Sema: Diagnose @backDeployed functions with missing bodies in swiftinterfaces.

A `@backDeployed` function printed in a `.swiftinterface` must have a function
body so that SILGen can emit a fallback copy to call when the back deployed
function is unavailable. Previously, the compiler would crash in SILGen when
compiling an interface containing a back deployed function without a body.

Resolves rdar://141593108.

include/swift/AST/DiagnosticsSema.def

@@ -7567,13 +7567,17 @@ ERROR(cannot_convert_default_value_type_to_argument_type, none,
 // MARK: Back deployment
 //------------------------------------------------------------------------------
 
-ERROR(attr_incompatible_with_back_deploy,none,
-      "'%0' cannot be applied to a back deployed %1",
-      (DeclAttribute, DescriptiveDeclKind))
+ERROR(attr_incompatible_with_back_deployed,none,
+      "'%0' cannot be applied to a back deployed %kind1",
+      (DeclAttribute, const Decl *))
 
-WARNING(backdeployed_opaque_result_not_supported,none,
-      "'%0' is unsupported on a %1 with a 'some' return type",
-      (DeclAttribute, DescriptiveDeclKind))
+WARNING(back_deployed_opaque_result_not_supported,none,
+      "'%0' cannot be applied to %kind1 because it has a 'some' return type",
+      (DeclAttribute, const ValueDecl *))
+
+ERROR(back_deployed_requires_body,none,
+      "'%0' requires that %kind1 have a body",
+      (DeclAttribute, const ValueDecl *))
 
 //------------------------------------------------------------------------------
 // MARK: Implicit opening of existential types

lib/Sema/TypeCheckAttr.cpp

@@ -4873,13 +4873,13 @@ void AttributeChecker::checkBackDeployedAttrs(
   // back deployment, which is to use the ABI version of the declaration when it
   // is available.
   if (auto *AEICA = D->getAttrs().getAttribute<AlwaysEmitIntoClientAttr>()) {
-    diagnoseAndRemoveAttr(AEICA, diag::attr_incompatible_with_back_deploy,
-                          AEICA, D->getDescriptiveKind());
+    diagnoseAndRemoveAttr(AEICA, diag::attr_incompatible_with_back_deployed,
+                          AEICA, D);
   }
 
   if (auto *TA = D->getAttrs().getAttribute<TransparentAttr>()) {
-    diagnoseAndRemoveAttr(TA, diag::attr_incompatible_with_back_deploy, TA,
-                          D->getDescriptiveKind());
+    diagnoseAndRemoveAttr(TA, diag::attr_incompatible_with_back_deployed, TA,
+                          D);
   }
 
   // Only functions, methods, computed properties, and subscripts are
@@ -4887,7 +4887,9 @@ void AttributeChecker::checkBackDeployedAttrs(
   auto *VD = cast<ValueDecl>(D);
   std::map<PlatformKind, SourceLoc> seenPlatforms;
 
-  auto *ActiveAttr = D->getAttrs().getBackDeployed(Ctx, false);
+  const BackDeployedAttr *ActiveAttr = nullptr;
+  if (D->getBackDeployedBeforeOSVersion(Ctx))
+    ActiveAttr = D->getAttrs().getBackDeployed(Ctx, false);
 
   for (auto *Attr : Attrs) {
     // Back deployment only makes sense for public declarations.
@@ -4931,19 +4933,9 @@ void AttributeChecker::checkBackDeployedAttrs(
       continue;
     }
 
-    if (auto *VarD = dyn_cast<VarDecl>(D)) {
-      // There must be a function body to back deploy so for vars we require
-      // that they be computed in order to allow back deployment.
-      if (VarD->hasStorageOrWrapsStorage()) {
-        diagnoseAndRemoveAttr(Attr, diag::attr_not_on_stored_properties, Attr);
-        continue;
-      }
-    }
-
     if (VD->getOpaqueResultTypeDecl()) {
-      diagnoseAndRemoveAttr(Attr,
-                            diag::backdeployed_opaque_result_not_supported,
-                            Attr, D->getDescriptiveKind())
+      diagnoseAndRemoveAttr(
+          Attr, diag::back_deployed_opaque_result_not_supported, Attr, VD)
           .warnInSwiftInterface(D->getDeclContext());
       continue;
     }
@@ -4960,12 +4952,29 @@ void AttributeChecker::checkBackDeployedAttrs(
       continue;
     }
 
-    if (Ctx.LangOpts.DisableAvailabilityChecking)
+    // The remaining diagnostics can only be diagnosed for attributes that
+    // apply to the active platform.
+    if (Attr != ActiveAttr)
       continue;
 
-    // Availability conflicts can only be diagnosed for attributes that apply
-    // to the active platform.
-    if (Attr != ActiveAttr)
+    if (auto *VarD = dyn_cast<VarDecl>(D)) {
+      // There must be a function body to back deploy so for vars we require
+      // that they be computed in order to allow back deployment.
+      if (VarD->hasStorageOrWrapsStorage()) {
+        diagnoseAndRemoveAttr(Attr, diag::attr_not_on_stored_properties, Attr);
+        continue;
+      }
+    }
+
+    if (auto *AFD = dyn_cast<AbstractFunctionDecl>(D)) {
+      if (!AFD->hasBody()) {
+        diagnoseAndRemoveAttr(Attr, diag::back_deployed_requires_body, Attr,
+                              VD);
+        continue;
+      }
+    }
+
+    if (Ctx.LangOpts.DisableAvailabilityChecking)
       continue;
 
     auto availability =

test/ModuleInterface/BackDeployedAttrBad.swiftinterface

@@ -0,0 +1,24 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: not %target-swift-typecheck-module-from-interface(%t/Test.swiftinterface) -module-name Test 2>&1 | %FileCheck %s
+
+// REQUIRES: OS=macosx || OS=ios || OS=tvos || OS=watchos || OS=visionos
+
+// This test uses split-file because the check lines cannot appear as comments
+// in the interface (they'd match themselves in the diagnostic output).
+// FIXME: -verify should work for -typecheck-module-from-interface
+
+// CHECK: Test.swiftinterface:5:2: error: '@backDeployed' requires that global function 'backDeployedFuncWithoutBody()' have a body
+// CHECK: Test.swiftinterface:9:2: error: '@backDeployed' must not be used on stored properties
+
+//--- Test.swiftinterface
+// swift-interface-format-version: 1.0
+// swift-module-flags:
+
+@available(macOS 14.4, iOS 17.4, watchOS 10.4, tvOS 17.4, visionOS 1.1, *)
+@backDeployed(before: macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0)
+public func backDeployedFuncWithoutBody()
+
+@available(macOS 14.4, iOS 17.4, watchOS 10.4, tvOS 17.4, visionOS 1.1, *)
+@backDeployed(before: macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0)
+public var backDeployedVarWithoutBody: Int

test/ModuleInterface/BackDeployedAttrGood.swiftinterface

@@ -0,0 +1,16 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags:
+
+// RUN: %target-swift-typecheck-module-from-interface(%s) -module-name Test
+// REQUIRES: OS=macosx
+
+// Since the following declarations are only back deployed on iOS, their bodies
+// should be missing in a `.swiftinterface` compiled for macOS
+
+@available(iOS 17.4, *)
+@backDeployed(before: iOS 18.0)
+public func backDeployedFuncOniOSWithoutBody()
+
+@available(iOS 17.4, *)
+@backDeployed(before: iOS 18.0)
+public var backDeployedVarWithoutBody: Int

test/Serialization/ignore-opaque-underlying-type-back-deploy.swift

@@ -57,7 +57,7 @@ public struct EV : V {
 @available(SwiftStdlib 5.1, *)
 public extension V {
 // CHECK: Loading underlying information for opaque type of 'backdeployedOpaqueFunc()'
-  @backDeployed(before: SwiftStdlib 5.1) // expected-warning 4 {{'@backDeployed' is unsupported on a instance method with a 'some' return type}}
+  @backDeployed(before: SwiftStdlib 5.1) // expected-warning 4 {{'@backDeployed' cannot be applied to instance method 'backdeployedOpaqueFunc()' because it has a 'some' return type}}
   func backdeployedOpaqueFunc() -> some V { EV() }
 }
 

test/attr/attr_backDeployed.swift

@@ -251,6 +251,9 @@ public enum CannotBackDeployEnum {
 @backDeployed(before: macOS 12.0) // expected-error {{'@backDeployed' must not be used on stored properties}}
 public var cannotBackDeployTopLevelVar = 79
 
+@backDeployed(before: iOS 15.0) // OK, this can only be diagnosed when compiling for iOS
+public var cannotBackDeployTopLevelVarOniOS = 79
+
 @backDeployed(before: macOS 12.0) // expected-error {{'@backDeployed' attribute cannot be applied to this declaration}}
 extension TopLevelStruct {}
 
@@ -266,13 +269,13 @@ public struct ConformsToTopLevelProtocol: TopLevelProtocol {
 }
 
 @available(SwiftStdlib 5.1, *)
-@backDeployed(before: macOS 12.0) // expected-warning {{'@backDeployed' is unsupported on a var with a 'some' return type}}
+@backDeployed(before: macOS 12.0) // expected-warning {{'@backDeployed' cannot be applied to var 'cannotBackDeployVarWithOpaqueResultType' because it has a 'some' return type}}
 public var cannotBackDeployVarWithOpaqueResultType: some TopLevelProtocol {
   return ConformsToTopLevelProtocol()
 }
 
 @available(SwiftStdlib 5.1, *)
-@backDeployed(before: macOS 12.0) // expected-warning {{'@backDeployed' is unsupported on a global function with a 'some' return type}}
+@backDeployed(before: macOS 12.0) // expected-warning {{'@backDeployed' cannot be applied to global function 'cannotBackDeployFuncWithOpaqueResultType()' because it has a 'some' return type}}
 public func cannotBackDeployFuncWithOpaqueResultType() -> some TopLevelProtocol {
   return ConformsToTopLevelProtocol()
 }