Sema: Diagnose @backDeployed on functions with opaque result types.

The compiler does not yet implement support for back deploying opaque result
types.

Resolves rdar://110806234

Author: tshortli

include/swift/AST/DiagnosticsSema.def

@@ -6976,6 +6976,10 @@ ERROR(attr_incompatible_with_back_deploy,none,
       "'%0' cannot be applied to a back deployed %1",
       (DeclAttribute, DescriptiveDeclKind))
 
+ERROR(backdeployed_opaque_result_not_supported,none,
+      "'%0' is unsupported on a %1 with a 'some' return type",
+      (DeclAttribute, DescriptiveDeclKind))
+
 //------------------------------------------------------------------------------
 // MARK: Implicit opening of existential types
 //------------------------------------------------------------------------------

lib/Sema/TypeCheckAttr.cpp

@@ -4324,6 +4324,14 @@ void AttributeChecker::checkBackDeployedAttrs(
       }
     }
 
+    if (VD->getOpaqueResultTypeDecl()) {
+      diagnoseAndRemoveAttr(Attr,
+                            diag::backdeployed_opaque_result_not_supported,
+                            Attr, D->getDescriptiveKind())
+          .warnInSwiftInterface(D->getDeclContext());
+      continue;
+    }
+
     auto AtLoc = Attr->AtLoc;
     auto Platform = Attr->Platform;
 

test/attr/attr_backDeployed.swift

@@ -261,6 +261,21 @@ protocol CannotBackDeployProtocol {}
 @backDeployed(before: macOS 12.0) // expected-error {{'@backDeployed' attribute cannot be applied to this declaration}}
 public actor CannotBackDeployActor {}
 
+public struct ConformsToTopLevelProtocol: TopLevelProtocol {
+  public init() {}
+}
+
+@available(SwiftStdlib 5.1, *)
+@backDeployed(before: macOS 12.0) // expected-error {{'@backDeployed' is unsupported on a var with a 'some' return type}}
+public var cannotBackDeployVarWithOpaqueResultType: some TopLevelProtocol {
+  return ConformsToTopLevelProtocol()
+}
+
+@available(SwiftStdlib 5.1, *)
+@backDeployed(before: macOS 12.0) // expected-error {{'@backDeployed' is unsupported on a global function with a 'some' return type}}
+public func cannotBackDeployFuncWithOpaqueResultType() -> some TopLevelProtocol {
+  return ConformsToTopLevelProtocol()
+}
 
 // MARK: - Function body diagnostics