some vars need unspecified isolation for initializing exprs

It's possible to create an impossible set of constraints for
instance-member stored properties of a type. For example:

@mainactor func getStatus() -> Int { /* ... */ }
@PIDActor func genPID() -> ProcessID { /* ... */ }

class Process {
  @mainactor var status: Int = getStatus()
  @PIDActor var pid: ProcessID = genPID()

  init() {} // Problem: what is the isolation of this init?
}

We cannot satisfy the isolation of the initilizing expressions,
which demand that genStatus and genPID are run with isolation
from a non-async designated initializer, which is not possible.

This patch changes the isolation for those initializer expressions
for instance members, saying that the isolation is unspecified.

fixes rdar://84225474

Author: kavon

lib/AST/Decl.cpp

@@ -8682,10 +8682,23 @@ ActorIsolation swift::getActorIsolationOfContext(DeclContext *dc) {
   if (auto *vd = dyn_cast_or_null<ValueDecl>(dc->getAsDecl()))
     return getActorIsolation(vd);
 
+  // In the context of the initializing or default-value expression of a
+  // stored property, the isolation varies between global and type members:
+  //   - For a static stored property, the isolation matches the VarDecl.
+  //   - For a field of a nominal type, the expression is not isolated.
+  // Without this distinction, a nominal can have non-async initializers
+  // with various kinds of isolation, so an impossible constraint can be
+  // created. See SE-0327 for details.
   if (auto *init = dyn_cast<PatternBindingInitializer>(dc)) {
-    if (auto *var = init->getBinding()->getAnchoringVarDecl(
-            init->getBindingIndex()))
+    if (auto *var =
+          init->getBinding()->getAnchoringVarDecl(init->getBindingIndex())) {
+
+      if (var->isInstanceMember() &&
+          !var->getAttrs().hasAttribute<LazyAttr>())
+        return ActorIsolation::forUnspecified();
+
       return getActorIsolation(var);
+    }
   }
 
   if (auto *closure = dyn_cast<AbstractClosureExpr>(dc)) {

test/Concurrency/property_initializers.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift -disable-availability-checking -warn-concurrency
+// REQUIRES: concurrency
+
+
+@MainActor
+func mainActorFn() -> Int { return 0 } // expected-note 2 {{calls to global function 'mainActorFn()' from outside of its actor context are implicitly asynchronous}}
+
+@MainActor
+class C {
+  var x: Int = mainActorFn() // expected-error {{call to main actor-isolated global function 'mainActorFn()' in a synchronous nonisolated context}}
+
+  lazy var y: Int = mainActorFn()
+
+  static var z: Int = mainActorFn()
+}
+
+@MainActor
+var x: Int = mainActorFn() // expected-error {{call to main actor-isolated global function 'mainActorFn()' in a synchronous nonisolated context}}