diff --git a/include/swift/Frontend/Frontend.h b/include/swift/Frontend/Frontend.h
index 11000540c69..328388da2c4 100644
--- a/include/swift/Frontend/Frontend.h
+++ b/include/swift/Frontend/Frontend.h
@@ -425,15 +425,15 @@ private:
   void setUpDiagnosticOptions();
   bool setUpModuleLoaders();
   Optional<unsigned> setUpCodeCompletionBuffer();
-  bool setupInputs(Optional<unsigned> codeCompletionBufferID);
+  bool setUpInputs(Optional<unsigned> codeCompletionBufferID);
   bool isInputSwift() {
     return Invocation.getInputKind() == InputFileKind::IFK_Swift;
   }
   bool isInSILMode() {
     return Invocation.getInputKind() == InputFileKind::IFK_SIL;
   }
-  bool setupForInput(const InputFile &input);
-  void setupForBuffer(llvm::MemoryBuffer *buffer, bool isPrimary);
+  bool setUpForInput(const InputFile &input);
+  void setUpForBuffer(llvm::MemoryBuffer *buffer, bool isPrimary);
   bool setUpForFile(StringRef file, bool isPrimary);
 
 public:
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 0f0033455c6..03394810f38 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -179,7 +179,8 @@ private:
         continue;
       hadDuplicates = addFile(A->getValue()) || hadDuplicates;
     }
-    return false; // Don't bail out for duplicates, too many tests depend on it.
+    return false; // FIXME: Don't bail out for duplicates, too many tests depend
+                  // on it.
   }
 
   bool doesCommandLineIncludeFilelist() { return FilelistPathArg; }
@@ -203,14 +204,13 @@ private:
     for (auto file : llvm::make_range(llvm::line_iterator(*FilelistBuffer),
                                       llvm::line_iterator()))
       hadDuplicates = addFile(file) || hadDuplicates;
-    return false; // Don't bail out for duplicates, too many tests depend on it.
+    return false; // FIXME: Don't bail out for duplicates, too many tests depend
+                  // on it.
   }
 
   bool addFile(StringRef file) {
-    if (Files.count(file) == 0) {
-      Files.insert(file);
+    if (Files.insert(file))
       return false;
-    }
     Diags.diagnose(SourceLoc(), diag::error_duplicate_input_file, file);
     return true;
   }
diff --git a/lib/Frontend/Frontend.cpp b/lib/Frontend/Frontend.cpp
index 2caac5ba7ed..6f288bfa00a 100644
--- a/lib/Frontend/Frontend.cpp
+++ b/lib/Frontend/Frontend.cpp
@@ -102,7 +102,7 @@ bool CompilerInstance::setup(const CompilerInvocation &Invok) {
   if (isInSILMode())
     Invocation.getLangOptions().EnableAccessControl = false;
 
-  return setupInputs(codeCompletionBufferID);
+  return setUpInputs(codeCompletionBufferID);
 }
 
 void CompilerInstance::setUpLLVMArguments() {
@@ -129,6 +129,7 @@ void CompilerInstance::setUpDiagnosticOptions() {
     Diagnostics.setWarningsAsErrors(true);
   }
 }
+
 bool CompilerInstance::setUpModuleLoaders() {
   if (hasSourceImport()) {
     bool immediate = FrontendOptions::isActionImmediate(
@@ -175,10 +176,10 @@ Optional<unsigned> CompilerInstance::setUpCodeCompletionBuffer() {
   return codeCompletionBufferID;
 }
 
-bool CompilerInstance::setupInputs(Optional<unsigned> codeCompletionBufferID) {
+bool CompilerInstance::setUpInputs(Optional<unsigned> codeCompletionBufferID) {
   for (const InputFile &input :
        Invocation.getFrontendOptions().Inputs.getAllFiles())
-    if (setupForInput(input))
+    if (setUpForInput(input))
       return true;
 
   // Set the primary file to the code-completion point if one exists.
@@ -196,14 +197,14 @@ bool CompilerInstance::setupInputs(Optional<unsigned> codeCompletionBufferID) {
   return false;
 }
 
-bool CompilerInstance::setupForInput(const InputFile &input) {
+bool CompilerInstance::setUpForInput(const InputFile &input) {
   if (llvm::MemoryBuffer *inputBuffer = input.buffer()) {
-    setupForBuffer(inputBuffer, input.isPrimary());
+    setUpForBuffer(inputBuffer, input.isPrimary());
     return false;
   }
   return setUpForFile(input.file(), input.isPrimary());
 }
-void CompilerInstance::setupForBuffer(llvm::MemoryBuffer *buffer,
+void CompilerInstance::setUpForBuffer(llvm::MemoryBuffer *buffer,
                                       bool isPrimary) {
   auto copy = llvm::MemoryBuffer::getMemBufferCopy(
       buffer->getBuffer(), buffer->getBufferIdentifier());
diff --git a/test/Driver/options.swift b/test/Driver/options.swift
index 6e2c1e0db75..e9807c404d6 100644
--- a/test/Driver/options.swift
+++ b/test/Driver/options.swift
@@ -7,6 +7,7 @@
 // SIL_FILES: this mode requires a single input file
 
 // RUN: not %target-swift-frontend -parse-sil -emit-sil %s %s 2>&1 | %FileCheck -check-prefix=DUPLICATE_FILES %s
+// RUN: not %target-swift-frontend -parse-sil -emit-sil %s %S/../Inputs/empty.swift 2>&1 | %FileCheck -check-prefix=SIL_FILES %s
 // DUPLICATE_FILES: duplicate input file 'SOURCE_DIR/test/Driver/options.swift'