Fix #14199 FN unusedVariable with templated type (regression) (#8013)

Author: chrchr-github

lib/checkunusedvar.cpp

@@ -148,6 +148,15 @@ class Variables {
     void clear() {
         mVarUsage.clear();
     }
+    void clearConst() {
+        std::set<int> keys;
+        for (const std::pair<const nonneg int, Variables::VariableUsage>& var : mVarUsage) {
+            if (var.second._var && var.second._var->isConst())
+                keys.emplace(var.first);
+        }
+        for (const int key : keys)
+            mVarUsage.erase(key);
+    }
     const std::map<nonneg int, VariableUsage> &varUsage() const {
         return mVarUsage;
     }
@@ -796,9 +805,8 @@ void CheckUnusedVar::checkFunctionVariableUsage_iterateScopes(const Scope* const
         // templates
         if (tok->isName() && endsWith(tok->str(), '>')) {
             // TODO: This is a quick fix to handle when constants are used
-            // as template parameters. Try to handle this better, perhaps
-            // only remove constants.
-            variables.clear();
+            // as template parameters.
+            variables.clearConst();
         }
 
         else if (Token::Match(tok->previous(), "[;{}]")) {

test/testunusedvar.cpp

@@ -6983,6 +6983,15 @@ class TestUnusedVar : public TestFixture {
                               "    return hash[0];\n"
                               "}\n");
         ASSERT_EQUALS("", errout_str());
+
+        functionVariableUsage("template <int N>\n" // #14199
+                              "struct S {\n"
+                              "    int i;\n"
+                              "};\n"
+                              "void f() {\n"
+                              "    S<0> s;\n"
+                              "}\n");
+        ASSERT_EQUALS("[test.cpp:6:10]: (style) Unused variable: s [unusedVariable]\n", errout_str());
     }
 
     void localvarFuncPtr() {