Ignore compiler-generated unused static variables

Exclude synthetic variables from cpp/unused-static-variable so empty parameter pack placeholders do not trigger alerts. Update the regression expectation and document the analysis change.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jketema

cpp/ql/src/Best Practices/Unused Entities/UnusedStaticVariables.ql

@@ -22,6 +22,7 @@ predicate declarationHasSideEffects(Variable v) {
 from Variable v
 where
   v.isStatic() and
+  not v.isCompilerGenerated() and
   v.hasDefinition() and
   not v.isConstexpr() and
   not exists(VariableAccess a | a.getTarget() = v) and

cpp/ql/src/change-notes/2026-10-30-unused-static-compiler-generated.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `cpp/unused-static-variable` query no longer reports compiler-generated variables, such as placeholders for empty function parameter packs.

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/UnusedStaticVariables.expected

@@ -2,4 +2,3 @@
 | test.cpp:8:12:8:21 | staticVar6 | Static variable staticVar6 is never read. |
 | test.cpp:10:11:10:19 | constVar8 | Static variable constVar8 is never read. |
 | test.cpp:12:12:12:22 | staticVar10 | Static variable staticVar10 is never read. |
-| test.cpp:53:25:53:30 | values | Static variable values is never read. |

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp

@@ -50,7 +50,7 @@ struct WithDestructor
 static WithConstructor constructed; // GOOD (constructor has a side effect)
 static WithDestructor destructed; // GOOD (destructor has a side effect)
 
-static auto sum(auto... values) // $ SPURIOUS: Alert
+static auto sum(auto... values)
 {
 	return (0 + ... + values);
 }