Check signatures for conformances from implementation-only imports

These also create a dependency on the implementation module, even if
both the type and the protocol are public. As John puts it, a
conformance is basically a declaration that we name as part of another
declaration.

More rdar://problem/48991061

Author: jrose-apple

include/swift/AST/AccessScopeChecker.h

@@ -54,13 +54,13 @@ class TypeDeclFinder : public TypeWalker {
   Action walkToTypePre(Type T) override;
 
 public:
-  virtual Action visitNominalType(const NominalType *ty) {
+  virtual Action visitNominalType(NominalType *ty) {
     return Action::Continue;
   }
-  virtual Action visitBoundGenericType(const BoundGenericType *ty) {
+  virtual Action visitBoundGenericType(BoundGenericType *ty) {
     return Action::Continue;
   }
-  virtual Action visitTypeAliasType(const TypeAliasType *ty) {
+  virtual Action visitTypeAliasType(TypeAliasType *ty) {
     return Action::Continue;
   }
 };
@@ -72,9 +72,9 @@ class SimpleTypeDeclFinder : public TypeDeclFinder {
   /// The function to call when a ComponentIdentTypeRepr is seen.
   llvm::function_ref<Action(const TypeDecl *)> Callback;
 
-  Action visitNominalType(const NominalType *ty) override;
-  Action visitBoundGenericType(const BoundGenericType *ty) override;
-  Action visitTypeAliasType(const TypeAliasType *ty) override;
+  Action visitNominalType(NominalType *ty) override;
+  Action visitBoundGenericType(BoundGenericType *ty) override;
+  Action visitTypeAliasType(TypeAliasType *ty) override;
 
 public:
   explicit SimpleTypeDeclFinder(

include/swift/AST/DiagnosticsSema.def

@@ -2391,6 +2391,10 @@ ERROR(decl_from_implementation_only_module,none,
       "cannot use %0 here; %1 has been imported as "
       "'@_implementationOnly'",
       (DeclName, Identifier))
+ERROR(conformance_from_implementation_only_module,none,
+      "cannot use conformance of %0 to %1 here; %2 has been imported as "
+      "'@_implementationOnly'",
+      (Type, DeclName, Identifier))
 
 // Derived conformances
 ERROR(cannot_synthesize_init_in_extension_of_nonfinal,none,

lib/AST/AccessScopeChecker.cpp

@@ -71,17 +71,17 @@ TypeWalker::Action TypeDeclFinder::walkToTypePre(Type T) {
 }
 
 TypeWalker::Action
-SimpleTypeDeclFinder::visitNominalType(const NominalType *ty) {
+SimpleTypeDeclFinder::visitNominalType(NominalType *ty) {
   return Callback(ty->getDecl());
 }
 
 TypeWalker::Action
-SimpleTypeDeclFinder::visitBoundGenericType(const BoundGenericType *ty) {
+SimpleTypeDeclFinder::visitBoundGenericType(BoundGenericType *ty) {
   return Callback(ty->getDecl());
 }
 
 TypeWalker::Action
-SimpleTypeDeclFinder::visitTypeAliasType(const TypeAliasType *ty) {
+SimpleTypeDeclFinder::visitTypeAliasType(TypeAliasType *ty) {
   return Callback(ty->getDecl());
 }