[Lldb-commits] [lldb] [lldb] Improve diagnostic when a variable being assigned isn't an lvalue (PR #216624)

[Alexsander Borges Damaceno via lldb-commits]

https://github.com/AlexsanderDamaceno created https://github.com/llvm/llvm-project/pull/216624

In some cases, when a binary is compiled with certain optimization flags, a variable may not have a memory location.

For example, a local variable may have its location represented in a register due to an optimization performed during compilation.

Since the current message is generic:

**"error: Couldn't apply expression side effects : couldn't write the new contents of var back into the variable"**

it could not make a clear why the write operation was not done, so this patch add a note for cases where the lvalue does not have a memory location.

Bug link: https://github.com/llvm/llvm-project/issues/130701

>From ca59ccf0a2534cd2485debeb415901e2ceced74e Mon Sep 17 00:00:00 2001
From: AlexsanderDamaceno <aemgbo at gmail.com>
Date: Sun, 16 Aug 2026 23:26:09 -0300
Subject: [PATCH] [lldb] Improve diagnostic when a variable being assigned
 isn't an lvalue

In some cases, when a binary is compiled with certain optimization flags, a variable may not have a memory location

For example, a local variable may have its location represented in a register due to an optimization performed during compilation.

Since the current message is generic:
"error: Couldn't apply expression side effects :
couldn't write the new contents of var back into the variable"

it could not make a clear why the write opertion was not done,
so this patch add a note for cases where the lvalue does not have a memory location.
---
 lldb/source/Expression/Materializer.cpp | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lldb/source/Expression/Materializer.cpp b/lldb/source/Expression/Materializer.cpp
index 51e95d3376f72..8a800d7b454ee 100644
--- a/lldb/source/Expression/Materializer.cpp
+++ b/lldb/source/Expression/Materializer.cpp
@@ -648,12 +648,21 @@ class EntityVariableBase : public Materializer::Entity {
       Status set_error;
 
       if (actually_write) {
+        if (!valobj_sp->CanSetValue()) {
+          err = Status::FromErrorStringWithFormatv(
+              "couldn't write the new contents of {0} back into the "
+              "variable\nnote: Left operand of assignment is not an lvalue",
+              GetName());
+          return;
+        }
+
         valobj_sp->SetData(data, set_error);
 
         if (!set_error.Success()) {
           err = Status::FromErrorStringWithFormatv(
-              "couldn't write the new contents of {0} back into the variable",
-              GetName());
+              "couldn't write the new contents of {0} back into the "
+              "variable\nnote: {1}",
+              GetName(), set_error.AsCString());
           return;
         }
       }