[SILGen] Emit unreachable in emitProlog() for uninhabited arguments after other codegen

Author: maxovtsin

lib/SILGen/SILGenProlog.cpp

@@ -555,19 +555,6 @@ void SILGenFunction::emitProlog(CaptureInfo captureInfo,
                          capture, ++ArgNo);
   }
 
-  // Emit an unreachable instruction if a parameter type is
-  // uninhabited
-  if (paramList) {
-    for (auto *param : *paramList) {
-      if (param->getType()->isStructurallyUninhabited()) {
-        SILLocation unreachableLoc(param);
-        unreachableLoc.markAsPrologue();
-        B.createUnreachable(unreachableLoc);
-        break;
-      }
-    }
-  }
-
   // Whether the given declaration context is nested within an actor's
   // destructor.
   auto isInActorDestructor = [](DeclContext *dc) {
@@ -718,6 +705,21 @@ void SILGenFunction::emitProlog(CaptureInfo captureInfo,
                     ExpectedExecutor);
     }
   }
+
+  // IMPORTANT: This block should be the last one in `emitProlog`, 
+  // since it terminates BB and no instructions should be insterted after it.
+  // Emit an unreachable instruction if a parameter type is
+  // uninhabited
+  if (paramList) {
+    for (auto *param : *paramList) {
+      if (param->getType()->isStructurallyUninhabited()) {
+        SILLocation unreachableLoc(param);
+        unreachableLoc.markAsPrologue();
+        B.createUnreachable(unreachableLoc);
+        break;
+      }
+    }
+  }
 }
 
 SILValue SILGenFunction::emitMainExecutor(SILLocation loc) {

test/SILGen/functions_uninhabited_param.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-emit-sil %s -o /dev/null -verify
+// RUN: %target-swift-emit-sil %s -o /dev/null -verify -enable-actor-data-race-checks
 
 //===--- Function declaration with uninhabited parameter type
 
@@ -19,11 +20,16 @@ map { arg in // expected-note {{'arg' is of type 'Never' which cannot be constru
   return 5 // expected-warning {{will never be executed}}
 }
 
-// We used to crash when emitting the closure below.
+// We used to crash when emitting the closures below.
 enum E {
   static func f(_: E) {}
 }
 
+@MainActor
+class Bar {
+  var foo: (E) -> Void = { _ in }
+}
+
 let _: (E.Type) -> (E) -> () = { s in { e in s.f(e) } }
 // expected-warning@-1 {{will never be executed}}
 // expected-note@-2 {{'e' is of type 'E' which cannot be constructed because it is an enum with no cases}}