[clang][CodeComplete] Add code completion for if constexpr and consteval

[FantasqueX]

C++17 supports if constexpr statement. C++23 supports if consteval statement. This patch implements this in code completion.

[llvmbot]

@llvm/pr-subscribers-clang

Author: FantasqueX (FantasqueX)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/124315.diff

3 Files Affected:

• (modified) clang/include/clang/Sema/SemaCodeCompletion.h (+1)
• (modified) clang/lib/Parse/ParseStmt.cpp (+8)
• (modified) clang/lib/Sema/SemaCodeComplete.cpp (+15)

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(),

[Lancern]

Thanks for working on this! Please add a test for your patch. You can take tests under clang/test/CodeCompletion for reference.

[Lancern]

LGTM.

[HighCommander4]

If I'm understanding correctly, this patch only completes the keywords themselves.

SemaCodeComplete also has the ability to produce "snippets", for example here is a snippet for if statements. In such a case, the inserted text is if (condition) { statements }, with condition and statements being marked as placeholders (in editors with snippet support, the cursor will jump to the placeholders to allow you to type over them).

What do you think about adding similar snippets for if constexpr and if consteval?

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[FantasqueX]

Hi @HighCommander4 thanks for your suggestions. I'm happy to add snippets support for if constexpr and if consteval. Since if <any number white space> constexp and if !consteval is valid, I decided to add code snippet completion in the specific code completion function instead of in statement completion. Do you think it is OK?

[HighCommander4]

Thanks! I tried out the patch with clangd and it seems to be working pretty well.

A further enhancement that occurs to me is that we could offer these patterns slightly earlier, e.g. when you're at the beginning of an empty line and invoke code completion and then type if to filter the results, you see the if (condition) {statements} pattern as the first result -- we could have if constexpr (condition) {statements} as another result there already. But that can be a future enhancement for another time.

The implementation looks pretty good, just have some minor comments.

[HighCommander4]

Looking at the similar function CodeCompleteAfterIf, I think mapCodeCompletionContext(SemaRef, PCC_Statement) would be more appropriate here than CodeCompletionContext::CCC_Other

[FantasqueX]

I think they are different. In CodeCompleteAfterIf, it's valid to provide a statement. While in the new added CodeCompleteKeywordAfterIf only two keywords are appropriate.

[FantasqueX]

@HighCommander4 @zyn0217 Really appreciate for your review. Thanks a lot.

[HighCommander4]

Thanks, LGTM!