From a31a1219a83f85cfeb9bb8fbf502835fb9e70572 Mon Sep 17 00:00:00 2001
From: Erik Eckstein <eeckstein@apple.com>
Date: Tue, 23 Sep 2025 11:37:58 +0200
Subject: [PATCH] GenericSpecializationMangler: don't assume a SIL function
 name can always be demangled

Demangling can fail if the user provides a custom not-demangable `@_silgen_name` (but still containing the "$s" mangling prefix).
---
 .../Utils/GenericSpecializationMangler.cpp    |  4 +++-
 test/SILOptimizer/specialize_ossa.sil         | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/lib/SIL/Utils/GenericSpecializationMangler.cpp b/lib/SIL/Utils/GenericSpecializationMangler.cpp
index 27cf65e1b8cc5..c2ce5017ec17d 100644
--- a/lib/SIL/Utils/GenericSpecializationMangler.cpp
+++ b/lib/SIL/Utils/GenericSpecializationMangler.cpp
@@ -63,8 +63,10 @@ std::string SpecializationMangler::finalize() {
   StringRef FuncName = Function ? Function->getName() : StringRef(FunctionName);
   NodePointer FuncTopLevel = nullptr;
   if (FuncName.starts_with(MANGLING_PREFIX_STR)) {
+    // Demangling can fail, i.e. FuncTopLevel == nullptr, if the user provides
+    // a custom not-demangable `@_silgen_name` (but still containing the "$s"
+    // mangling prefix).
     FuncTopLevel = D.demangleSymbol(FuncName);
-    assert(FuncTopLevel);
   }
   else if (FuncName.starts_with(MANGLING_PREFIX_EMBEDDED_STR)) {
     FuncTopLevel = D.demangleSymbol(FuncName);
diff --git a/test/SILOptimizer/specialize_ossa.sil b/test/SILOptimizer/specialize_ossa.sil
index 72de52e5b8661..3a15dac511c06 100644
--- a/test/SILOptimizer/specialize_ossa.sil
+++ b/test/SILOptimizer/specialize_ossa.sil
@@ -1665,3 +1665,22 @@ bb0(%0 : $Int):
   dealloc_stack %1
   return %11
 }
+
+
+sil hidden [ossa] @$s_custom_not_demangable_name : $@convention(thin) <T> (@thick T.Type) -> () {
+bb0(%0 : $@thick T.Type):
+  %8 = tuple ()
+  return %8 : $()
+}
+
+// CHECK-LABEL: sil [ossa] @call_custom_name :
+// CHECK:         function_ref @$s29$s_custom_not_demangable_nameSi_Ttg5 : $@convention(thin) () -> ()
+// CHECK:       } // end sil function 'call_custom_name'
+sil [ossa] @call_custom_name : $@convention(thin) () -> () {
+bb0:
+  %2 = function_ref @$s_custom_not_demangable_name : $@convention(thin) <τ_0_0> (@thick τ_0_0.Type) -> ()
+  %3 = metatype $@thick Int.Type
+  %4 = apply %2<Int>(%3) : $@convention(thin) <τ_0_0> (@thick τ_0_0.Type) -> ()
+  %5 = tuple ()
+  return %5 : $()
+}