Fix crash when module in selector doesn’t exist

# Conflicts:
#	lib/AST/UnqualifiedLookup.cpp

Author: beccadax

lib/AST/UnqualifiedLookup.cpp

@@ -381,6 +381,8 @@ void UnqualifiedLookupFactory::setAsideUnavailableResults(
 }
 
 void UnqualifiedLookupFactory::addImportedResults(const DeclContext *const dc) {
+  assert(dc);
+
   using namespace namelookup;
   SmallVector<ValueDecl *, 8> CurModuleResults;
   auto resolutionKind = isOriginallyTypeLookup ? ResolutionKind::TypesOnly
@@ -391,6 +393,10 @@ void UnqualifiedLookupFactory::addImportedResults(const DeclContext *const dc) {
     // We'd need a new ResolutionKind.
     moduleToLookIn =
         dc->getASTContext().getLoadedModule(Name.getModuleSelector());
+  
+  // If we didn't find the module, it obviously can't have any results.
+  if (!moduleToLookIn)
+    return;
 
   auto nlOptions = NL_UnqualifiedDefault;
   if (options.contains(Flags::IncludeUsableFromInline))

test/NameLookup/module_selector.swift

@@ -448,3 +448,19 @@ precedencegroup main::PG1 {
   higherThan: Swift::AdditionPrecedence
   // expected-error@-1 {{precedence group specifier cannot be qualified with module selector}}
 }
+
+func badModuleNames() {
+  NonexistentModule::print()
+  // expected-error@-1 {{declaration 'print' is not imported through module 'NonexistentModule'}}
+  // expected-note@-2 {{did you mean module 'Swift'?}} {{3-20=Swift}}
+  // FIXME redundant: expected-note@-3  {{did you mean module 'Swift'?}}
+
+  _ = "foo".NonexistentModule::count
+  // FIXME improve: expected-error@-1 {{value of type 'String' has no member 'NonexistentModule::count'}}
+
+  let x: NonexistentModule::MyType = NonexistentModule::MyType()
+  // expected-error@-1 {{use of undeclared type 'NonexistentModule::MyType'}}
+
+  let y: A.NonexistentModule::MyChildType = fatalError()
+  // expected-error@-1 {{'NonexistentModule::MyChildType' is not a member type of 'A'}}
+}