Assertion failure: `GenericEnv && "Expected a GenericEnv"` when building in Release mode.

[fibrechannelscsi]

Description
The compiler generates an assertion failure when attempting to compile the code shown below. The error message is:

Assertion failed: (GenericEnv && "Expected a GenericEnv"), function isLargeLoadableType, file LoadableByAddress.cpp, line 111.

Steps to reproduce
Build the following code in Release mode:

import _Differentiation; import Foundation
@differentiable(reverse)
func f(array: [Double]) -> Double {
    var array = array
    for index in withoutDerivative(at: 0 ..< array.count) {array.update(at: index, byCalling: { (element: inout Double) in let initialElement = element; for _ in withoutDerivative(at: 0 ..< index) {element *= initialElement}})}
    return 0.0
}
public func valueWithPullback<T>(at x: T, of f: @differentiable(reverse) (inout T) -> Void) -> (value: Void, pullback: (inout T.TangentVector) -> Void) {fatalError()}
public func pullback<T>(at x: T, of f: @differentiable(reverse) (inout T) -> Void) -> (inout T.TangentVector) -> Void {return valueWithPullback(at: x, of: f).pullback}
public extension Array {@differentiable(reverse) mutating func update(at index: Int, byCalling closure: @differentiable(reverse) (inout Element) -> Void) where Element: Differentiable {closure(&self[index])}}
public extension Array where Element: Differentiable {
    @derivative(of: update(at:byCalling:))
    mutating func vjpUpdate(at index: Int,byCalling closure: @differentiable(reverse) (inout Element) -> Void) -> (value: Void, pullback: (inout Self.TangentVector) -> Void){
        let closurePullback = pullback(at: self[index], of: closure)
        return (value: (), pullback: { closurePullback(&$0.base[index]) })
    }
}
public struct D<I: Equatable, D> {public subscript(_ index: I) -> D? {get {fatalError()} set {fatalError()}}}

Expected behavior
The code should compile successfully.

Environment

• Swift compiler version info: 2023-04-04a toolchain.
• Xcode version info: Version 14.2.
• Deployment target: M1

Additional context
On an older toolchain (2022-05-23a), the assertion failure is different:
Assertion failed: (type->mayHaveMembers()), function lookupMember, file TypeCheckNameLookup.cpp, line 311.

Here is the stack trace from the 2023-04-04a toolchain:

1.	Apple Swift version 5.9-dev (LLVM aa434c9ab379e8f, Swift 387b73f79813d8e)
2.	Compiling with the current language version
3.	While evaluating request ExecuteSILPipelineRequest(Run pipelines { IRGen Preparation } on SIL for genericEnv3)
4.	While running pass #58 SILModuleTransform "LoadableByAddress".
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           0x000000010534a210 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1  swift-frontend           0x0000000105349514 llvm::sys::RunSignalHandlers() + 128
2  swift-frontend           0x000000010534a850 SignalHandler(int) + 304
3  libsystem_platform.dylib 0x00000001944842a4 _sigtramp + 56
4  libsystem_pthread.dylib  0x0000000194455cec pthread_kill + 288
5  libsystem_c.dylib        0x000000019438e2c8 abort + 180
6  libsystem_c.dylib        0x000000019438d620 err + 0
7  swift-frontend           0x00000001053ffb28 isLargeLoadableType(swift::GenericEnvironment*, swift::SILType, swift::irgen::IRGenModule&) (.cold.7) + 0
8  swift-frontend           0x0000000100b28d18 isLargeLoadableType(swift::GenericEnvironment*, swift::SILType, swift::irgen::IRGenModule&) + 308
9  swift-frontend           0x0000000100b284bc modNonFuncTypeResultType(swift::GenericEnvironment*, swift::CanTypeWrapper<swift::SILFunctionType>, swift::irgen::IRGenModule&) + 176
10 swift-frontend           0x0000000100b2825c LargeSILTypeMapper::getNewResults(swift::GenericEnvironment*, swift::CanTypeWrapper<swift::SILFunctionType>, swift::irgen::IRGenModule&) + 224
11 swift-frontend           0x0000000100b28664 LargeSILTypeMapper::getNewSILFunctionType(swift::GenericEnvironment*, swift::CanTypeWrapper<swift::SILFunctionType>, swift::irgen::IRGenModule&) + 356
12 swift-frontend           0x0000000100b2c2e8 (anonymous namespace)::LoadableByAddress::run() + 10156
13 swift-frontend           0x0000000100eda55c swift::SILPassManager::runModulePass(unsigned int) + 992
14 swift-frontend           0x0000000100ee0474 swift::SILPassManager::execute() + 624
15 swift-frontend           0x0000000100ed74e4 swift::SILPassManager::executePassPipelinePlan(swift::SILPassPipelinePlan const&) + 68
16 swift-frontend           0x0000000100ed746c swift::ExecuteSILPipelineRequest::evaluate(swift::Evaluator&, swift::SILPipelineExecutionDescriptor) const + 68
17 swift-frontend           0x0000000100ef8b00 swift::SimpleRequest<swift::ExecuteSILPipelineRequest, std::__1::tuple<> (swift::SILPipelineExecutionDescriptor), (swift::RequestFlags)1>::evaluateRequest(swift::ExecuteSILPipelineRequest const&, swift::Evaluator&) + 28
18 swift-frontend           0x0000000100ee2edc llvm::Expected<swift::ExecuteSILPipelineRequest::OutputType> swift::Evaluator::getResultUncached<swift::ExecuteSILPipelineRequest>(swift::ExecuteSILPipelineRequest const&) + 252
19 swift-frontend           0x0000000100ed76e0 swift::executePassPipelinePlan(swift::SILModule*, swift::SILPassPipelinePlan const&, bool, swift::irgen::IRGenModule*) + 84
20 swift-frontend           0x0000000100aaebb0 swift::performIRGeneration(swift::ModuleDecl*, swift::IRGenOptions const&, swift::TBDGenOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, llvm::GlobalVariable**) + 1048
21 swift-frontend           0x000000010069c710 performCompileStepsPostSILGen(swift::CompilerInstance&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, int&, swift::FrontendObserver*) + 1608
22 swift-frontend           0x000000010069bd30 swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*) + 1104
23 swift-frontend           0x00000001006aaa28 withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool) + 160
24 swift-frontend           0x000000010069d814 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3212
25 swift-frontend           0x0000000100529f80 swift::mainEntry(int, char const**) + 3304
26 dyld                     0x000000019412be50 start + 2544

With respect to 2023-04-04a only, in the source code posted above, removing struct D, or changing the return type on the subscript operator from D? to D will cause the compilation to succeed in Release mode.

[BradLarson]

cc @asl

[asl]

Maybe related to #63086 somehow

[asl]

f could be further simplified down to:

@differentiable(reverse)
func f(array: [Double]) -> Double {
  var array = array
  array.update(at: 1,
    byCalling: {
      (element: inout Double) in
      let initialElement = element;
      element *= initialElement
    }
  )
  
  return 0.0
}

[asl]

We are having the following after sil-combine:

  %19 = convert_function %16 : $@noescape @callee_guaranteed (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () to $@noescape @callee_guaranteed <τ_0_0> (@inout τ_0_0) -> @owned @callee_guaranteed @
substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector> // user: %21
  %20 = convert_function %17 : $@noescape @callee_guaranteed (@inout Double) -> @owned @callee_guaranteed (@inout Double) -> () to $@noescape @callee_guaranteed <τ_0_0> (@inout τ_0_0) -> @owned @callee_guaranteed @
substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector> // user: %21
  %21 = differentiable_function [parameters 0] [results 0] %18 : $@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Double> with_derivative {%19 : $@noescape @callee_guaranteed <τ_0_0> (@i
nout τ_0_0) -> @owned @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector>, %20 : $@noescape @callee_guaranteed <τ_0_0> (@inout τ_0_0) -> @owned @callee_guaranteed @substituted <τ_
0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector>} // user: %23

Note that τ_0_0 is left unspecified here (the function is not generic). This transformation is somehow exposed by (convert_escape_to_noescape (thin_to_thick_function x)) => (thin_to_thick_function [noescape] x) transform, however, the latter one creates instructions with correct types.

[asl]

So, it seems we're calculating pullback / differential types wrong for substituted types.

E.g. for the following function type:

(sil_function_type type=@noescape @callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <Double>
  (input=generic_type_param_type depth=0 index=0)
  (substitution_map generic_signature=<τ_0_0>
    (substitution τ_0_0 -> Double))
  (substitution_map generic_signature=<nullptr>))

we end with

(sil_function_type type=@callee_guaranteed @substituted <τ_0_0> (@inout τ_0_0) -> () for <τ_0_0.TangentVector>
  (input=generic_type_param_type depth=0 index=0)
  (substitution_map generic_signature=<τ_0_0>
    (substitution τ_0_0 -> τ_0_0.TangentVector))
  (substitution_map generic_signature=<nullptr>))

which is not quite correct to me

Tagging @rxwei @dan-zheng

[asl]

Ok, so the way how we're calculating a derivative type looks a bit confusing to me:

• We are ignoring all pattern substitutions (both in function type and tangent vector type)
• We are using a newly-created substitution map to materialize a TangentVector associate type in case of archetype / type parameter

@rxwei @dan-zheng Do you happen to remember why it was done this way?