Make sure protocol witness errors don't leave the conformance context

That is, if there's a problem with a witness, and the witness comes
from a different extension from the conformance (or the original type,
when the conformance is on an extension), put the main diagnostic on
the conformance, with a note on the witness. This involves some
shuffling and rephrasing of existing diagnostics too.

There's a few reasons for this change:

- More context. It may not be obvious why a declaration in file
  A.swift needs to be marked 'public' if you can't see the conformance
  in B.swift.

- Better locations for imported declarations. If you're checking a
  conformance in a source file but the witness came from an imported
  module, it's better to put the diagnostic on the part you have
  control over. (This is especially true in Xcode, which can't display
  diagnostics on imported declarations in the source editor.)

- Plays better with batch mode. Without this change, you can have
  diagnostics being reported in file A.swift that are tied to a
  conformance declared in file B.swift. Of course the contents of
  A.swift also affect the diagnostic, but compiling A.swift on its
  own wouldn't produce the diagnostic, and so putting it there is
  problematic.

The change does in some cases make for a worse user experience,
though; if you just want to apply the changes and move on, the main
diagnostic isn't in the "right place". It's the note that has the info
and possible fix-it. It's also a slightly more complicated
implementation.

Author: jrose-apple

include/swift/AST/DiagnosticsSema.def

@@ -1553,6 +1553,9 @@ ERROR(type_witness_objc_generic_parameter,none,
       "type %0 involving Objective-C type parameter%select{| %1}2 cannot be "
       "used for associated type %3 of protocol %4",
       (Type, Type, bool, DeclName, DeclName))
+NOTE(witness_fix_access,none,
+     "mark the %0 as '%select{%error|fileprivate|internal|public|%error}1' to "
+     "satisfy the requirement", (DescriptiveDeclKind, AccessLevel))
 
 ERROR(protocol_refine_access,none,
       "%select{protocol must be declared %select{"
@@ -3793,9 +3796,6 @@ ERROR(availability_protocol_requires_version,
 NOTE(availability_protocol_requirement_here, none,
      "protocol requirement here", ())
 
-NOTE(availability_conformance_introduced_here, none,
-     "conformance introduced here", ())
-
 // This doesn't display as an availability diagnostic, but it's
 // implemented there and fires when these subscripts are marked
 // unavailable, so it seems appropriate to put it here.

lib/AST/ASTContext.cpp

@@ -2250,6 +2250,9 @@ bool swift::fixDeclarationName(InFlightDiagnostic &diag, ValueDecl *decl,
 bool swift::fixDeclarationObjCName(InFlightDiagnostic &diag, ValueDecl *decl,
                                    Optional<ObjCSelector> targetNameOpt,
                                    bool ignoreImpliedName) {
+  if (decl->isImplicit())
+    return false;
+
   // Subscripts cannot be renamed, so handle them directly.
   if (isa<SubscriptDecl>(decl)) {
     diag.fixItInsert(decl->getAttributeInsertionLoc(/*forModifier=*/false),

lib/AST/Decl.cpp

@@ -2095,6 +2095,9 @@ bool ValueDecl::canInferObjCFromRequirement(ValueDecl *requirement) {
 }
 
 SourceLoc ValueDecl::getAttributeInsertionLoc(bool forModifier) const {
+  if (isImplicit())
+    return SourceLoc();
+
   if (auto var = dyn_cast<VarDecl>(this)) {
     // [attrs] var ...
     // The attributes are part of the VarDecl, but the 'var' is part of the PBD.

lib/Sema/TypeCheckProtocol.cpp

@@ -24,6 +24,16 @@ extension GenericClass : WithAssocOther {
   typealias Other = [T] // expected-error{{type 'GenericClass<T>.Other' involving Objective-C type parameter 'T' cannot be used for associated type 'Other' of protocol 'WithAssocOther'}}
 }
 
+protocol WithAssocSeparate {
+  associatedtype Separate
+}
+
+extension GenericClass {
+  typealias Separate = T // expected-note {{'Separate' declared here}}
+}
+extension GenericClass : WithAssocSeparate { // expected-error {{type 'GenericClass<T>.Separate' involving Objective-C type parameter 'T' cannot be used for associated type 'Separate' of protocol 'WithAssocSeparate'}}
+}
+
 protocol WithAssocElement {
   associatedtype Element
 }

test/ClangImporter/objc_generics_conformance.swift

@@ -118,18 +118,21 @@ protocol VeryImportantProto {
 }
 
 private struct VIPPrivateType : VeryImportantProto {
-  private typealias Assoc = Int // expected-error {{type alias 'Assoc' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}}
+  private typealias Assoc = Int // expected-error {{type alias 'Assoc' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}} {{none}}
+  // expected-note@-1 {{mark the type alias as 'fileprivate' to satisfy the requirement}} {{3-10=fileprivate}}
   var value: Int
 }
 
 private struct VIPPrivateProp : VeryImportantProto {
   typealias Assoc = Int
-  private var value: Int // expected-error {{property 'value' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}} {{3-10=fileprivate}}
+  private var value: Int // expected-error {{property 'value' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}} {{none}}
+  // expected-note@-1 {{mark the var as 'fileprivate' to satisfy the requirement}} {{3-10=fileprivate}}
 }
 
 private struct VIPPrivateSetProp : VeryImportantProto {
   typealias Assoc = Int
-  private(set) var value: Int // expected-error {{setter for property 'value' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}} {{3-10=fileprivate}}
+  private(set) var value: Int // expected-error {{setter for property 'value' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}} {{none}}
+  // expected-note@-1 {{mark the var as 'fileprivate' to satisfy the requirement}} {{3-10=fileprivate}}
 }
 
 private class VIPPrivateSetBase {
@@ -140,9 +143,11 @@ private class VIPPrivateSetSub : VIPPrivateSetBase, VeryImportantProto { // expe
 }
 
 private class VIPPrivateSetPropBase {
-  private(set) var value: Int = 0 // expected-error {{setter for property 'value' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}} {{3-10=fileprivate}}
+  private(set) var value: Int = 0
+  // expected-note@-1 {{mark the var as 'fileprivate' to satisfy the requirement}} {{3-10=fileprivate}}
 }
 private class VIPPrivateSetPropSub : VIPPrivateSetPropBase, VeryImportantProto {
+  // expected-error@-1 {{setter for property 'value' must be as accessible as its enclosing type because it matches a requirement in protocol 'VeryImportantProto'}} {{none}}
   typealias Assoc = Int
 }
 

test/Compatibility/accessibility.swift

@@ -132,19 +132,22 @@ func privateInOtherFile() {}
 
 #if !ACCESS_DISABLED
 struct ConformerByTypeAlias : TypeProto {
-  private typealias TheType = Int // expected-error {{type alias 'TheType' must be declared internal because it matches a requirement in internal protocol 'TypeProto'}} {{3-10=internal}}
+  private typealias TheType = Int // expected-error {{type alias 'TheType' must be declared internal because it matches a requirement in internal protocol 'TypeProto'}} {{none}}
+  // expected-note@-1 {{mark the type alias as 'internal' to satisfy the requirement}} {{3-10=internal}}
 }
 
 struct ConformerByLocalType : TypeProto {
-  private struct TheType {} // expected-error {{struct 'TheType' must be declared internal because it matches a requirement in internal protocol 'TypeProto'}} {{3-10=internal}}
+  private struct TheType {} // expected-error {{struct 'TheType' must be declared internal because it matches a requirement in internal protocol 'TypeProto'}} {{none}}
+  // expected-note@-1 {{mark the struct as 'internal' to satisfy the requirement}} {{3-10=internal}}
 }
 
 private struct PrivateConformerByLocalType : TypeProto {
   struct TheType {} // okay
 }
 
 private struct PrivateConformerByLocalTypeBad : TypeProto {
-  private struct TheType {} // expected-error {{struct 'TheType' must be as accessible as its enclosing type because it matches a requirement in protocol 'TypeProto'}} {{3-10=fileprivate}}
+  private struct TheType {} // expected-error {{struct 'TheType' must be as accessible as its enclosing type because it matches a requirement in protocol 'TypeProto'}} {{none}}
+  // expected-note@-1 {{mark the struct as 'fileprivate' to satisfy the requirement}} {{3-10=fileprivate}}
 }
 #endif