[SILGen] Emit _checkExpectedExecutor only if its available on a deployment target

Don't emit `_checkExpectedExecutor` check if deployment target is
not sufficiently new otherwise it would result in crash.

Resolves: rdar://106827064

Author: xedin

lib/SILGen/SILGenConcurrency.cpp

@@ -15,6 +15,7 @@
 #include "RValue.h"
 #include "Scope.h"
 #include "swift/AST/ASTContext.h"
+#include "swift/AST/Availability.h"
 #include "swift/AST/ProtocolConformance.h"
 #include "swift/Basic/Range.h"
 
@@ -595,6 +596,17 @@ void SILGenFunction::emitPreconditionCheckExpectedExecutor(
   if (!checkExecutor)
     return;
 
+  // Forego a check if instrinsic is unavailable, this could happen
+  // in main-actor context.
+  auto &C = checkExecutor->getASTContext();
+  if (!C.LangOpts.DisableAvailabilityChecking) {
+    auto deploymentAvailability = AvailabilityContext::forDeploymentTarget(C);
+    auto declAvailability =
+        AvailabilityInference::availableRange(checkExecutor, C);
+    if (!deploymentAvailability.isContainedIn(declAvailability))
+      return;
+  }
+
   // We don't want the debugger to step into these.
   loc.markAutoGenerated();
 

test/SILGen/preconcurrency_conformances_backdeploy.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend -primary-file %s -target %target-cpu-apple-macosx10.14 -enable-experimental-feature DynamicActorIsolation -emit-silgen -o - | %FileCheck %s
+
+// REQUIRES: asserts
+// REQUIRES: concurrency
+// REQUIRES: OS=macosx
+
+protocol P {
+  associatedtype T
+
+  func fn() -> T?
+}
+
+@MainActor
+struct IsolatedType<T> : @preconcurrency P {
+  func fn() -> T? { nil }
+}
+
+// CHECK-LABEL: sil private [transparent] [thunk] [ossa] @$s38preconcurrency_conformances_backdeploy12IsolatedTypeVyxGAA1PA2aEP2fn1TQzSgyFTW
+// CHECK-NOT: function_ref @$ss22_checkExpectedExecutor14_filenameStart01_D6Length01_D7IsASCII5_line9_executoryBp_BwBi1_BwBetF