Merge pull request #66673 from tshortli/diagnose-back-deployed-opaque-result-types

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

include/swift/AST/DiagnosticEngine.h

@@ -564,6 +564,15 @@ namespace swift {
     /// until the next major language version.
     InFlightDiagnostic &warnUntilSwiftVersion(unsigned majorVersion);
 
+    /// Limit the diagnostic behavior to warning if the context is a
+    /// swiftinterface.
+    ///
+    /// This is useful for diagnostics for restrictions that may be lifted by a
+    /// future version of the compiler. In such cases, it may be helpful to
+    /// avoid failing to build a module from its interface if the interface was
+    /// emitted using a compiler that no longer has the restriction.
+    InFlightDiagnostic &warnInSwiftInterface(const DeclContext *context);
+
     /// Conditionally limit the diagnostic behavior to warning until
     /// the specified version.  If the condition is false, no limit is
     /// imposed, meaning (presumably) it is treated as an error.

include/swift/AST/DiagnosticsSema.def

@@ -7053,6 +7053,10 @@ 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
 //------------------------------------------------------------------------------