[clangd] Show lambda signature for lambda autocompletions

The original bug report can be found
[here](https://github.com/clangd/clangd/issues/85)

Given the following code:

```c++
void function() {
  auto Lambda = [](int a, double &b) {return 1.f;};
  La^
}
```

Triggering the completion at `^` would show `(lambda)` before this patch
and would show signature `(int a, double &b) const`, build a snippet etc
with this patch.

Reviewers: sammccall

Reviewed by: sammccall

Differential revision: https://reviews.llvm.org/D70445

1 files changed, 27 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index 81eed4330cd..e75aca4d81e 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -3315,6 +3315,18 @@ CodeCompletionResult::createCodeCompletionStringForOverride(
   return Result.TakeString();
 }
 
+// FIXME: Right now this works well with lambdas. Add support for other functor
+// types like std::function.
+static const NamedDecl *extractFunctorCallOperator(const NamedDecl *ND) {
+  const auto *VD = dyn_cast<VarDecl>(ND);
+  if (!VD)
+    return nullptr;
+  const auto *RecordDecl = VD->getType()->getAsCXXRecordDecl();
+  if (!RecordDecl || !RecordDecl->isLambda())
+    return nullptr;
+  return RecordDecl->getLambdaCallOperator();
+}
+
 CodeCompletionString *CodeCompletionResult::createCodeCompletionStringForDecl(
     Preprocessor &PP, ASTContext &Ctx, CodeCompletionBuilder &Result,
     bool IncludeBriefComments, const CodeCompletionContext &CCContext,
@@ -3339,9 +3351,8 @@ CodeCompletionString *CodeCompletionResult::createCodeCompletionStringForDecl(
   for (const auto *I : ND->specific_attrs<AnnotateAttr>())
     Result.AddAnnotation(Result.getAllocator().CopyString(I->getAnnotation()));
 
-  AddResultTypeChunk(Ctx, Policy, ND, CCContext.getBaseType(), Result);
-
-  if (const auto *Function = dyn_cast<FunctionDecl>(ND)) {
+  auto AddFunctionTypeAndResult = [&](const FunctionDecl *Function) {
+    AddResultTypeChunk(Ctx, Policy, Function, CCContext.getBaseType(), Result);
     AddQualifierToCompletionString(Result, Qualifier, QualifierIsInformative,
                                    Ctx, Policy);
     AddTypedNameChunk(Ctx, Policy, ND, Result);
@@ -3349,9 +3360,21 @@ CodeCompletionString *CodeCompletionResult::createCodeCompletionStringForDecl(
     AddFunctionParameterChunks(PP, Policy, Function, Result);
     Result.AddChunk(CodeCompletionString::CK_RightParen);
     AddFunctionTypeQualsToCompletionString(Result, Function);
+  };
+
+  if (const auto *Function = dyn_cast<FunctionDecl>(ND)) {
+    AddFunctionTypeAndResult(Function);
+    return Result.TakeString();
+  }
+
+  if (const auto *CallOperator =
+          dyn_cast_or_null<FunctionDecl>(extractFunctorCallOperator(ND))) {
+    AddFunctionTypeAndResult(CallOperator);
     return Result.TakeString();
   }
 
+  AddResultTypeChunk(Ctx, Policy, ND, CCContext.getBaseType(), Result);
+
   if (const FunctionTemplateDecl *FunTmpl =
           dyn_cast<FunctionTemplateDecl>(ND)) {
     AddQualifierToCompletionString(Result, Qualifier, QualifierIsInformative,
@@ -3417,6 +3440,7 @@ CodeCompletionString *CodeCompletionResult::createCodeCompletionStringForDecl(
     Result.AddChunk(CodeCompletionString::CK_RightAngle);
     return Result.TakeString();
   }
+
   if (const auto *Method = dyn_cast<ObjCMethodDecl>(ND)) {
     Selector Sel = Method->getSelector();
     if (Sel.isUnarySelector()) {