[lldb][test] Fix libstdc++ std::variant formatter tests for valueless variants (#147283)

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](https://en.cppreference.com/w/cpp/utility/variant/valueless_by_exception.html)
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

Author: Michael137

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 *
@@ -62,17 +61,13 @@ 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"])
+        self.expect("frame variable v_valueless", substrs=["v_valueless =  No Value"])
 
         self.expect(
-            "frame variable v_many_types_no_value",
-            substrs=["v_many_types_no_value =  No Value"],
+            "frame variable v_many_types_valueless",
+            substrs=["v_many_types_valueless =  No Value"],
         )
-        """
 
     @add_test_categories(["libstdcxx"])
     def test_invalid_variant_index(self):

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/main.cpp

@@ -4,7 +4,9 @@
 #include <vector>
 
 struct S {
-  operator int() { throw 42; }
+  S() = default;
+  S(S &&) { throw 42; }
+  S &operator=(S &&) = default;
 };
 
 int main() {
@@ -21,7 +23,7 @@ int main() {
   std::variant<int, double, char> v2;
   std::variant<int, double, char> v3;
   std::variant<std::variant<int, double, char>> v_v1;
-  std::variant<int, double, char> v_no_value;
+  std::variant<int, char, S> v_valueless = 5;
   // The next variant has many types, meaning the type index does not fit in
   // a byte and must be `unsigned short` instead of `unsigned char` when
   // using the unstable libc++ ABI. With stable libc++ ABI, the type index
@@ -43,8 +45,11 @@ int main() {
       int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
       int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
       int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
-      int, int, int, int, int, int, int, int, int, int, int, int>
-      v_many_types_no_value;
+      int, int, int, int, int, int, int, int, int, int, int, int, S>
+      v_many_types_valueless;
+
+  v_valueless = 5;
+  v_many_types_valueless.emplace<0>(10);
 
   v1 = 12; // v contains int
   v1_typedef = v1;
@@ -67,18 +72,22 @@ int main() {
   printf("%f\n", d); // break here
 
   try {
-    v_no_value.emplace<0>(S());
+    // Exception in type-changing move-assignment is guaranteed to put
+    // std::variant into a valueless state.
+    v_valueless = S();
   } catch (...) {
   }
 
-  printf("%zu\n", v_no_value.index());
+  printf("%d\n", v_valueless.valueless_by_exception());
 
   try {
-    v_many_types_no_value.emplace<0>(S());
+    // Exception in move-assignment is guaranteed to put std::variant into a
+    // valueless state.
+    v_many_types_valueless = S();
   } catch (...) {
   }
 
-  printf("%zu\n", v_many_types_no_value.index());
+  printf("%d\n", v_many_types_valueless.valueless_by_exception());
 
   return 0; // break here
 }