From 37d9edc0c5cfeae8b2e7dac173129ae6ab2c9429 Mon Sep 17 00:00:00 2001
From: Ilya Biryukov <ibiryukov@google.com>
Date: Thu, 23 May 2019 07:45:35 +0000
Subject: [CodeComplete] Complete a lambda when preferred type is a function

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

Reviewers: kadircet

Reviewed By: kadircet

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62238

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361461 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Sema/SemaCodeComplete.cpp | 71 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)

(limited to 'lib/Sema/SemaCodeComplete.cpp')

diff --git a/lib/Sema/SemaCodeComplete.cpp b/lib/Sema/SemaCodeComplete.cpp
index 0f33442e4a..e6093141ab 100644
--- a/lib/Sema/SemaCodeComplete.cpp
+++ b/lib/Sema/SemaCodeComplete.cpp
@@ -4108,6 +4108,69 @@ static void AddEnumerators(ResultBuilder &Results, ASTContext &Context,
   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,
@@ -4169,6 +4232,14 @@ void Sema::CodeCompleteExpression(Scope *S,
   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());
 }
-- 
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