improve unnecessarycatchthrow plugin

it is not legal to eliminate a catch/re-throw where the re-throw
expliciting mentions the exception variable and the exception variable
is a non-final class

Change-Id: I7fd88b0d004d2efa66aef2c0876e07f203da3c28
Reviewed-on: https://gerrit.libreoffice.org/42782
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/test/unnecessarycatchthrow.cxx

@@ -30,6 +30,29 @@ int main()
         std::cout << "test";
         throw;
     }
+
+}
+
+void test1()
+{
+    // cannot remove catch/throw where the throw is of a non-final class
+    struct B {};
+    struct D: B {};
+    try {
+        throw D();
+    } catch (B & b) {
+        throw b; // must not be removed
+    }
+};
+
+void test2()
+{
+    struct F final {};
+    try {
+        throw F();
+    } catch (F const & f) { // expected-error {{unnecessary catch and throw [loplugin:unnecessarycatchthrow]}}
+        throw f;
+    }
 };
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */

compilerplugins/clang/unnecessarycatchthrow.cxx

@@ -58,6 +58,15 @@ bool UnnecessaryCatchThrow::VisitCXXTryStmt(CXXTryStmt const * tryStmt)
     auto subExpr = throwExpr->getSubExpr();
     if (subExpr)
     {
+        if (auto cxxConstructExpr = dyn_cast<CXXConstructExpr>(subExpr)) {
+            if (!cxxConstructExpr->getConstructor()->isCopyConstructor())
+                return true;
+            if (!cxxConstructExpr->getConstructor()->getParent()->hasAttr<FinalAttr>())
+                return true;
+            if (cxxConstructExpr->getNumArgs() != 1)
+                return true;
+            subExpr = cxxConstructExpr->getArg(0);
+        }
         auto declRefExpr = dyn_cast<DeclRefExpr>(subExpr->IgnoreImpCasts());
         if (!declRefExpr)
             return true;