[clang] [clang][constexpr] Allow IndirectGotoStmt in constexpr functions for C++23 (PR #213449)

[via cfe-commits]

llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: babadany2999

<details>
<summary>Changes</summary>

### Summary
P2242R3 relaxed `constexpr` function requirements in C++23 to allow jump statements such as `goto` and labels inside `constexpr` functions. In `CheckConstexprFunctionStmt`, `GotoStmtClass` and `LabelStmtClass` were added, but `IndirectGotoStmtClass` (GNU computed goto) was missed.

This patch adds `case Stmt::IndirectGotoStmtClass:` alongside `GotoStmtClass` in `CheckConstexprFunctionStmt`.

### Details
- Before this patch, writing `goto *p;` inside a `constexpr` function resulted in `error: statement not allowed in constexpr function` because `IndirectGotoStmtClass` fell through to the `default` case in `CheckConstexprFunctionStmt`.
- With this patch, indirect gotos behave identically to standard gotos in `constexpr` function bodies:
  - Allowed in C++23 mode.
  - Issued as a C++23 extension warning in earlier modes (C++11 through C++20).
  - Executing an indirect goto during constant evaluation continues to produce an error during evaluation, while valid runtime paths (e.g. guarded by `if consteval`) succeed.

### Test Plan
- Added test case covering indirect gotos (`goto *(&&x);`) to `clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp`.
- Verified test fails without the patch and passes with the patch across all standard modes (`-std=c++11` through C++23).

---
Full diff: https://github.com/llvm/llvm-project/pull/213449.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaDeclCXX.cpp (+1) 
- (modified) clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp (+8) 


``````````diff
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 47b01b913b428..76b3d8cca9701 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -2343,6 +2343,7 @@ CheckConstexprFunctionStmt(Sema &SemaRef, const FunctionDecl *Dcl, Stmt *S,
 
   case Stmt::LabelStmtClass:
   case Stmt::GotoStmtClass:
+  case Stmt::IndirectGotoStmtClass:
     if (Cxx2bLoc.isInvalid())
       Cxx2bLoc = S->getBeginLoc();
     for (Stmt *SubStmt : S->children()) {
diff --git a/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp b/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp
index 51990ee4341d2..508941ae0429d 100644
--- a/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp
+++ b/clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp
@@ -167,6 +167,14 @@ constexpr int DisallowedStmtsCXX14_7() {
   int n; // beforecxx20-warning {{uninitialized variable in a constexpr function}}
 }
 
+constexpr int DisallowedStmtsCXX14_8() {
+  return 0; // beforecxx14-note {{previous}}
+  //  - an indirect goto statement
+  goto *(&&x); // beforecxx23-warning {{use of this statement in a constexpr function is a C++23 extension}}
+  x:;
+    return 0; // beforecxx14-warning {{multiple return}}
+}
+
 constexpr int ForStmt() {
   for (int n = 0; n < 10; ++n) {} // beforecxx14-error {{statement not allowed in constexpr function}}
     return 0;

``````````

</details>


https://github.com/llvm/llvm-project/pull/213449