[cxx-interop] Prohibit weak references to foreign reference types

This fixes a runtime crash when a `weak` reference to a C++ foreign reference type is used.

Instead of a runtime crash, Swift would now emit a compiler error saying that `weak` keyword is incompatible with foreign reference types.

rdar://124040825 / resolves swiftlang#83080

Author: egorzhdan

include/swift/AST/DiagnosticsSema.def

@@ -6357,6 +6357,9 @@ ERROR(invalid_ownership_protocol_type,none,
 ERROR(invalid_ownership_incompatible_class,none,
       "%0 is incompatible with %1 references",
       (Type, ReferenceOwnership))
+ERROR(invalid_ownership_incompatible_foreign_reference_type,none,
+      "%0 is incompatible with 'weak' references, because it is a foreign reference type",
+      (Type))
 ERROR(invalid_ownership_with_optional,none,
       "%0 variable cannot have optional type", (ReferenceOwnership))
 ERROR(invalid_ownership_not_optional,none,

lib/Sema/TypeCheckAttr.cpp

@@ -5451,13 +5451,28 @@ Type TypeChecker::checkReferenceOwnershipAttr(VarDecl *var, Type type,
   }
 
   ClassDecl *underlyingClass = underlyingType->getClassOrBoundGenericClass();
-  if (underlyingClass && underlyingClass->isIncompatibleWithWeakReferences()) {
-    Diags
-        .diagnose(attr->getLocation(),
-                  diag::invalid_ownership_incompatible_class, underlyingType,
-                  ownershipKind)
-        .fixItRemove(attr->getRange());
-    attr->setInvalid();
+  if (underlyingClass) {
+    if (underlyingClass->isIncompatibleWithWeakReferences()) {
+      Diags
+          .diagnose(attr->getLocation(),
+                    diag::invalid_ownership_incompatible_class, underlyingType,
+                    ownershipKind)
+          .fixItRemove(attr->getRange());
+      attr->setInvalid();
+    }
+    // Foreign reference types cannot be held as weak references, since the
+    // Swift runtime has no way to make the pointer null when the object is
+    // deallocated.
+    // Unowned references to foreign reference types are supported.
+    if (ownershipKind == ReferenceOwnership::Weak &&
+        underlyingClass->isForeignReferenceType()) {
+      Diags
+          .diagnose(attr->getLocation(),
+                    diag::invalid_ownership_incompatible_foreign_reference_type,
+                    underlyingType)
+          .fixItRemove(attr->getRange());
+      attr->setInvalid();
+    }
   }
 
   auto PDC = dyn_cast<ProtocolDecl>(dc);

test/Interop/Cxx/foreign-reference/weak-reference-typechecker.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift -cxx-interoperability-mode=default -I %S/Inputs -disable-availability-checking
+
+import POD
+
+struct HasWeakReference {
+  weak var x: Empty? // expected-error {{'Empty' is incompatible with 'weak' references, because it is a foreign reference type}}
+}
+
+struct HasUnownedReference {
+  unowned var x: Empty!
+}