diff --git a/lib/SIL/IR/SILFunctionType.cpp b/lib/SIL/IR/SILFunctionType.cpp
index 83400b833e382..468ede36a1d20 100644
--- a/lib/SIL/IR/SILFunctionType.cpp
+++ b/lib/SIL/IR/SILFunctionType.cpp
@@ -3605,7 +3605,9 @@ CanSILFunctionType swift::buildSILFunctionThunkType(
 
   if (!capturedEnvs.empty() ||
       expectedType->hasPrimaryArchetype() ||
-      sourceType->hasPrimaryArchetype()) {
+      sourceType->hasPrimaryArchetype() ||
+      (inputSubstType && inputSubstType->hasPrimaryArchetype()) ||
+      (outputSubstType && outputSubstType->hasPrimaryArchetype())) {
     // Get the existing generic signature.
     baseGenericSig = fn->getLoweredFunctionType()
         ->getInvocationGenericSignature();
diff --git a/test/SILGen/opaque_result_type_thunk.swift b/test/SILGen/opaque_result_type_thunk.swift
new file mode 100644
index 0000000000000..dfacbabb2dd34
--- /dev/null
+++ b/test/SILGen/opaque_result_type_thunk.swift
@@ -0,0 +1,19 @@
+// RUN: %target-swift-emit-silgen %s
+
+// https://github.com/swiftlang/swift/issues/86118
+
+// The AST type of the thunk depends on the generic signature, but the
+// lowered type does not, because we can see the opaque return type's
+// underlying type from the expansion point, and it does not involve
+// type parameters. Make sure this does not cause us to assert.
+
+public struct G<T> {
+  public static func f(_: Any, _: Any) -> some Any {
+    return 123
+  }
+}
+
+public func g<T>(_: T) {
+  let fn: (Any, Any) -> _ = G<T>.f
+  let fn2: (Int, Int) -> _ = fn
+}