0.U
+      dontTouch(r)
+    }
+    val exc = intercept[Throwable] {
+      ChiselStage.emitSystemVerilog(new TestTop)
+    }
+    exc should not be a[StackOverflowError]
+  }
 }