Return type of void functions breaks MLIR invariants

[keryell]

I work on a project involving some CIR micro-surgery and I was banging my head on having mlir::inlineCall() working and it actually was due to the precondition on https://github.com/llvm/llvm-project/blob/main/mlir/lib/Transforms/Utils/InliningUtils.cpp#L458 failing: a
The reason is that C++ void (...) functions say they return a !cir.void type in CIR but actually they return no value.
The ClangIR infrastructure hides this in various ways.
Some random sampling:

bool FuncType::isVoid() const { return mlir::isa<VoidType>(getReturnType()); }

In clang/lib/CIR/Dialect/IR/CIRDialect.cpp:

ParseResult cir::FuncOp::parse() {
...
  // Fetch return type or set it to void if empty/ommited.
  mlir::Type returnType =
      (resultTypes.empty() ? cir::VoidType::get(builder.getContext())
                           : resultTypes.front());
}
static LogicalResult
verifyCallCommInSymbolUses(...) {
...
  // Void function must not return any results.
  if (fnType.isVoid() && op->getNumResults() != 0)
    return op->emitOpError("callee returns void but call has results");

  // Non-void function calls must return exactly one result.
  if (!fnType.isVoid() && op->getNumResults() != 1)
    return op->emitOpError("incorrect number of results for callee");

  // Parent function and return value types must match.
  if (!fnType.isVoid() &&
      op->getResultTypes().front() != fnType.getReturnType()) {
    return op->emitOpError("result type mismatch: expected ")
           << fnType.getReturnType() << ", but provided "
           << op->getResult(0).getType();

I thought about different ways to fix this but at the end I think the most reasonable solution is not to go against MLIR. 😄
Just remove the return type of void functions and adapt the infrastructure accordingly.
I have seen some breakage when using --remove-dead-values and --symbol-dce which might be related.
I am curious about how we make the difference in between K&R C f() and C99 f(void) but this is an orthogonal story.