Sema: Allow unqualified type lookup to find typealiases in protocols

Not really specific to typealiases; but we don't allow nominal types
to be nested inside protocols yet, and all other types of protocol
members have witnesses in the concrete type.

Fixes <https://bugs.swift.org/browse/SR-2792>.

Author: slavapestov

lib/Sema/TypeCheckNameLookup.cpp

@@ -214,18 +214,38 @@ SmallVector<TypeDecl *, 1>
 TypeChecker::lookupUnqualifiedType(DeclContext *dc, DeclName name,
                                    SourceLoc loc,
                                    NameLookupOptions options) {
+  SmallVector<TypeDecl *, 1> decls;
+
+  // Try lookup without ProtocolMembers first.
   UnqualifiedLookup lookup(
       name, dc, this,
       options.contains(NameLookupFlags::KnownPrivate),
       loc,
       /*OnlyTypes=*/true,
-      options.contains(NameLookupFlags::ProtocolMembers),
+      /*ProtocolMembers=*/false,
       options.contains(NameLookupFlags::IgnoreAccessibility));
-
-  LookupResult result;
-  SmallVector<TypeDecl *, 1> decls;
   for (auto found : lookup.Results)
     decls.push_back(cast<TypeDecl>(found.getValueDecl()));
+
+  if (decls.empty() &&
+      options.contains(NameLookupFlags::ProtocolMembers)) {
+    // Try again, this time with protocol members.
+    //
+    // FIXME: Fix the problem where if NominalTypeDecl::getAllProtocols()
+    // is called too early, we start resolving extensions -- even those
+    // which do provide conformances.
+    UnqualifiedLookup lookup(
+        name, dc, this,
+        options.contains(NameLookupFlags::KnownPrivate),
+        loc,
+        /*OnlyTypes=*/true,
+        /*ProtocolMembers=*/true,
+        options.contains(NameLookupFlags::IgnoreAccessibility));
+
+    for (auto found : lookup.Results)
+      decls.push_back(cast<TypeDecl>(found.getValueDecl()));
+  }
+
   return decls;
 }
 

lib/Sema/TypeCheckType.cpp

@@ -1031,9 +1031,6 @@ resolveTopLevelIdentTypeComponent(TypeChecker &TC, DeclContext *DC,
   NameLookupOptions lookupOptions = defaultUnqualifiedLookupOptions;
   if (options.contains(TR_KnownNonCascadingDependency))
     lookupOptions |= NameLookupFlags::KnownPrivate;
-  // FIXME: Eliminate this once we can handle finding protocol members
-  // in resolveTypeInContext.
-  lookupOptions -= NameLookupFlags::ProtocolMembers;
   auto globals = TC.lookupUnqualifiedType(lookupDC,
                                           comp->getIdentifier(),
                                           comp->getIdLoc(),

test/Generics/associated_types.swift

@@ -21,8 +21,7 @@ struct Z : Fooable {
     var a : AssocType // expected-warning {{variable 'a' was never used; consider replacing with '_' or removing it}} {{9-10=_}}
   }
 
-  // FIXME: We should be able to find this.
-  func blarg() -> AssocType {} // expected-error{{use of undeclared type 'AssocType'}}
+  func blarg() -> AssocType {}
 
   func wonka() -> Z.AssocType {}
 }

test/decl/ext/generic.swift

@@ -152,3 +152,9 @@ struct S4<Q>: P4 {
 }
 
 extension S4 where T : P5 {}
+
+struct S5<Q> {
+  init(_: Q) { }
+}
+
+extension S5 : P4 {}

test/decl/typealias/protocol.swift

@@ -154,9 +154,9 @@ struct T5 : P5 {
   var v2: P5.T2 // expected-error {{typealias 'T2' can only be used with a concrete type or generic parameter base}}
   var v3: P5.X // expected-error {{typealias 'X' can only be used with a concrete type or generic parameter base}}
 
-  // FIXME: Unqualified reference to typealias from a protocol conformance
-  var v4: T1 // expected-error {{use of undeclared type 'T1'}}
-  var v5: T2 // expected-error {{use of undeclared type 'T2'}}
+  // Unqualified reference to typealias from a protocol conformance
+  var v4: T1 // OK
+  var v5: T2 // OK
 
   // Qualified reference
   var v6: T5.T1 // OK
@@ -178,8 +178,7 @@ extension P7 {
 struct S7 : P7 {
   typealias A = Int
 
-  // FIXME
-  func inTypeContext(y: Y) // expected-error {{use of undeclared type 'Y'}}
+  func inTypeContext(y: Y) { }
 
   func inExpressionContext() {
     _ = Y.self