Sema: Diagnose availability of accessors in chained assignments.

In the following example, the writeback via the property `b` should be
diagnosed since `b`'s setter is unavailable:

```
struct A {
  struct B {
    var x: Int = 0
  }

  private var _b: B = B()
  var b: B {
    get { _b }
    @available(*, unavailable) set {
      _b = newValue
    }
  }
}

var a = A()
a.b.x = 1
```

Resolves rdar://125019717

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -3537,10 +3537,20 @@ class ExprAvailabilityWalker : public ASTWalker {
 
   /// Walk a member reference expression, checking for availability.
   void walkMemberRef(MemberRefExpr *E) {
-    // Walk the base in a getter context.
-    // FIXME: We may need to look at the setter too, if we're going to do
-    // writeback. The AST should have this information.
-    walkInContext(E, E->getBase(), MemberAccessContext::Getter);
+    // Walk the base. If the access context is currently `Setter`, then we must
+    // be diagnosing the destination of an assignment. When recursing, diagnose
+    // any remaining member refs as if they were in an InOutExpr, since there is
+    // a writeback occurring through them as a result of the assignment.
+    //
+    //   someVar.x.y = 1
+    //           │ ╰─ MemberAccessContext::Setter
+    //           ╰─── MemberAccessContext::InOut
+    //
+    MemberAccessContext accessContext =
+        (AccessContext == MemberAccessContext::Setter)
+            ? MemberAccessContext::InOut
+            : AccessContext;
+    walkInContext(E, E->getBase(), accessContext);
 
     ConcreteDeclRef DR = E->getMember();
     // Diagnose for the member declaration itself.

test/Sema/availability_accessors.swift

@@ -41,14 +41,14 @@ struct BaseStruct {
   var unavailableSetter: Value {
     get { .defaultValue }
     @available(*, unavailable)
-    set { fatalError() } // expected-note 16 {{setter for 'unavailableSetter' has been explicitly marked unavailable here}}
+    set { fatalError() } // expected-note 21 {{setter for 'unavailableSetter' has been explicitly marked unavailable here}}
   }
 
   var unavailableGetterAndSetter: Value {
     @available(*, unavailable)
     get { fatalError() } // expected-note 28 {{getter for 'unavailableGetterAndSetter' has been explicitly marked unavailable here}}
     @available(*, unavailable)
-    set { fatalError() } // expected-note 16 {{setter for 'unavailableGetterAndSetter' has been explicitly marked unavailable here}}
+    set { fatalError() } // expected-note 21 {{setter for 'unavailableGetterAndSetter' has been explicitly marked unavailable here}}
   }
 }
 
@@ -96,12 +96,12 @@ func testLValueAssignments() {
   x.unavailableGetter[0].b = 1 // expected-error {{getter for 'unavailableGetter' is unavailable}}
 
   x.unavailableSetter = someValue // expected-error {{setter for 'unavailableSetter' is unavailable}}
-  x.unavailableSetter.a = someValue.a // FIXME: missing diagnostic for setter
+  x.unavailableSetter.a = someValue.a // expected-error {{setter for 'unavailableSetter' is unavailable}}
   x.unavailableSetter[0] = someValue.a // expected-error {{setter for 'unavailableSetter' is unavailable}}
   x.unavailableSetter[0].b = 1 // expected-error {{setter for 'unavailableSetter' is unavailable}}
 
   x.unavailableGetterAndSetter = someValue // expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
-  x.unavailableGetterAndSetter.a = someValue.a // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} FIXME: missing diagnostic for setter
+  x.unavailableGetterAndSetter.a = someValue.a // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   x.unavailableGetterAndSetter[0] = someValue.a // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   x.unavailableGetterAndSetter[0].b = 1 // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
 }
@@ -128,14 +128,14 @@ func testKeyPathLoads() {
   _ = x[keyPath: \.unavailableSetter.a]
   _ = x[keyPath: \.unavailableSetter[0]]
   _ = x[keyPath: \.unavailableSetter[0].b]
-  _ = a[keyPath: \.[takesIntInOut(&x.unavailableSetter.a.b)]] // FIXME: missing diagnostic for setter
+  _ = a[keyPath: \.[takesIntInOut(&x.unavailableSetter.a.b)]] // expected-error {{setter for 'unavailableSetter' is unavailable}}
   _ = a[keyPath: \.[takesIntInOut(&x.unavailableSetter[0].b)]] // expected-error {{setter for 'unavailableSetter' is unavailable}}
 
   _ = x[keyPath: \.unavailableGetterAndSetter] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
   _ = x[keyPath: \.unavailableGetterAndSetter.a] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
   _ = x[keyPath: \.unavailableGetterAndSetter[0]] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
   _ = x[keyPath: \.unavailableGetterAndSetter[0].b] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
-  _ = a[keyPath: \.[takesIntInOut(&x.unavailableGetterAndSetter.a.b)]] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} FIXME: missing diagnostic for setter
+  _ = a[keyPath: \.[takesIntInOut(&x.unavailableGetterAndSetter.a.b)]] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} // expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   _ = a[keyPath: \.[takesIntInOut(&x.unavailableGetterAndSetter[0].b)]] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
 }
 
@@ -162,14 +162,14 @@ func testKeyPathAssignments() {
   x[keyPath: \.unavailableSetter.a] = someValue.a // expected-error {{setter for 'unavailableSetter' is unavailable}}
   x[keyPath: \.unavailableSetter[0]] = someValue.a // expected-error {{setter for 'unavailableSetter' is unavailable}}
   x[keyPath: \.unavailableSetter[0].b] = 1 // expected-error {{setter for 'unavailableSetter' is unavailable}}
-  a[keyPath: \.[takesIntInOut(&x.unavailableSetter.a.b)]] = 0 // FIXME: missing diagnostic for setter
+  a[keyPath: \.[takesIntInOut(&x.unavailableSetter.a.b)]] = 0 // expected-error {{setter for 'unavailableSetter' is unavailable}}
   a[keyPath: \.[takesIntInOut(&x.unavailableSetter[0].b)]] = 0 // expected-error {{setter for 'unavailableSetter' is unavailable}}
 
   x[keyPath: \.unavailableGetterAndSetter] = someValue // expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   x[keyPath: \.unavailableGetterAndSetter.a] = someValue.a // expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   x[keyPath: \.unavailableGetterAndSetter[0]] = someValue.a // expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   x[keyPath: \.unavailableGetterAndSetter[0].b] = 1 // expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
-  a[keyPath: \.[takesIntInOut(&x.unavailableGetterAndSetter.a.b)]] = 0 // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} FIXME: missing diagnostic for setter
+  a[keyPath: \.[takesIntInOut(&x.unavailableGetterAndSetter.a.b)]] = 0 // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   a[keyPath: \.[takesIntInOut(&x.unavailableGetterAndSetter[0].b)]] = 0 // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
 }
 
@@ -192,13 +192,13 @@ func testMutatingMember() {
   x.unavailableSetter.setToZero() // expected-error {{setter for 'unavailableSetter' is unavailable}}
   x.unavailableSetter[0].setToZero() // expected-error {{setter for 'unavailableSetter' is unavailable}}
   _ = a[x.unavailableSetter.setToZero()] // expected-error {{setter for 'unavailableSetter' is unavailable}}
-  _ = a[x.unavailableSetter.a.setToZero()] // FIXME: missing diagnostic for setter
+  _ = a[x.unavailableSetter.a.setToZero()] // expected-error {{setter for 'unavailableSetter' is unavailable}}
   _ = a[x.unavailableSetter[0].setToZero()] // expected-error {{setter for 'unavailableSetter' is unavailable}}
 
   x.unavailableGetterAndSetter.setToZero() // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   x.unavailableGetterAndSetter[0].setToZero() // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   _ = a[x.unavailableGetterAndSetter.setToZero()] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
-  _ = a[x.unavailableGetterAndSetter.a.setToZero()] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} FIXME: should diagnose setter
+  _ = a[x.unavailableGetterAndSetter.a.setToZero()] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   _ = a[x.unavailableGetterAndSetter[0].setToZero()] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
 }
 
@@ -218,12 +218,12 @@ func testPassAsInOutParameter() {
   takesInOut(&x.unavailableGetter[0].b) // expected-error {{getter for 'unavailableGetter' is unavailable}}
 
   takesInOut(&x.unavailableSetter) // expected-error {{setter for 'unavailableSetter' is unavailable}}
-  takesInOut(&x.unavailableSetter.a) //
+  takesInOut(&x.unavailableSetter.a) // expected-error {{setter for 'unavailableSetter' is unavailable}}
   takesInOut(&x.unavailableSetter[0]) // expected-error {{setter for 'unavailableSetter' is unavailable}}
   takesInOut(&x.unavailableSetter[0].b) // expected-error {{setter for 'unavailableSetter' is unavailable}}
 
   takesInOut(&x.unavailableGetterAndSetter) // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
-  takesInOut(&x.unavailableGetterAndSetter.a) // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} FIXME: missing diagnostic for setter
+  takesInOut(&x.unavailableGetterAndSetter.a) // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   takesInOut(&x.unavailableGetterAndSetter[0]) // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
   takesInOut(&x.unavailableGetterAndSetter[0].b) // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}}
 }