Allow Deriving Type Witnesses Across Files

A recent change to witness matching in #32578 suddenly made the
following construction illegal:

// File1.swift
enum MyEnumInAnotherFile { /**/ }

// File2.swift
extension MyEnumInAnotherFile {
  static var allCases: [MyEnumInAnotherFile] { /**/ }
}

Because it was no longer possible to derive the type witness for
`AllCases`. This is because, when inference ran before synthesis, we
would use the value witness to pick out the type witness and thus had no
need for synthesis. Now that we run synthesis first, we ought to just
allow deriving type witnesses across files, but still ban deriving value
witnesses. In general, if you can utter a type in a different file to
extend it, you should be able to see the requirements necessary to
derive a default type witness.

rdar://66279278, rdar://66279375, rdar://66279384, rdar://66279415, rdar://66279503

Author: CodaFi

lib/Sema/DerivedConformanceCaseIterable.cpp

@@ -113,9 +113,6 @@ ValueDecl *DerivedConformance::deriveCaseIterable(ValueDecl *requirement) {
 }
 
 Type DerivedConformance::deriveCaseIterable(AssociatedTypeDecl *assocType) {
-  if (checkAndDiagnoseDisallowedContext(assocType))
-    return nullptr;
-
   // Check that we can actually derive CaseIterable for this type.
   if (!canDeriveConformance(Nominal))
     return nullptr;

lib/Sema/DerivedConformanceDifferentiable.cpp

@@ -895,9 +895,6 @@ DerivedConformance::deriveDifferentiable(AssociatedTypeDecl *requirement) {
                            diag::broken_differentiable_requirement);
     return std::make_pair(nullptr, nullptr);
   }
-  // Diagnose conformances in disallowed contexts.
-  if (checkAndDiagnoseDisallowedContext(requirement))
-    return std::make_pair(nullptr, nullptr);
 
   // Start an error diagnostic before attempting derivation.
   // If derivation succeeds, cancel the diagnostic.

test/AutoDiff/Sema/DerivedConformances/Inputs/class_differentiable_other_module.swift

@@ -1,9 +1,9 @@
 import _Differentiation
 
-// expected-note @+1 {{type declared here}}
+// expected-note @+1 2 {{type declared here}}
 class OtherFileNonconforming {}
 
-// expected-note @+1 {{type declared here}}
+// expected-note @+1 2 {{type declared here}}
 class GenericOtherFileNonconforming<T: Differentiable> {
   var x: T
 }

test/AutoDiff/Sema/DerivedConformances/Inputs/struct_differentiable_other_module.swift

@@ -1,9 +1,9 @@
 import _Differentiation
 
-// expected-note @+1 {{type declared here}}
+// expected-note @+1 2 {{type declared here}}
 struct OtherFileNonconforming {}
 
-// expected-note @+1 {{type declared here}}
+// expected-note @+1 2 {{type declared here}}
 struct GenericOtherFileNonconforming<T: Differentiable> {
   var x: T
 }

test/AutoDiff/Sema/DerivedConformances/class_differentiable.swift

@@ -570,10 +570,8 @@ class WrappedProperties: Differentiable {
 
 // Test derived conformances in disallowed contexts.
 
-// expected-error @+2 {{type 'OtherFileNonconforming' does not conform to protocol 'Differentiable'}}
-// expected-error @+1 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
+// expected-error @+1 2 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
 extension OtherFileNonconforming: Differentiable {}
 
-// expected-error @+2 {{type 'GenericOtherFileNonconforming<T>' does not conform to protocol 'Differentiable'}}
-// expected-error @+1 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
+// expected-error @+1 2 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
 extension GenericOtherFileNonconforming: Differentiable {}

test/AutoDiff/Sema/DerivedConformances/struct_differentiable.swift

@@ -383,10 +383,8 @@ struct WrappedProperties: Differentiable {
 
 // Verify that cross-file derived conformances are disallowed.
 
-// expected-error @+2 {{type 'OtherFileNonconforming' does not conform to protocol 'Differentiable'}}
-// expected-error @+1 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
+// expected-error @+1 2 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
 extension OtherFileNonconforming: Differentiable {}
 
-// expected-error @+2 {{type 'GenericOtherFileNonconforming<T>' does not conform to protocol 'Differentiable'}}
-// expected-error @+1 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
+// expected-error @+1 2 {{implementation of 'Differentiable' cannot be automatically synthesized in an extension in a different file to the type}}
 extension GenericOtherFileNonconforming: Differentiable {}

test/Sema/Inputs/enum_conformance_synthesis_other.swift

@@ -29,3 +29,7 @@ extension ImpliedMain: ImplierMain {}
 enum ImpliedOther: ImplierOther {
     case a(Int)
 }
+
+enum CaseIterableAcrossFiles {
+  case A
+}

test/Sema/enum_conformance_synthesis.swift

@@ -224,7 +224,12 @@ enum Complex2 {
 }
 extension Complex2 : Hashable {}
 extension Complex2 : CaseIterable {}  // expected-error {{type 'Complex2' does not conform to protocol 'CaseIterable'}}
-extension FromOtherFile: CaseIterable {} // expected-error {{cannot be automatically synthesized in an extension in a different file to the type}} expected-error {{does not conform to protocol 'CaseIterable'}}
+extension FromOtherFile: CaseIterable {} // expected-error {{cannot be automatically synthesized in an extension in a different file to the type}}
+extension CaseIterableAcrossFiles: CaseIterable {
+  public static var allCases: [CaseIterableAcrossFiles] {
+    return [ .A ]
+  }
+}
 
 // No explicit conformance and it cannot be derived.
 enum NotExplicitlyHashableAndCannotDerive {