SIL: Fix bug in AbstractionPattern::getFunctionThrownErrorType()

When storing a closure with type `() throws(any Error) -> ()` as
a fully opaque error, we want to leave it unchanged, instead of
re-abstracting it to throw the error indirectly. Thus, we had
a special carveout here.

However, the carveout was too broad, because if the thrown error
type contained type parameters, the resulting AbstractionPattern
was invalid.

While fixing this I realized this entire hack is unsound in some
cases, if you view the same value as a `() throws(U) -> ()` vs
an `() -> throws(any Error) -> ()`.

Perhaps we should always box the thrown error when maximally
abstracting a closure, but that would also be an ABI break.

- Fixes https://github.com/swiftlang/swift/issues/84051

lib/SIL/IR/AbstractionPattern.cpp

@@ -1366,7 +1366,14 @@ AbstractionPattern::getFunctionThrownErrorType(
     if (!substErrorType)
       return std::nullopt;
 
-    return std::make_pair(AbstractionPattern(*substErrorType),
+    // FIXME: This is actually unsound. The most opaque form of
+    // `(T) throws(U) -> V` should actually be
+    // `(T) throws(any Error) -> V`.
+    auto pattern = ((*substErrorType)->isErrorExistentialType()
+                    ? AbstractionPattern(*substErrorType)
+                    : AbstractionPattern::getOpaque());
+
+    return std::make_pair(pattern,
                           (*substErrorType)->getCanonicalType());
   }
 

test/SILGen/keypath_typed_throws.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swift-emit-silgen %s
+// RUN: %target-swift-emit-silgen %s -enable-library-evolution
+// RUN: %target-swift-emit-silgen %s -enable-testing
+// RUN: %target-swift-emit-silgen %s -enable-library-evolution -enable-testing
+
+public struct Visitor<Node, Failure: Error> {
+  public var visit: (Node) throws(Failure) -> Void
+}