Fix LifetimeDependence type inference for setters.

A setter on a non-Escapable type may have a dependency on both it's incoming
'self' and 'newValue'. If the 'newValue' dependency does not match the getter's
dependency, then lifetime diagnostics will not accept the generated '_modify'
accessor:

error: lifetime-dependent value returned by generated accessor '_modify'

To fix this, make sure that we don't (conservatively) infer a borrow
dependency on 'newValue'.

Fixes rdar://150444400

Author: atrick

lib/AST/LifetimeDependence.cpp

@@ -1126,12 +1126,19 @@ class LifetimeDependenceChecker {
       if (paramTypeInContext->hasError()) {
         return;
       }
-      auto kind = inferLifetimeDependenceKind(paramTypeInContext,
-                                              param->getValueOwnership());
-
-      pushDeps(createDeps(selfIndex)
-                   .add(selfIndex, LifetimeDependenceKind::Inherit)
-                   .add(newValIdx, *kind));
+      auto targetDeps =
+        createDeps(selfIndex).add(selfIndex, LifetimeDependenceKind::Inherit);
+
+      // The 'newValue' dependence kind must match the getter's dependence kind
+      // because generated the implementation '_modify' accessor composes the
+      // getter's result with the setter's 'newValue'. In particular, if the
+      // result type is non-Escapable then the setter must not depend on
+      // 'newValue'.
+      if (!paramTypeInContext->isEscapable()) {
+        targetDeps = std::move(targetDeps)
+          .add(newValIdx, LifetimeDependenceKind::Inherit);
+      }
+      pushDeps(std::move(targetDeps));
       break;
     }
     case AccessorKind::MutableAddress:

test/SILOptimizer/lifetime_dependence/Inputs/lifetime_depend_diagnose.swiftinterface

@@ -24,4 +24,24 @@ extension NE {
     return NE(pointer: _pointer)
   }
 }
+
+public struct NCNE<Element>: ~Swift.Copyable & ~Swift.Escapable {
+  var e: Element
+}
+
+extension NCNE where Element : Swift.BitwiseCopyable {
+  // Ensure that lifetime dependence diagnostics accessp the generated _modify accessor:
+  // the getter dependency must match the setter's mewValue dependence.
+  // In this case, the getter has no dependency because the result is BitwiseCopyable. The setter cannot, therefore,
+  // have a borrow dependency no 'newValue' which is produced by the getter.
+  public subscript() -> Element {
+    get {
+      return e
+    }
+    //@lifetime(self: copy self, self: copy newValue)
+    set {
+      e = newValue
+    }
+  }
+}
 #endif