Sema: Do not diagnose set accessor availability for InOutExprs inside of LoadExprs.

The recent deprecation of the set accessor for `CommandLine.arguments` exposed
a bug in availability checking:

```
 for arg in CommandLine.arguments[1...] {
                      ╰─ warning: setter for 'arguments' is deprecated: ...
```

The parser generates an `InOutExpr` in the AST for the subscript access in the
code above and the availabilty checker was unconditionally diagnosing set
accessors for `InOutExprs`, resulting in spurious availability warnings for
read-only accesses of properties. The fix is to check whether there is an
enclosing `LoadExpr` in the AST and only diagnose the getter accessor in that
case.

Resolves rdar://124566405

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -3496,6 +3496,20 @@ class ExprAvailabilityWalker : public ASTWalker {
     return call;
   }
 
+  /// Walks up to the first enclosing LoadExpr and returns it.
+  const LoadExpr *getEnclosingLoadExpr() const {
+    assert(!ExprStack.empty() && "must be called while visiting an expression");
+    ArrayRef<const Expr *> stack = ExprStack;
+    stack = stack.drop_back();
+
+    for (auto expr : llvm::reverse(stack)) {
+      if (auto loadExpr = dyn_cast<LoadExpr>(expr))
+        return loadExpr;
+    }
+
+    return nullptr;
+  }
+
   /// Walk an assignment expression, checking for availability.
   void walkAssignExpr(AssignExpr *E) {
     // We take over recursive walking of assignment expressions in order to
@@ -3575,7 +3589,12 @@ class ExprAvailabilityWalker : public ASTWalker {
 
   /// Walk an inout expression, checking for availability.
   void walkInOutExpr(InOutExpr *E) {
-    walkInContext(E, E->getSubExpr(), MemberAccessContext::InOut);
+    // If there is a LoadExpr in the stack, then this InOutExpr is not actually
+    // indicative of any mutation so the access context should just be Getter.
+    auto accessContext = getEnclosingLoadExpr() ? MemberAccessContext::Getter
+                                                : MemberAccessContext::InOut;
+
+    walkInContext(E, E->getSubExpr(), accessContext);
   }
 
   bool shouldWalkIntoClosure(AbstractClosureExpr *closure) const {

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 12 {{setter for 'unavailableSetter' has been explicitly marked unavailable here}}
   }
 
   var unavailableGetterAndSetter: Value {
     @available(*, unavailable)
     get { fatalError() } // expected-note 24 {{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 12 {{setter for 'unavailableGetterAndSetter' has been explicitly marked unavailable here}}
   }
 }
 
@@ -73,13 +73,13 @@ func testRValueLoads() {
 
   _ = x.unavailableSetter
   _ = x.unavailableSetter.a
-  _ = x.unavailableSetter[0] // expected-error {{setter for 'unavailableSetter' is unavailable}} FIXME: setter should not be diagnosed
-  _ = x.unavailableSetter[0].b // expected-error {{setter for 'unavailableSetter' is unavailable}} FIXME: setter should not be diagnosed
+  _ = x.unavailableSetter[0]
+  _ = x.unavailableSetter[0].b
 
   _ = x.unavailableGetterAndSetter // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
   _ = x.unavailableGetterAndSetter.a // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
-  _ = x.unavailableGetterAndSetter[0] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}} FIXME: setter should not be diagnosed
-  _ = x.unavailableGetterAndSetter[0].b // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}} FIXME: setter should not be diagnosed
+  _ = x.unavailableGetterAndSetter[0] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
+  _ = x.unavailableGetterAndSetter[0].b // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
 }
 
 func testLValueAssignments() {
@@ -243,11 +243,11 @@ struct TestPatternBindingInitExprs {
 
   var unavailableSetter = global.unavailableSetter
   var unavailableSetter_a = global.unavailableSetter.a
-  var unavailableSetter_0 = global.unavailableSetter[0] // expected-error {{setter for 'unavailableSetter' is unavailable}} FIXME: setter should not be diagnosed
-  var unavailableSetter_0_b = global.unavailableSetter[0].b // expected-error {{setter for 'unavailableSetter' is unavailable}}
+  var unavailableSetter_0 = global.unavailableSetter[0]
+  var unavailableSetter_0_b = global.unavailableSetter[0].b
 
   var unavailableGetterAndSetter = global.unavailableGetterAndSetter // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
   var unavailableGetterAndSetter_a = global.unavailableGetterAndSetter.a // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
-  var unavailableGetterAndSetter_0 = global.unavailableGetterAndSetter[0] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}} FIXME: setter should not be diagnosed
-  var unavailableGetterAndSetter_0_b = global.unavailableGetterAndSetter[0].b // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}} expected-error {{setter for 'unavailableGetterAndSetter' is unavailable}} FIXME: setter should not be diagnosed
+  var unavailableGetterAndSetter_0 = global.unavailableGetterAndSetter[0] // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
+  var unavailableGetterAndSetter_0_b = global.unavailableGetterAndSetter[0].b // expected-error {{getter for 'unavailableGetterAndSetter' is unavailable}}
 }