[clang] [CIR] Better handling of `void` function return (PR #128089)

[Ronan Keryell via cfe-commits]

================
@@ -375,11 +409,48 @@ void printFuncTypeArgs(mlir::AsmPrinter &p, mlir::ArrayRef<mlir::Type> params,
   p << ')';
 }
 
+// Use a custom parser to handle the optional return and argument types without
+// an optional anchor.
+static mlir::ParseResult parseFuncType(mlir::AsmParser &p,
+                                       mlir::Type &optionalReturnTypes,
+                                       llvm::SmallVector<mlir::Type> &params,
+                                       bool &isVarArg) {
+  if (failed(parseFuncTypeReturn(p, optionalReturnTypes)))
+    return failure();
+  return parseFuncTypeArgs(p, params, isVarArg);
+}
+
+static void printFuncType(mlir::AsmPrinter &p, mlir::Type optionalReturnTypes,
+                          mlir::ArrayRef<mlir::Type> params, bool isVarArg) {
+  printFuncTypeReturn(p, optionalReturnTypes);
+  printFuncTypeArgs(p, params, isVarArg);
+}
+
+// Return the actual return type or an explicit !cir.void if the function does
+// not return anything
+mlir::Type FuncType::getReturnType() const {
+  if (isVoid())
+    return cir::VoidType::get(getContext());
+  return static_cast<detail::FuncTypeStorage *>(getImpl())->optionalReturnType;
+}
+
+/// Returns the result type of the function as an ArrayRef, enabling better
+/// integration with generic MLIR utilities.
 llvm::ArrayRef<mlir::Type> FuncType::getReturnTypes() const {
-  return static_cast<detail::FuncTypeStorage *>(getImpl())->returnType;
+  if (isVoid())
+    return {};
+  return static_cast<detail::FuncTypeStorage *>(getImpl())->optionalReturnType;
----------------
keryell wrote:

No because it returns the real return types (plural) that any real MLIR user is waiting for: the actual return types, which in the case of C/C++ is 0 or 1 type.
The whole point of this PR is to fix this bug and not to return some synthetic `!cir.void` which was breaking the neighborhood.

https://github.com/llvm/llvm-project/pull/128089