SILGen: Stub unavailable protocol witness thunks.

Part of rdar://107388493

Author: tshortli

lib/SILGen/SILGenConstructor.cpp

@@ -750,6 +750,9 @@ void SILGenFunction::emitEnumConstructor(EnumElementDecl *element) {
   auto &enumTI =
       SGM.Types.getTypeLowering(enumTy, TypeExpansionContext::minimal());
 
+  if (shouldLowerToUnavailableCodeStub(element))
+    emitApplyOfUnavailableCodeReached();
+
   RegularLocation Loc(element);
   CleanupLocation CleanupLoc(element);
   Loc.markAutoGenerated();

lib/SILGen/SILGenPoly.cpp

@@ -4966,6 +4966,9 @@ void SILGenFunction::emitProtocolWitness(
   SmallVector<ManagedValue, 8> origParams;
   collectThunkParams(loc, origParams);
 
+  if (shouldLowerToUnavailableCodeStub(witness.getDecl()))
+    emitApplyOfUnavailableCodeReached();
+
   if (enterIsolation) {
     // If we are supposed to enter the actor, do so now by hopping to the
     // actor.

test/SILGen/unavailable_decl_optimization_stub_protocol_witness.swift

@@ -0,0 +1,31 @@
+// RUN: %target-swift-emit-silgen -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=stub | %FileCheck %s --check-prefixes=CHECK
+
+public struct S {}
+
+public protocol P {
+  static func requirement(_ s: S) -> Self
+}
+
+public enum EnumWithProtocolWitness {
+  @available(*, unavailable)
+  case requirement(_ s: S)
+}
+
+@available(*, unavailable)
+extension EnumWithProtocolWitness: P {}
+
+// protocol witness for static P.requirement(_:) in conformance EnumWithProtocolWitness
+//
+// CHECK-LABEL: sil {{.*}} @$s4Test23EnumWithProtocolWitnessOAA1PA2aDP11requirementyxAA1SVFZTW
+// CHECK:         [[FNREF:%.*]] = function_ref @$ss31_diagnoseUnavailableCodeReacheds5NeverOyF
+// CHECK-NEXT:    [[APPLY:%.*]] = apply [[FNREF]]()
+// CHECK:         {{%.*}} = function_ref @$s4Test23EnumWithProtocolWitnessO11requirementyAcA1SVcACmF
+// CHECK:       } // end sil function '$s4Test23EnumWithProtocolWitnessOAA1PA2aDP11requirementyxAA1SVFZTW'
+//
+// EnumWithProtocolWitness.requirement(_:)
+//
+// CHECK-LABEL: sil {{.*}} @$s4Test23EnumWithProtocolWitnessO11requirementyAcA1SVcACmF
+// CHECK:         [[FNREF:%.*]] = function_ref @$ss31_diagnoseUnavailableCodeReacheds5NeverOyF
+// CHECK-NEXT:    [[APPLY:%.*]] = apply [[FNREF]]()
+// CHECK:         enum $EnumWithProtocolWitness, #EnumWithProtocolWitness.requirement!enumelt
+// CHECK:       } // end sil function '$s4Test23EnumWithProtocolWitnessO11requirementyAcA1SVcACmF'