SIL: Always give @_silgen_name forward declarations public linkage.

When `@_silgen_name` is applied to a function with no body, it is a forward
declaration. It therefore must be treated as an external (public) declaration
regardless of the access level it was given in source.

Resolves rdar://141436934.

Author: tshortli

lib/SIL/IR/SILDeclRef.cpp

@@ -651,6 +651,14 @@ SILLinkage SILDeclRef::getDefinitionLinkage() const {
       effectiveAccess = std::max(effectiveAccess, AccessLevel::Internal);
   }
 
+  // Declarations with a @_silgen_name attribute and no body may be forward
+  // declarations of functions defined in another module. Therefore they must
+  // always have external (public) linkage, regardless of declared access level.
+  if (auto afd = getAbstractFunctionDecl()) {
+    if (!afd->hasBody() && afd->getAttrs().hasAttribute<SILGenNameAttr>())
+      effectiveAccess = AccessLevel::Public;
+  }
+
   switch (effectiveAccess) {
   case AccessLevel::Private:
   case AccessLevel::FilePrivate:

test/IRGen/silgen_name_linkage.swift

@@ -0,0 +1,49 @@
+// RUN: %target-swift-frontend -parse-as-library -emit-ir %s | %FileCheck %s
+
+// Since this test depends on weak linking based on availability, it only
+// applies to Apple platforms.
+// REQUIRES: OS=macosx || OS=ios || OS=tvos || OS=watchos || OS=visionos
+
+@available(SwiftStdlib 9999, *)
+@_silgen_name("privateForwardDecl")
+private func privateForwardDecl()
+
+@available(SwiftStdlib 9999, *)
+@_silgen_name("internalForwardDecl")
+internal func internalForwardDecl()
+
+@available(SwiftStdlib 9999, *)
+@_silgen_name("publicForwardDecl")
+public func publicForwardDecl()
+
+@available(SwiftStdlib 9999, *)
+@_silgen_name("privateDefined")
+private func privateDefined() {}
+
+// CHECK: define internal swiftcc void @privateDefined()
+
+@available(SwiftStdlib 9999, *)
+@_silgen_name("internalDefined")
+internal func internalDefined() {}
+
+// CHECK: define hidden swiftcc void @internalDefined()
+
+@available(SwiftStdlib 9999, *)
+@_silgen_name("publicDefined")
+public func publicDefined() {}
+
+// CHECK: define swiftcc void @publicDefined()
+
+public func test() {
+  guard #available(SwiftStdlib 9999, *) else { return }
+  privateForwardDecl()
+  internalForwardDecl()
+  publicForwardDecl()
+  privateDefined()
+  internalDefined()
+  publicDefined()
+}
+
+// CHECK: declare extern_weak swiftcc void @privateForwardDecl()
+// CHECK: declare extern_weak swiftcc void @internalForwardDecl()
+// CHECK: declare extern_weak swiftcc void @publicForwardDecl()