[clang] [clang][bytecode] Fix crash in void functions returning non-void expr… (PR #176550)

[via cfe-commits]

https://github.com/Serosh-commits created https://github.com/llvm/llvm-project/pull/176550

This patch addresses a crash in the new bytecode interpreter that occurs when a void function contains a return statement with a non-void expression (for example, a conditional operator where one branch has a side-effect and the other returns a value).

The Problem: The crash was triggered by an assertion failure in 
RVOPtr
: Assertion S.Current->getFunction()->hasRVO() failed.

In the bytecode compiler’s 
visitReturnStmt
, the logic was previously checking the type of the return expression rather than the return type of the function itself. If the expression had a non-void type (like the 
int
 in the reproducer return n > 1 ? foo(n-1) : 0;), the compiler would attempt to emit an 
RVOPtr
 opcode. Since void functions do not (and should not) have RVO metadata, the interpreter would hit the assertion.

The Fix: I've updated 
visitReturnStmt
 to explicitly check CompilingFunction->getReturnType()->isVoidType().

If the function is void, we now use 
discard()
 on the return expression. This ensures the expression is still evaluated for its side effects (if any) but its result is properly popped from the stack, and no RVO pointer is expected.
This prevents the compiler from falling through to the RVO initialization path for functions that cannot return a value.
Testing: I've added a regression test in 
clang/test/AST/ByteCode/gh176536.cpp
 using the reported reproducer. The test confirms that we now correctly handle the mismatch and emit the appropriate diagnostic instead of crashing the frontend.

Fixes #176536



>From 7f3db33e42835b3bf6b04ce64509d4fd4fc660fe Mon Sep 17 00:00:00 2001
From: Serosh <janmejayapanda400 at gmail.com>
Date: Sat, 17 Jan 2026 15:57:11 +0530
Subject: [PATCH] [clang][bytecode] Fix crash in void functions returning
 non-void expressions

The bytecode compiler was erroneously emitting an RVOPtr opcode for
void functions when they contained a non-void return expression.
This led to an assertion failure in the interpreter.

This fix ensures that discard() is used instead of visit() for such
expressions in void functions, and prevents the emission of RVO pointers.

Fixes #176536
---
 clang/lib/AST/ByteCode/Compiler.cpp  | 5 +++--
 clang/test/AST/ByteCode/gh176536.cpp | 6 ++++++
 2 files changed, 9 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/AST/ByteCode/gh176536.cpp

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 21f8db06919ed..623ba7380e1f3 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -5678,8 +5678,9 @@ bool Compiler<Emitter>::visitReturnStmt(const ReturnStmt *RS) {
       return this->emitRet(*ReturnType, RS);
     }
 
-    if (RE->getType()->isVoidType()) {
-      if (!this->visit(RE))
+    if (RE->getType()->isVoidType() ||
+        (CompilingFunction && CompilingFunction->getReturnType()->isVoidType())) {
+      if (!this->discard(RE))
         return false;
     } else {
       InitLinkScope<Emitter> ILS(this, InitLink::RVO());
diff --git a/clang/test/AST/ByteCode/gh176536.cpp b/clang/test/AST/ByteCode/gh176536.cpp
new file mode 100644
index 0000000000000..2ee45c14cae48
--- /dev/null
+++ b/clang/test/AST/ByteCode/gh176536.cpp
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -verify %s
+
+constexpr void foo(int n) {
+  return n > 1 ? foo(n - 1) : 0; // expected-error {{return type 'void' must match the return type 'int' of the expression}}
+}
+static_assert((foo(2), true), "");