[clang] [clangd] Add code completion for if constexpr (PR #124315)

[via cfe-commits]

https://github.com/FantasqueX created https://github.com/llvm/llvm-project/pull/124315

C++17 supports `if constexpr` statement. This patch implements this in code completion.

>From 47f97a5ca5964d4b970be2ee13e3dc4ffc95d3b6 Mon Sep 17 00:00:00 2001
From: Letu Ren <fantasquex at gmail.com>
Date: Sat, 25 Jan 2025 01:23:52 +0800
Subject: [PATCH] [clangd] Add code completion for if constexpr

C++17 supports `if constexpr` statement. This patch implements this in
code completion.
---
 clang/include/clang/Sema/SemaCodeCompletion.h |  1 +
 clang/lib/Parse/ParseStmt.cpp                 |  8 ++++++++
 clang/lib/Sema/SemaCodeComplete.cpp           | 15 +++++++++++++++
 3 files changed, 24 insertions(+)

diff --git a/clang/include/clang/Sema/SemaCodeCompletion.h b/clang/include/clang/Sema/SemaCodeCompletion.h
index e931596c215d31..af44745d5d1239 100644
--- a/clang/include/clang/Sema/SemaCodeCompletion.h
+++ b/clang/include/clang/Sema/SemaCodeCompletion.h
@@ -152,6 +152,7 @@ class SemaCodeCompletion : public SemaBase {
   void CodeCompleteDesignator(const QualType BaseType,
                               llvm::ArrayRef<Expr *> InitExprs,
                               const Designation &D);
+  void CodeCompleteIfConstExpr(Scope *S) const;
   void CodeCompleteAfterIf(Scope *S, bool IsBracedThen);
 
   void CodeCompleteQualifiedId(Scope *S, CXXScopeSpec &SS, bool EnteringContext,
diff --git a/clang/lib/Parse/ParseStmt.cpp b/clang/lib/Parse/ParseStmt.cpp
index cd4504630f8719..3f9900dd997ada 100644
--- a/clang/lib/Parse/ParseStmt.cpp
+++ b/clang/lib/Parse/ParseStmt.cpp
@@ -1553,6 +1553,14 @@ StmtResult Parser::ParseIfStatement(SourceLocation *TrailingElseLoc) {
       IsConsteval = true;
       ConstevalLoc = ConsumeToken();
     }
+
+    if (Tok.is(tok::code_completion)) {
+      if (getLangOpts().CPlusPlus17) {
+        cutOffParsing();
+        Actions.CodeCompletion().CodeCompleteIfConstExpr(getCurScope());
+        return StmtError();
+      }
+    }
   }
   if (!IsConsteval && (NotLocation.isValid() || Tok.isNot(tok::l_paren))) {
     Diag(Tok, diag::err_expected_lparen_after) << "if";
diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index 58f3efbe0daf89..b159fd26a45208 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -6762,6 +6762,21 @@ void SemaCodeCompletion::CodeCompleteInitializer(Scope *S, Decl *D) {
   CodeCompleteExpression(S, Data);
 }
 
+void SemaCodeCompletion::CodeCompleteIfConstExpr(Scope *S) const {
+  ResultBuilder Results(SemaRef, CodeCompleter->getAllocator(),
+                        CodeCompleter->getCodeCompletionTUInfo(),
+                        CodeCompletionContext::CCC_SymbolOrNewName);
+  Results.EnterNewScope();
+
+  Results.AddResult(CodeCompletionResult("constexpr"));
+
+  Results.ExitScope();
+
+  HandleCodeCompleteResults(&SemaRef, CodeCompleter,
+                            Results.getCompletionContext(), Results.data(),
+                            Results.size());
+}
+
 void SemaCodeCompletion::CodeCompleteAfterIf(Scope *S, bool IsBracedThen) {
   ResultBuilder Results(SemaRef, CodeCompleter->getAllocator(),
                         CodeCompleter->getCodeCompletionTUInfo(),