clang-tidy fixes (mostly manual) related to PR pybind#3166

Author: rwgk

include/pybind11/detail/smart_holder_poc.h

@@ -309,7 +309,7 @@ struct smart_holder {
             hld.vptr.reset(static_cast<void *>(unq_ptr.get()), std::move(gd));
         else
             hld.vptr.reset(unq_ptr.get(), std::move(gd));
-        unq_ptr.release();
+        (void) unq_ptr.release();
         hld.is_populated = true;
         return hld;
     }

include/pybind11/detail/smart_holder_type_casters.h

@@ -704,13 +704,11 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
     static handle cast(const std::shared_ptr<T> &src, return_value_policy policy, handle parent) {
         switch (policy) {
             case return_value_policy::automatic:
-                break;
             case return_value_policy::automatic_reference:
                 break;
             case return_value_policy::take_ownership:
                 throw cast_error("Invalid return_value_policy for shared_ptr (take_ownership).");
             case return_value_policy::copy:
-                break;
             case return_value_policy::move:
                 break;
             case return_value_policy::reference:
@@ -809,7 +807,7 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
                     // Critical transfer-of-ownership section. This must stay together.
                     self_life_support->deactivate_life_support();
                     holder.reclaim_disowned();
-                    src.release();
+                    (void) src.release();
                     // Critical section end.
                     return existing_inst;
                 }

tests/pure_cpp/smart_holder_poc_test.cpp

@@ -137,8 +137,8 @@ TEST_CASE("from_raw_ptr_take_ownership+disown+reclaim_disowned", "[S]") {
     REQUIRE(*new_owner == 19);
     hld.reclaim_disowned(); // Manually veriified: without this, clang++ -fsanitize=address reports
                             // "detected memory leaks".
-    new_owner.release();    // Manually verified: without this, clang++ -fsanitize=address reports
-                            // "attempting double-free".
+    (void) new_owner.release(); // Manually verified: without this, clang++ -fsanitize=address
+                                // reports "attempting double-free".
     REQUIRE(hld.as_lvalue_ref<int>() == 19);
     REQUIRE(new_owner.get() == nullptr);
 }

tests/test_class_sh_trampoline_shared_from_this.cpp

@@ -21,7 +21,7 @@ struct Sft : std::enable_shared_from_this<Sft> {
     // history in case something goes wrong.
     // However, compilers other than clang have a variety of issues. It is not
     // worth the trouble covering all platforms.
-    Sft(const Sft &other) { history = other.history + "_CpCtor"; }
+    Sft(const Sft &other) : enable_shared_from_this(other) { history = other.history + "_CpCtor"; }
 
     Sft(Sft &&other) noexcept { history = other.history + "_MvCtor"; }