From 0619a83380703bac86eb8a762ab0931762e30759 Mon Sep 17 00:00:00 2001
From: Allan Shortlidge <shortlidge@apple.com>
Date: Fri, 13 Dec 2024 16:41:17 -0800
Subject: [PATCH] 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.
---
 lib/SIL/IR/SILDeclRef.cpp            |  8 +++++
 test/IRGen/silgen_name_linkage.swift | 49 ++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)
 create mode 100644 test/IRGen/silgen_name_linkage.swift

diff --git a/lib/SIL/IR/SILDeclRef.cpp b/lib/SIL/IR/SILDeclRef.cpp
index ae96bd37a6cdf..3180203db0b99 100644
--- a/lib/SIL/IR/SILDeclRef.cpp
+++ b/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:
diff --git a/test/IRGen/silgen_name_linkage.swift b/test/IRGen/silgen_name_linkage.swift
new file mode 100644
index 0000000000000..9993727edd0e0
--- /dev/null
+++ b/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()