Do not minimize memberwise initializer access level with nominal access level.

It is important for private declarations to have internal memberwise initializers:
https://forums.swift.org/t/undiagnosed-private-structs-classes-with-public-members/31940/5

Author: dan-zheng

lib/Sema/CodeSynthesis.cpp

@@ -282,10 +282,11 @@ ConstructorDecl *swift::createMemberwiseImplicitConstructor(
 ConstructorDecl *swift::getOrCreateEffectiveMemberwiseInitializer(
     ASTContext &ctx, NominalTypeDecl *nominal) {
   // Compute the access level for the memberwise initializer: the minimum of:
-  // - The access level of the nominal type declaration itself.
+  // - Public, by default. This enables public nominal types to have public
+  //   memberwise initializers.
   // - The access level of each memberwise-initialized property in the nominal
   //   type declaration.
-  auto accessLevel = std::min(AccessLevel::Public, nominal->getFormalAccess());
+  auto accessLevel = AccessLevel::Public;
   for (auto *member : nominal->getMembers()) {
     auto var = dyn_cast<VarDecl>(member);
     if (!var ||

lib/Sema/CodeSynthesis.h

@@ -70,7 +70,12 @@ ConstructorDecl *createMemberwiseImplicitConstructor(ASTContext &ctx,
 // Get the effective memberwise initializer of the given nominal type, or create
 // it if it does not exist.
 // Sets the access level of the memberwise initializer to the minimum of:
-// - The access level of the nominal type declaration itself.
+// - Public, by default. This enables public nominal types to have public
+//   memberwise initializers.
+//   - NOTE(TF-1077): The `public` default is important for `TangentVector`
+//     structs synthesized during `Differentiable` derived conformances.
+//     Manually extending `TangentVector` structs to define a public
+//     memberwise initializer causes a redeclaration error.
 // - The access level of each memberwise-initialized property in the nominal
 //   type declaration.
 ConstructorDecl *getOrCreateEffectiveMemberwiseInitializer(

test/Sema/differentiable_access_level.swift

@@ -19,14 +19,14 @@ private struct PrivateStruct: Differentiable {}
 // CHECK-LABEL: internal struct InternalStruct : Differentiable {
 // CHECK:   internal init()
 // CHECK:   internal struct TangentVector : Differentiable, AdditiveArithmetic, PointwiseMultiplicative, ElementaryFunctions {
-// CHECK:     internal init()
+// CHECK:     public init()
 // CHECK:   }
 // CHECK: }
 
 // CHECK-LABEL: private struct PrivateStruct : Differentiable {
 // CHECK:   internal init()
 // CHECK:   fileprivate struct TangentVector : Differentiable, AdditiveArithmetic, PointwiseMultiplicative, ElementaryFunctions {
-// CHECK:     fileprivate init()
+// CHECK:     public init()
 // CHECK:   }
 // CHECK: }
 
@@ -44,13 +44,13 @@ private class PrivateClass: Differentiable {}
 // CHECK-LABEL: internal class InternalClass : Differentiable {
 // CHECK:   internal init()
 // CHECK:   internal struct TangentVector : Differentiable, AdditiveArithmetic, PointwiseMultiplicative, ElementaryFunctions {
-// CHECK:     internal init()
+// CHECK:     public init()
 // CHECK:   }
 // CHECK: }
 
 // CHECK-LABEL: private class PrivateClass : Differentiable {
 // CHECK:   internal init()
 // CHECK:   fileprivate struct TangentVector : Differentiable, AdditiveArithmetic, PointwiseMultiplicative, ElementaryFunctions {
-// CHECK:     fileprivate init()
+// CHECK:     public init()
 // CHECK:   }
 // CHECK: }