[clang][CodeComplete] skip explicit obj param in code completion string (#146258)

Fixes clangd/clangd#2339

Author: MythreyaK

clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp

@@ -4363,6 +4363,36 @@ TEST(CompletionTest, PreambleFromDifferentTarget) {
   EXPECT_THAT(Result.Completions, Not(testing::IsEmpty()));
   EXPECT_THAT(Signatures.signatures, Not(testing::IsEmpty()));
 }
+
+TEST(CompletionTest, SkipExplicitObjectParameter) {
+  Annotations Code(R"cpp(
+    struct A {
+      void foo(this auto&& self, int arg); 
+    };
+
+    int main() {
+      A a {};
+      a.^
+    }
+  )cpp");
+
+  auto TU = TestTU::withCode(Code.code());
+  TU.ExtraArgs = {"-std=c++23"};
+
+  auto Preamble = TU.preamble();
+  ASSERT_TRUE(Preamble);
+
+  CodeCompleteOptions Opts{};
+
+  MockFS FS;
+  auto Inputs = TU.inputs(FS);
+  auto Result = codeComplete(testPath(TU.Filename), Code.point(),
+                             Preamble.get(), Inputs, Opts);
+
+  EXPECT_THAT(Result.Completions,
+              ElementsAre(AllOf(named("foo"), signature("(int arg)"),
+                                snippetSuffix("(${1:int arg})"))));
+}
 } // namespace
 } // namespace clangd
 } // namespace clang

clang/lib/Sema/SemaCodeComplete.cpp

@@ -3260,6 +3260,13 @@ static void AddFunctionParameterChunks(Preprocessor &PP,
       break;
     }
 
+    // C++23 introduces an explicit object parameter, a.k.a. "deducing this"
+    // Skip it for autocomplete and treat the next parameter as the first
+    // parameter
+    if (FirstParameter && Param->isExplicitObjectParameter()) {
+      continue;
+    }
+
     if (FirstParameter)
       FirstParameter = false;
     else

clang/test/CodeCompletion/skip-explicit-object-parameter.cpp

@@ -0,0 +1,14 @@
+struct A {
+  void foo(this A self, int arg);
+};
+
+int main() {
+  A a {};
+  a.
+}
+// RUN: %clang_cc1 -cc1 -fsyntax-only -code-completion-at=%s:%(line-2):5 -std=c++23 %s | FileCheck %s
+// CHECK: COMPLETION: A : A::
+// CHECK-NEXT: COMPLETION: foo : [#void#]foo(<#int arg#>)
+// CHECK-NEXT: COMPLETION: operator= : [#A &#]operator=(<#const A &#>)
+// CHECK-NEXT: COMPLETION: operator= : [#A &#]operator=(<#A &&#>)
+// CHECK-NEXT: COMPLETION: ~A : [#void#]~A()