[CodeComplete] Complete a lambda when preferred type is a function

[ilya-biryukov]

Summary

Uses a heuristic to detect std::function and friends.

[kadircet]

clang/lib/Sema/SemaCodeComplete.cpp

4135

This looks cheesy, do we really want to perform this operation only for templates with "function" in their names?

As discussed offline maybe perform this for any template with a single(non-defaulted?) argument, or maybe even better perform a type-check as suggested by you. But I believe there would be too many false positives with the current state.

4154

maybe also add a placeholder for captures?

[ilya-biryukov]

• Add placeholder for captures.
• Only accept classes that have exactly one template argument.

[ilya-biryukov]

clang/lib/Sema/SemaCodeComplete.cpp

4135

WDYT about "function" + only template with a single arg?
FWIW, I think function in template arguments are very rare, so I'm more optimistic about false-positives even in the current form.

We have an option of making it super-restrictive and only firing on whitelisted things like std::function, boost::function, etc.
But we don't have a mechanism to configure those from the outside, so that will probably turn out too restrictive.

4154

Done. With = as a default.
I'd personally rather have less placeholders, but I guess that might be useful.

[kadircet]

clang/lib/Sema/SemaCodeComplete.cpp

4125

I thought you decided to get rid of this branch after the offline discussions, sorry for not mentioning in earlier revision.

4135

I don't think adding "function" as a restriction is helping much on reducing false positives. I would rather get rid of that check to increase usability.

The real problem is rather checking constructors to make sure there is at least one for which we can provide a callable with the signature we found. Anything else just seems cheesy and might result in people adding more and more cases over time.

It is up to you, I am OK if you choose to keep this restriction as well.

4142

what about referencetype?

void test();
void (&y)() = test;

[ilya-biryukov]

• Only work on sugared types
• Do not check for 'function' in the name

clang/lib/Sema/SemaCodeComplete.cpp

4125

Sorry, my bad, forgot about it.

4135

Removed the restriction on the function way.
I don't think there's an easy way to check for conversions, though, I hope the false-positive rate would super-small.
If not, I'll find a way to check conversions or whitelist only known function names.

4142

Lambdas are r-values, so suggesting them for l-value references would lead to an error.
R-value references to functions are so rare that I feel handling those is not worth it.

[kadircet]

LGTM, thanks!

clang/lib/Sema/SemaCodeComplete.cpp

4142

ah right

[ilya-biryukov]

Thanks for a quick review!

[ilya-biryukov]

• Remove a leftover from name check.

clang/lib/Sema/SemaCodeComplete.cpp

if (Enumerators.Seen.count(E))
       continue;
 
     CodeCompletionResult R(E, CCP_EnumInCase, Qualifier);
     Results.AddResult(R, CurContext, nullptr, false);
   }
   Results.ExitScope();
 }
 
+/// Try to find a corresponding FunctionProtoType for function-like types (e.g.
+/// function pointers, std::function, etc).
+static const FunctionProtoType *TryDeconstructFunctionLike(QualType T) {
+  assert(!T.isNull());
+  // Try to extract first template argument from std::function<> and similar.
+  // Note we only handle the sugared types, they closely match what users wrote.
+  // We explicitly choose to not handle ClassTemplateSpecializationDecl.
+  if (auto *Specialization = T->getAs<TemplateSpecializationType>()) {
+    if (Specialization->getNumArgs() != 1)
+      return nullptr;
+    const TemplateArgument &Argument = Specialization->getArg(0);
+    if (Argument.getKind() != TemplateArgument::Type)
+      return nullptr;
+    return Argument.getAsType()->getAs<FunctionProtoType>();
+  }
+  // Handle other cases.
+  if (T->isPointerType())
+    T = T->getPointeeType();
+  return T->getAs<FunctionProtoType>();
+}
+
+/// Adds a pattern completion for a lambda expression with the specified
+/// parameter types and placeholders for parameter names.
+static void AddLambdaCompletion(ResultBuilder &Results,
+                                llvm::ArrayRef<QualType> Parameters,
+                                const LangOptions &LangOpts) {
+  CodeCompletionBuilder Completion(Results.getAllocator(),
+                                   Results.getCodeCompletionTUInfo());
+  // [](<parameters>) {}
+  Completion.AddChunk(CodeCompletionString::CK_LeftBracket);
+  Completion.AddPlaceholderChunk("=");
+  Completion.AddChunk(CodeCompletionString::CK_RightBracket);
+  if (!Parameters.empty()) {
+    Completion.AddChunk(CodeCompletionString::CK_LeftParen);
+    bool First = true;
+    for (auto Parameter : Parameters) {
+      if (!First)
+        Completion.AddChunk(CodeCompletionString::ChunkKind::CK_Comma);
+      else
+        First = false;
+
+      constexpr llvm::StringLiteral NamePlaceholder = "!#!NAME_GOES_HERE!#!";
+      std::string Type = NamePlaceholder;
+      Parameter.getAsStringInternal(Type, PrintingPolicy(LangOpts));
+      llvm::StringRef Prefix, Suffix;
+      std::tie(Prefix, Suffix) = llvm::StringRef(Type).split(NamePlaceholder);
+      Prefix = Prefix.rtrim();
+      Suffix = Suffix.ltrim();
+
+      Completion.AddTextChunk(Completion.getAllocator().CopyString(Prefix));
+      Completion.AddChunk(CodeCompletionString::CK_HorizontalSpace);
+      Completion.AddPlaceholderChunk("parameter");
+      Completion.AddTextChunk(Completion.getAllocator().CopyString(Suffix));
+    };
+    Completion.AddChunk(CodeCompletionString::CK_RightParen);
+  }
+  Completion.AddChunk(CodeCompletionString::CK_LeftBrace);
+  Completion.AddPlaceholderChunk("body");
+  Completion.AddChunk(CodeCompletionString::CK_RightBrace);
+
+  Results.AddResult(Completion.TakeString());
+}
+
 /// Perform code-completion in an expression context when we know what
 /// type we're looking for.
 void Sema::CodeCompleteExpression(Scope *S,
                                   const CodeCompleteExpressionData &Data) {
   ResultBuilder Results(
       *this, CodeCompleter->getAllocator(),
       CodeCompleter->getCodeCompletionTUInfo(),
       CodeCompletionContext(
void Sema::CodeCompleteExpression(Scope *S,
 
   if (S->getFnParent() && !Data.ObjCCollection &&
       !Data.IntegralConstantExpression)
     AddPrettyFunctionResults(getLangOpts(), Results);
 
   if (CodeCompleter->includeMacros())
     AddMacroResults(PP, Results, CodeCompleter->loadExternal(), false,
                     PreferredTypeIsPointer);
+
+  // Complete a lambda expression when preferred type is a function.
+  if (!Data.PreferredType.isNull() && getLangOpts().CPlusPlus11) {
+    if (const FunctionProtoType *F =
+            TryDeconstructFunctionLike(Data.PreferredType))
+      AddLambdaCompletion(Results, F->getParamTypes(), getLangOpts());
+  }
+
   HandleCodeCompleteResults(this, CodeCompleter, Results.getCompletionContext(),
                             Results.data(), Results.size());
 }
 
 void Sema::CodeCompleteExpression(Scope *S, QualType PreferredType,
                                   bool IsParenthesized) {
   return CodeCompleteExpression(
       S, CodeCompleteExpressionData(PreferredType, IsParenthesized));

clang/test/CodeCompletion/lambdas.cpp

+template <class T>
+struct function {
+};
+
+
+void test() {
+  void (*x)(int, double) = nullptr;
+
+  function<void(int, double)> y = {};
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:7:28 %s -o - | FileCheck -check-prefix=CHECK-1 %s
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:9:35 %s -o - | FileCheck -check-prefix=CHECK-1 %s
+  // CHECK-1: COMPLETION: Pattern : [<#=#>](int <#parameter#>, double <#parameter#>){<#body#>}
+
+  // == Placeholders for suffix types must be placed properly.
+  function<void(void(*)(int))> z = {};
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:15:36 %s -o - | FileCheck -check-prefix=CHECK-2 %s
+  // CHECK-2: COMPLETION: Pattern : [<#=#>](void (* <#parameter#>)(int)){<#body#>}
+
+  // == No need for a parameter list if function has no parameters.
+  function<void()> a = {};
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:20:24 %s -o - | FileCheck -check-prefix=CHECK-3 %s
+  // CHECK-3: COMPLETION: Pattern : [<#=#>]{<#body#>}
+}
+
+template <class T, class Allocator = int>
+struct vector {};
+
+void test2() {
+  // == Try to preserve types as written.
+  function<void(vector<int>)> a = {};
+
+  using function_typedef = function<void(vector<int>)>;
+  function_typedef b = {};
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:30:35 %s -o - | FileCheck -check-prefix=CHECK-4 %s
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:33:24 %s -o - | FileCheck -check-prefix=CHECK-4 %s
+  // CHECK-4: COMPLETION: Pattern : [<#=#>](vector<int> <#parameter#>){<#body#>}
+}
+
+// Check another common function wrapper name.
+template <class T> struct unique_function {};
+
+void test3() {
+  unique_function<void()> a = {};
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:43:31 %s -o - | FileCheck -check-prefix=CHECK-5 %s
+  // CHECK-5: COMPLETION: Pattern : [<#=#>]{<#body#>}
+}
+
+template <class T, class U> struct weird_function {};
+void test4() {
+  weird_function<void(), int> b = {};
+  // RUN: %clang_cc1 -fsyntax-only -code-completion-patterns -code-completion-at=%s:50:35 %s -o - | FileCheck -check-prefix=CHECK-6 %s
+  // CHECK-6-NOT: COMPLETION: Pattern : [<#=
+}