From e0139bcc0ca8825c1b932e40306ee46daf041ad3 Mon Sep 17 00:00:00 2001
From: Doug Gregor <dgregor@apple.com>
Date: Thu, 13 Nov 2025 11:34:01 -0800
Subject: [PATCH 1/2] Retain the Swift calling convention when it's on an
 imported C declaration

Fixes #69264.
---
 lib/SIL/IR/SILFunctionType.cpp         | 27 +++++++++++++++++++++-----
 test/IRGen/c_calling_conventions.swift | 26 +++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 5 deletions(-)
 create mode 100644 test/IRGen/c_calling_conventions.swift

diff --git a/lib/SIL/IR/SILFunctionType.cpp b/lib/SIL/IR/SILFunctionType.cpp
index 7bf39297ee23e..6c8ed0bbeb0c6 100644
--- a/lib/SIL/IR/SILFunctionType.cpp
+++ b/lib/SIL/IR/SILFunctionType.cpp
@@ -4424,12 +4424,29 @@ TypeConverter::getDeclRefRepresentation(SILDeclRef c) {
     if (!c.hasDecl())
       return SILFunctionTypeRepresentation::CFunctionPointer;
 
-    if (auto method =
-            dyn_cast_or_null<clang::CXXMethodDecl>(c.getDecl()->getClangDecl()))
-      return isa<clang::CXXConstructorDecl>(method) || method->isStatic()
-                 ? SILFunctionTypeRepresentation::CFunctionPointer
-                 : SILFunctionTypeRepresentation::CXXMethod;
+    if (auto clangDecl = c.getDecl()->getClangDecl()) {
+      if (auto method = dyn_cast<clang::CXXMethodDecl>(clangDecl)) {
+        return isa<clang::CXXConstructorDecl>(method) || method->isStatic()
+                   ? SILFunctionTypeRepresentation::CFunctionPointer
+                   : SILFunctionTypeRepresentation::CXXMethod;
+      }
+
+      if (auto function = dyn_cast<clang::FunctionDecl>(clangDecl)) {
+        if (auto fnType = function->getType()->getAs<clang::FunctionType>()) {
+          switch (fnType->getCallConv()) {
+            case clang::CC_Swift:
+              return SILFunctionTypeRepresentation::Thin;
 
+            case clang::CC_C:
+              return SILFunctionTypeRepresentation::CFunctionPointer;
+
+            default:
+              // Fall back to heuristics below.
+              break;
+          }
+        }
+      }
+    }
 
     // For example, if we have a function in a namespace:
     if (c.getDecl()->isImportAsMember())
diff --git a/test/IRGen/c_calling_conventions.swift b/test/IRGen/c_calling_conventions.swift
new file mode 100644
index 0000000000000..805cd148ee4cd
--- /dev/null
+++ b/test/IRGen/c_calling_conventions.swift
@@ -0,0 +1,26 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -I%t %t/caller.swift -emit-ir | %FileCheck %s
+
+//--- c_funcs.h
+
+__attribute__((swiftcall))
+extern void with_swiftcc(void);
+
+//--- module.modulemap
+
+module c_funcs {
+  header "c_funcs.h"
+}
+
+//--- caller.swift
+
+import c_funcs
+
+func test() {
+  // CHECK: call swiftcc void @with_swiftcc()
+  with_swiftcc()
+}
+// CHECK: declare swiftcc void @with_swiftcc()
+
+test()

From 7517425398968924320f026d938eb3d3c1fd2697 Mon Sep 17 00:00:00 2001
From: Doug Gregor <dgregor@apple.com>
Date: Thu, 13 Nov 2025 16:35:02 -0800
Subject: [PATCH 2/2] Update test for Windows

---
 test/IRGen/c_calling_conventions.swift | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/IRGen/c_calling_conventions.swift b/test/IRGen/c_calling_conventions.swift
index 805cd148ee4cd..f33580e86f623 100644
--- a/test/IRGen/c_calling_conventions.swift
+++ b/test/IRGen/c_calling_conventions.swift
@@ -21,6 +21,6 @@ func test() {
   // CHECK: call swiftcc void @with_swiftcc()
   with_swiftcc()
 }
-// CHECK: declare swiftcc void @with_swiftcc()
+// CHECK: declare {{.*}}swiftcc void @with_swiftcc()
 
 test()