SIL verification failure with certain conformances to `Collection`.

[fibrechannelscsi]

Description
The following code fails to compile in Debug mode. The error message is:
SIL verification failed: public/shared function must have a body: F->isDefinition() || F->hasForeignBody()

Steps to reproduce
This reproducer requires four files: two Package.swift files (one is for an internal package), and two regular Swift source files.
The directory tree looks like this:

./Package.swift
./Sources/InternalPackage/Package.swift
./Sources/InternalPackage/Sources/InternalPackage/U.swift
./Sources/ReproducerServer/A.swift

Listing for ./Package.swift:

// swift-tools-version: 5.6
import PackageDescription
let package = Package(
    name: "Reproducer",
    platforms: [.macOS(.v12),],
    products: [.library(name: "Reproducer", targets: ["ReproducerServer"]),],
    dependencies: [.package(name: "InternalPackage", path: "./Sources/InternalPackage")],
    targets: [.target(name: "ReproducerServer", dependencies: [.product(name: "InternalPackage", package: "InternalPackage")])]
)

Listing for ./Sources/InternalPackage/Package.swift:

// swift-tools-version: 5.6
// The swift-tools-version declares the minimum version of Swift required to build this package.
import PackageDescription
let package = Package(
    name: "InternalPackage",
    platforms: [.macOS(.v12), .iOS(.v15)],
    products: [.library(name: "InternalPackage",targets: ["InternalPackage"])],
    dependencies: [.package(url: "https://github.com/apple/swift-collections.git",from: "1.0.4")],
    targets: [.target(name: "InternalPackage",dependencies: [.product(name: "Collections", package: "swift-collections")])]
)

Listing for ./Sources/InternalPackage/Sources/InternalPackage/U.swift:

import Foundation; import OrderedCollections
public struct L<Root>{}
public typealias LV<V> = OrderedDictionary<String, B<V>>
public typealias B<V> = OrderedDictionary<String, V>
public protocol LP: Collection where Index == OrderedDictionary<Key, L<Root>>.Index, Element == OrderedDictionary<Key, L<Root>>.Element{
    associatedtype Root
    associatedtype Key: Hashable
    var l: OrderedDictionary<Key, L<Root>> { get set }
}
public extension LP {
    var startIndex: Index {self.l.elements.startIndex}
    var endIndex: Index {self.l.elements.endIndex}
    var indices: Range<Index> {self.l.elements.indices}
    func index(after index: Index) -> Index {fatalError()}
    subscript(index: Index) -> Element {self.l.elements[index]}
}
public protocol BGP: LP {}
public struct BG<Root, Key: Hashable>: BGP {
    public typealias Index = OrderedDictionary<Key, L<Root>>.Index
    public typealias Element = OrderedDictionary<Key, L<Root>>.Element
    public var l = OrderedDictionary<Key, L<Root>>()
}

Listing for ./Sources/ReproducerServer/A.swift:

import Foundation; import Collections; import InternalPackage
public final class C<Graph: A>: U<Graph.Root>, LP {
    public var l: OrderedDictionary<QOT, L<Root>>
    public typealias Index = OrderedDictionary<QOT, L<Root>>.Index
    public typealias Element = OrderedDictionary<QOT, L<Root>>.Element
    public typealias Root = Graph.Root
    public init(){fatalError()}
    required init(from decoder: any Decoder) throws {fatalError("")}
}
public enum QOT: String, Codable, Hashable, Comparable, CaseIterable {
    case A
    public static func < (lhs: QOT, rhs: QOT) -> Bool {return true}
}
public protocol A: Q {
    associatedtype Root: QO
    var root: Root { get }
}
public class Q: Codable {}
public protocol QO: AnyObject{}
public class U<Root: QO>: Q, A {
    public var root: Root
    public init (root: Root){fatalError()}
    required init(from decoder: any Decoder) throws {fatalError()}
}

Note that condensing the contents of A.swift and U.swift into a single file causes the build to succeed.

Expected behavior
The compilation should complete successfully.

Environment

• Swift compiler version info: Toolchain 2023-05-31a exhibits this issue. Note that the default Xcode 14.2 toolchain is able to compile this successfully.
• Xcode version info: 14.2
• Deployment target: M1

Additional context
Stack trace:

1.	Apple Swift version 5.9-dev (LLVM fd15f782007e262, Swift 65bb599048dbd75)
2.	Compiling with the current language version
3.	While evaluating request ASTLoweringRequest(Lowering AST to SIL for file "/Users/user/playground/A/Sources/ReproducerServer/A.swift")
4.	While verifying SIL function "@$s15InternalPackage2LPPAAEy3KeyQz3key_AA1LVy4RootQzG5valuetSicir".
 for read for subscript(_:) (in module 'InternalPackage')
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  swift-frontend           0x00000001071c869c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x00000001071c7a40 llvm::sys::RunSignalHandlers() + 112
2  swift-frontend           0x00000001071c8cdc SignalHandler(int) + 304
3  libsystem_platform.dylib 0x0000000197f0ea24 _sigtramp + 56
4  libsystem_pthread.dylib  0x0000000197edfc28 pthread_kill + 288
5  libsystem_c.dylib        0x0000000197dedae8 abort + 180
6  swift-frontend           0x00000001032845a8 (anonymous namespace)::SILVerifier::_require(bool, llvm::Twine const&, std::__1::function<void ()> const&) + 1472
7  swift-frontend           0x0000000103284e88 (anonymous namespace)::SILVerifier::visitSILFunction(swift::SILFunction*) + 640
8  swift-frontend           0x0000000103280ba8 swift::SILFunction::verify(swift::SILPassManager*, bool, bool, bool) const + 204
9  swift-frontend           0x0000000103283a38 swift::SILModule::verify(swift::SILPassManager*, bool, bool) const + 184
10 swift-frontend           0x0000000103283948 swift::SILModule::verify(bool, bool) const + 88
11 swift-frontend           0x0000000102a31b50 swift::Lowering::SILGenModule::~SILGenModule() + 168
12 swift-frontend           0x0000000102a3ae94 swift::ASTLoweringRequest::evaluate(swift::Evaluator&, swift::ASTLoweringDescriptor) const + 5136
13 swift-frontend           0x0000000102afeda8 swift::SimpleRequest<swift::ASTLoweringRequest, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>> (swift::ASTLoweringDescriptor), (swift::RequestFlags)9>::evaluateRequest(swift::ASTLoweringRequest const&, swift::Evaluator&) + 148
14 swift-frontend           0x0000000102a3db98 llvm::Expected<swift::ASTLoweringRequest::OutputType> swift::Evaluator::getResultUncached<swift::ASTLoweringRequest>(swift::ASTLoweringRequest const&) + 396
15 swift-frontend           0x0000000102a3b120 swift::performASTLowering(swift::FileUnit&, swift::Lowering::TypeConverter&, swift::SILOptions const&, swift::IRGenOptions const*) + 100
16 swift-frontend           0x000000010251476c swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 524
17 swift-frontend           0x0000000102523988 withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) + 160
18 swift-frontend           0x0000000102517278 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 724
19 swift-frontend           0x000000010251620c swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2456
20 swift-frontend           0x0000000102360940 swift::mainEntry(int, char const**) + 2144
21 dyld                     0x0000000197b87f28 start + 2236

Additional SIL-related information:

SIL verification failed: public/shared function must have a body: F->isDefinition() || F->hasForeignBody()
In function:
// LP.subscript.read
sil shared [serialized] @$s15InternalPackage2LPPAAEy3KeyQz3key_AA1LVy4RootQzG5valuetSicir : $@yield_once @convention(method) <τ_0_0 where τ_0_0 : LP> (Int, @in_guaranteed τ_0_0) -> (@yields @in_guaranteed τ_0_0.Key, @yields L<τ_0_0.Root>)

If we remove the conformance of LP to Collection, that is, we change line 5 of U.swift to:
public protocol LP {
and then delete the extension of LP (that is, deleting lines 10 to 16 of the same file), then the compilation succeeds.

[asl]

Non-differentiability diagnostics is missed somewhere. We do not support differentiating read semantic members accessors and co-routines.

[asl]

So, likely similar to #54404 and around

[asavonic]

This issue is not related to differentiation, and the reproducer does not use it at all.

When the module U.swift defines a coroutine read accessor (subscript), and another module (A.swift) imports it, the accessor decl is deserialized without a body and it does not pass the SIL Verifier:

    SIL verification failed: public/shared function must have a body: F->isDefinition() || F->hasForeignBody()
    In function:
    // LP.subscript.read
    sil shared [serialized] @$s15InternalPackage2LPPAAEy3KeyQz3key_AA1LVy4RootQzG5valuetSicir :
      $@yield_once @convention(method) <τ_0_0 where τ_0_0 : LP> (Int, @in_guaranteed τ_0_0) ->
      (@yields @in_guaranteed τ_0_0.Key, @yields L<τ_0_0.Root>)

It is not clear to me how A.swift is supposed to define it. U.swift does it in getSynthesizedAccessor, but this function early-exists for A.swift, because the accessor already exists (but doesn't have a body)

  frame #0: 0x0000555558f0772e swift-frontend`swift::AbstractStorageDecl::getSynthesizedAccessor(this=0x0000555566aa90f8, kind=Read) const at Decl.cpp:2639:12
     2636
     2637	AccessorDecl *AbstractStorageDecl::getSynthesizedAccessor(AccessorKind kind) const {
     2638	  if (auto *accessor = getAccessor(kind))
  -> 2639	    return accessor;

I've tried the following patch, but it crashes later in IRGen, and I'm not sure if this is a correct way to fix this issue.
If anyone knows how imported accessors are supposed to be defined, please let me know.

diff --git a/lib/AST/Decl.cpp b/lib/AST/Decl.cpp
index fb73ad1454a..9ff761dc98f 100644
--- a/lib/AST/Decl.cpp
+++ b/lib/AST/Decl.cpp
@@ -2636,7 +2636,8 @@ bool AbstractStorageDecl::requiresOpaqueModifyCoroutine() const {
 
 AccessorDecl *AbstractStorageDecl::getSynthesizedAccessor(AccessorKind kind) const {
   if (auto *accessor = getAccessor(kind))
-    return accessor;
+    if (accessor->getBodyKind() != AccessorDecl::BodyKind::None)
+      return accessor;
 
   ASTContext &ctx = getASTContext();
   return evaluateOrDefault(ctx.evaluator,
diff --git a/lib/AST/TypeCheckRequests.cpp b/lib/AST/TypeCheckRequests.cpp
index c4bcb953087..0ade927c8e8 100644
--- a/lib/AST/TypeCheckRequests.cpp
+++ b/lib/AST/TypeCheckRequests.cpp
@@ -759,7 +759,7 @@ SynthesizeAccessorRequest::getCachedResult() const {
   auto *storage = std::get<0>(getStorage());
   auto kind = std::get<1>(getStorage());
   auto *accessor = storage->getAccessor(kind);
-  if (accessor)
+  if (accessor && accessor->getBodyKind() != AccessorDecl::BodyKind::None)
     return accessor;
   return llvm::None;
 }
@@ -767,7 +767,10 @@ SynthesizeAccessorRequest::getCachedResult() const {
 void SynthesizeAccessorRequest::cacheResult(AccessorDecl *accessor) const {
   auto *storage = std::get<0>(getStorage());
   auto kind = std::get<1>(getStorage());
-
+  auto *existingAccessor = storage->getAccessor(kind);
+  if (existingAccessor) {
+    return;
+  }
   storage->setSynthesizedAccessor(kind, accessor);
 }
 
diff --git a/lib/Sema/TypeCheckStorage.cpp b/lib/Sema/TypeCheckStorage.cpp
index 89d46decbaf..afc486f5ea3 100644
--- a/lib/Sema/TypeCheckStorage.cpp
+++ b/lib/Sema/TypeCheckStorage.cpp
@@ -2359,12 +2359,15 @@ createCoroutineAccessorPrototype(AbstractStorageDecl *storage,
   // Coroutine accessors always return ().
   const Type retTy = TupleType::getEmpty(ctx);
 
-  auto *accessor = AccessorDecl::create(
+  auto *accessor = storage->getAccessor(kind);
+  if (!accessor) {
+    accessor = AccessorDecl::create(
       ctx, loc, /*AccessorKeywordLoc=*/SourceLoc(), kind, storage,
       /*StaticLoc=*/SourceLoc(), StaticSpellingKind::None,
       /*Async=*/false, /*AsyncLoc=*/SourceLoc(),
       /*Throws=*/false, /*ThrowsLoc=*/SourceLoc(), params, retTy, dc);
-  accessor->setSynthesized();
+    accessor->setSynthesized();
+  }
 
   if (isMutating)
     accessor->setSelfAccessKind(SelfAccessKind::Mutating);

[tbkka]

Can you simplify the reproducer further?

[fibrechannelscsi]

I can get A.swift down to:

import Foundation; import Collections; import InternalPackage
public final class C<Graph: A>: LP {
    public var l: OrderedDictionary<String, L<Graph.Root>>
    public typealias Index = OrderedDictionary<String, L<Graph.Root>>.Index
    required init(from decoder: any Decoder) throws {fatalError("")}
}
public protocol A {associatedtype Root: AnyObject; var root: Root { get }}

and U.swift down to:

import Foundation; import OrderedCollections
public struct L<Root>{}
public protocol LP: Collection where Index == OrderedDictionary<Key, L<Root>>.Index, Element == OrderedDictionary<Key, L<Root>>.Element{
    associatedtype Root
    associatedtype Key: Hashable
    var l: OrderedDictionary<Key, L<Root>> { get set }
}
public extension LP {
    var startIndex: Index {self.l.elements.startIndex}
    var endIndex: Index {self.l.elements.endIndex}
    var indices: Range<Index> {self.l.elements.indices}
    func index(after index: Index) -> Index {fatalError()}
    subscript(index: Index) -> Element {self.l.elements[index]}
}
public struct BG<Root, Key: Hashable>: LP {
    public typealias Index = OrderedDictionary<Key, L<Root>>.Index
    public typealias Element = OrderedDictionary<Key, L<Root>>.Element
    public var l = OrderedDictionary<Key, L<Root>>()
}

Note that deleting all of struct BG causes the compilation to succeed.