[SR-13070] AST verification failure regarding `@differentiable` attribute: "checked decl cannot have error type"

[dan-zheng]

Previous ID	SR-13070
Radar	rdar://problem/74094401
Original Reporter	@dan-zheng
Type	Bug

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Compiler | |Labels | Bug, AutoDiff, CompilerCrash | |Assignee | @rxwei | |Priority | Medium | md5: 0b10d7cbd6875867fcb054de325ea6ee

relates to:

• SR-11838 Cycle involving implicit initializers and witness resolution

Issue Description:

import _Differentiation

// NOTE: No crash if `class` is changed to `struct`.
class Class: Differentiable {
  // NOTE: No crash if `@differentiable` attribute is removed.
  @differentiable
  var x: Float = 10

  var y: NonExistentType = 30
}

$ swiftc error.swift
error.swift:11:3: warning: synthesis of the 'Differentiable.move(along:)' requirement for 'Class' requires 'wrappedValue' in property wrapper 'Wrapper' to be mutable; add an explicit '@noDerivative' attribute
  @differentiable
  ^
  @noDerivative
checked decl cannot have error type
(accessor_decl implicit range=[error.swift:14:7 - line:14:7] 'anonname=0x7fd7258992a0' interface type='(Class) -> () -> <<error type>>' access=internal get_for=y
  (parameter "self" type='Class' interface type='Class')
  (parameter_list)
  (brace_stmt implicit range=[error.swift:14:7 - line:14:7]
    (return_stmt implicit
      (member_ref_expr implicit type='<<error type>>' decl=error.(file).Class.y@error.swift:14:7 direct_to_storage
        (declref_expr implicit type='Class' decl=error.(file).Class.<anonymous>.self@error.swift:14:7 function_ref=unapplied)))))
Stack dump:
0.  Program arguments: /Users/danielzheng/swift-tf/build/Ninja-ReleaseAssert/swift-macosx-x86_64/bin/swift -frontend -c -primary-file error.swift -target x86_64-apple-darwin19.4.0 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -color-diagnostics -target-sdk-version 10.15.4 -module-name error -o /var/folders/m_/6f7q8zfs3n9fr0c_4gy8840m00hc_q/T/error-df9d65.o
1.  Swift version 5.3-dev (LLVM 537023d5f0, Swift f2b25f7355)
2.  While walking into decl 'Class' (at error.swift:9:1)
3.  While verifying AccessorDecl getter for y (at error.swift:14:7)
4.  While verifying FuncDecl getter for y (at error.swift:14:7)
5.  While verifying AbstractFunctionDecl getter for y (at error.swift:14:7)
0  swift                    0x000000010fd2f218 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x000000010fd2e198 llvm::sys::RunSignalHandlers() + 248
2  swift                    0x000000010fd2f80d SignalHandler(int) + 285
3  libsystem_platform.dylib 0x00007fff6a3e15fd _sigtramp + 29
4  libsystem_platform.dylib 000000000000000000 _sigtramp + 18446603338733709856
5  libsystem_c.dylib        0x00007fff6a2b7808 abort + 120
6  swift                    0x00000001101f41b0 (anonymous namespace)::Verifier::verifyChecked(swift::VarDecl*) (.cold.1) + 0
7  swift                    0x000000010c78efae (anonymous namespace)::Verifier::verifyChecked(swift::ValueDecl*) + 174
8  swift                    0x000000010c78fc08 (anonymous namespace)::Verifier::verifyChecked(swift::AbstractFunctionDecl*) + 888
9  swift                    0x000000010c78fdda (anonymous namespace)::Verifier::verifyChecked(swift::FuncDecl*) + 314

What's expected is a "type not found in scope" diagnostic:

error.swift:9:10: error: cannot find type 'NonExistentType' in scope
  var y: NonExistentType = 30
         ^~~~~~~~~~~~~~~

[rxwei]

@swift-ci create