[lldb][test] Fix libstdc++ std::variant formatter tests for valueless variants

[Michael137]

A default-constructed variant has a valid index (being the first element of the variant). The only case where the index is variant_npos is when the variant is "valueless", which according to cppreference only happens when an exception is thrown during assignment to the variant.

I adjusted the test to test that scenario, and the formatter seems to work.

Unblocks #147253

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Michael Buch (Michael137)

Changes

Instead of using the byte-size to make a guess at what the std::variant_npos value is, just look it up in debug-info.

Unblocks #147253

---

Full diff: https://github.com/llvm/llvm-project/pull/147283.diff

3 Files Affected:

• (modified) lldb/examples/synthetic/gnu_libstdcpp.py (+4-9)
• (modified) lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py (-5)
• (modified) lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/main.cpp (+1)

diff --git a/lldb/examples/synthetic/gnu_libstdcpp.py b/lldb/examples/synthetic/gnu_libstdcpp.py
index 20b9488af5597..96b11322db775 100644
--- a/lldb/examples/synthetic/gnu_libstdcpp.py
+++ b/lldb/examples/synthetic/gnu_libstdcpp.py
@@ -889,17 +889,12 @@ def VariantSummaryProvider(valobj, dict):
     if not (index_obj and index_obj.IsValid() and data_obj and data_obj.IsValid()):
         return "<Can't find _M_index or _M_u>"
 
-    def get_variant_npos_value(index_byte_size):
-        if index_byte_size == 1:
-            return 0xFF
-        elif index_byte_size == 2:
-            return 0xFFFF
-        else:
-            return 0xFFFFFFFF
+    npos = valobj.GetTarget().FindFirstGlobalVariable("std::variant_npos")
+    if not npos:
+        return "<Can't find std::variant_npos sentinel>"
 
-    npos_value = get_variant_npos_value(index_obj.GetByteSize())
     index = index_obj.GetValueAsUnsigned(0)
-    if index == npos_value:
+    if index == npos.GetValueAsUnsigned(0):
         return " No Value"
 
     # Strip references and typedefs.
diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py
index 394e221809f7c..c3325c9e73cb9 100644
--- a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py
@@ -2,7 +2,6 @@
 Test lldb data formatter for LibStdC++ std::variant.
 """
 
-
 import lldb
 from lldbsuite.test.decorators import *
 from lldbsuite.test.lldbtest import *
@@ -62,9 +61,6 @@ def test_with_run_command(self):
             "frame variable v3",
             substrs=["v3 =  Active Type = char  {", "Value = 'A'", "}"],
         )
-        """
-        TODO: temporarily disable No Value tests as they seem to fail on ubuntu/debian
-        bots. Pending reproduce and investigation.
 
         self.expect("frame variable v_no_value", substrs=["v_no_value =  No Value"])
 
@@ -72,7 +68,6 @@ def test_with_run_command(self):
             "frame variable v_many_types_no_value",
             substrs=["v_many_types_no_value =  No Value"],
         )
-        """
 
     @add_test_categories(["libstdcxx"])
     def test_invalid_variant_index(self):
diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/main.cpp b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/main.cpp
index 36e0f74f831f8..acb07d5700b8a 100644
--- a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/main.cpp
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/main.cpp
@@ -49,6 +49,7 @@ int main() {
   v1 = 12; // v contains int
   v1_typedef = v1;
   v_v1 = v1;
+  v1_typedef = v1;
   int i = std::get<int>(v1);
   printf("%d\n", i); // break here
 

[labath]

Looking up global variables is not completely ideal (can be slow and look up unrelated things). Would index_obj.GetValueAsSigned() == -1 work by any chance?

[Michael137]

Looking up global variables is not completely ideal (can be slow and look up unrelated things). Would index_obj.GetValueAsSigned() == -1 work by any chance?

Actually after some more investigating it turns out the test was just flawed. A default-constructed variant has a valid index (being the first element of the variant). The only case where the index is variant_npos is when the variant is "valueless", which according to cppreference only happens when an exception is thrown during assignment to the variant.

I adjusted the test to test that scenario, and the formatter seems to work.

[labath]

Cool. Thanks for looking into this.