[Diagnostics] Improve the diagnostics for invalid associatedtype or

typealias access on existential types by mentioning the base type in
the error message.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -2465,11 +2465,13 @@ WARNING(append_interpolation_access_control,none,
 
 // Protocols and existentials
 ERROR(assoc_type_outside_of_protocol,none,
-      "associated type %0 can only be used with a concrete type or "
-      "generic parameter base", (DeclNameRef))
+      "cannot access associated type %0 from %1; use a concrete type or "
+      "generic parameter base instead",
+      (DeclNameRef, Type))
 ERROR(typealias_outside_of_protocol,none,
-      "type alias %0 can only be used with a concrete type or "
-      "generic parameter base", (DeclNameRef))
+      "cannot access type alias %0 from %1; use a concrete type or "
+      "generic parameter base instead",
+      (DeclNameRef, Type))
 
 ERROR(objc_protocol_inherits_non_objc_protocol,none,
       "@objc protocol %0 cannot refine non-@objc protocol %1", (Type, Type))

lib/Sema/CSDiagnostics.cpp

@@ -4405,10 +4405,10 @@ bool AllowTypeOrInstanceMemberFailure::diagnoseAsError() {
         // static members doesn't make a whole lot of sense
         if (isa<TypeAliasDecl>(Member)) {
           Diag.emplace(
-              emitDiagnostic(diag::typealias_outside_of_protocol, Name));
+              emitDiagnostic(diag::typealias_outside_of_protocol, Name, instanceTy));
         } else if (isa<AssociatedTypeDecl>(Member)) {
           Diag.emplace(
-              emitDiagnostic(diag::assoc_type_outside_of_protocol, Name));
+              emitDiagnostic(diag::assoc_type_outside_of_protocol, Name, instanceTy));
         } else if (isa<ConstructorDecl>(Member)) {
           Diag.emplace(
               emitDiagnostic(diag::construct_protocol_by_name, instanceTy));

lib/Sema/TypeCheckType.cpp

@@ -1656,12 +1656,12 @@ static Type resolveNestedIdentTypeComponent(TypeResolution resolution,
 
     case TypeChecker::UnsupportedMemberTypeAccessKind::TypeAliasOfExistential:
       diags.diagnose(comp->getNameLoc(), diag::typealias_outside_of_protocol,
-                     comp->getNameRef());
+                     comp->getNameRef(), parentTy);
       return ErrorType::get(ctx);
 
     case TypeChecker::UnsupportedMemberTypeAccessKind::AssociatedTypeOfExistential:
       diags.diagnose(comp->getNameLoc(), diag::assoc_type_outside_of_protocol,
-                     comp->getNameRef());
+                     comp->getNameRef(), parentTy);
       return ErrorType::get(ctx);
     }
 

test/decl/typealias/associated_types.swift

@@ -4,16 +4,16 @@ protocol BaseProto {
   associatedtype AssocTy
 }
 var a: BaseProto.AssocTy = 4
-// expected-error@-1{{associated type 'AssocTy' can only be used with a concrete type or generic parameter base}}
+// expected-error@-1{{cannot access associated type 'AssocTy' from 'BaseProto'; use a concrete type or generic parameter base instead}}
 
 var a = BaseProto.AssocTy.self
-// expected-error@-1{{associated type 'AssocTy' can only be used with a concrete type or generic parameter base}}
+// expected-error@-1{{cannot access associated type 'AssocTy' from 'BaseProto'; use a concrete type or generic parameter base instead}}
 
 protocol DerivedProto : BaseProto {
   func associated() -> AssocTy // no-warning
 
   func existential() -> BaseProto.AssocTy
-  // expected-error@-1{{associated type 'AssocTy' can only be used with a concrete type or generic parameter base}}
+  // expected-error@-1{{cannot access associated type 'AssocTy' from 'BaseProto'; use a concrete type or generic parameter base instead}}
 }
 
 

test/decl/typealias/protocol.swift

@@ -176,8 +176,8 @@ struct T5 : P5 {
   var a: P5.T1 // OK
 
   // Invalid -- cannot represent associated type of existential
-  var v2: P5.T2 // expected-error {{type alias 'T2' can only be used with a concrete type or generic parameter base}}
-  var v3: P5.X // expected-error {{type alias 'X' can only be used with a concrete type or generic parameter base}}
+  var v2: P5.T2 // expected-error {{cannot access type alias 'T2' from 'P5'; use a concrete type or generic parameter base instead}}
+  var v3: P5.X // expected-error {{cannot access type alias 'X' from 'P5'; use a concrete type or generic parameter base instead}}
 
   // Unqualified reference to typealias from a protocol conformance
   var v4: T1 // OK
@@ -188,7 +188,7 @@ struct T5 : P5 {
   var v7: T5.T2 // OK
 
   var v8 = P6.A.self
-  var v9 = P6.B.self // expected-error {{type alias 'B' can only be used with a concrete type or generic parameter base}}
+  var v9 = P6.B.self // expected-error {{cannot access type alias 'B' from 'P6'; use a concrete type or generic parameter base instead}}
 }
 
 // Unqualified lookup finds typealiases in protocol extensions

test/type/subclass_composition.swift

@@ -322,7 +322,7 @@ func dependentMemberTypes<T : BaseIntAndP2>(
   _: T.FullyConcrete,
 
   _: BaseIntAndP2.DependentInConcreteConformance, // FIXME expected-error {{}}
-  _: BaseIntAndP2.DependentProtocol, // expected-error {{type alias 'DependentProtocol' can only be used with a concrete type or generic parameter base}}
+  _: BaseIntAndP2.DependentProtocol, // expected-error {{cannot access type alias 'DependentProtocol' from 'BaseIntAndP2' (aka 'Base<Int> & P2'); use a concrete type or generic parameter base instead}}
   _: BaseIntAndP2.DependentClass,
   _: BaseIntAndP2.FullyConcrete) {}