♻️ miscellaneous changes pulled from #622 (#669)

## Description

This PR pulls out a couple of changes from #662 that can go into the
library immediately, while that PR is still in draft.
Mostly fixes a couple of linter/compiler warnings and improves the
handling of Boost / boost_multiprecision.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
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- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

cmake/ExternalDependencies.cmake

@@ -39,12 +39,13 @@ else()
   endif()
 endif()
 
-option(USE_SYSTEM_BOOST "Whether to try to use the system Boost installation" ON)
+option(USE_SYSTEM_BOOST "Whether to try to use the system Boost installation" OFF)
 set(BOOST_MIN_VERSION
     1.80.0
     CACHE STRING "Minimum required Boost version")
-find_package(Boost ${BOOST_MIN_VERSION} CONFIG QUIET)
-if(NOT Boost_FOUND OR NOT USE_SYSTEM_BOOST)
+if(USE_SYSTEM_BOOST)
+  find_package(Boost ${BOOST_MIN_VERSION} CONFIG REQUIRED)
+else()
   set(BOOST_MP_STANDALONE
       ON
       CACHE INTERNAL "Use standalone boost multiprecision")
@@ -54,14 +55,14 @@ if(NOT Boost_FOUND OR NOT USE_SYSTEM_BOOST)
   set(BOOST_URL
       https://github.com/boostorg/multiprecision/archive/refs/tags/Boost_${BOOST_VERSION}.tar.gz)
   if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.24)
-    FetchContent_Declare(boost_multiprecision URL ${BOOST_URL} FIND_PACKAGE_ARGS
-                                                  ${BOOST_MIN_VERSION} CONFIG)
-    list(APPEND FETCH_PACKAGES boost_multiprecision)
+    FetchContent_Declare(boost_mp URL ${BOOST_URL} FIND_PACKAGE_ARGS ${BOOST_MIN_VERSION} CONFIG
+                                      NAMES boost_multiprecision)
+    list(APPEND FETCH_PACKAGES boost_mp)
   else()
-    find_package(boost_multiprecision ${BOOST_MIN_VERSION} QUIET CONFIG)
-    if(NOT boost_multiprecision_FOUND)
-      FetchContent_Declare(boost_multiprecision URL ${BOOST_URL})
-      list(APPEND FETCH_PACKAGES boost_multiprecision)
+    find_package(boost_mp ${BOOST_MIN_VERSION} QUIET CONFIG NAMES boost_multiprecision)
+    if(NOT boost_mp_FOUND)
+      FetchContent_Declare(boost_mp URL ${BOOST_URL})
+      list(APPEND FETCH_PACKAGES boost_mp)
     endif()
   endif()
 endif()

include/mqt-core/ir/operations/CompoundOperation.hpp

@@ -41,7 +41,7 @@ class CompoundOperation final : public Operation {
 
   [[nodiscard]] bool isNonUnitaryOperation() const override;
 
-  [[nodiscard]] inline bool isSymbolicOperation() const override;
+  [[nodiscard]] bool isSymbolicOperation() const override;
 
   [[nodiscard]] bool isCustomGate() const;
 

src/ir/parsers/QASM3Parser.cpp

@@ -109,7 +109,7 @@ class OpenQasm3Parser final : public InstVisitor {
     }
 
     for (uint64_t i = 0; i < qubit.second; ++i) {
-      qubits.emplace_back(qubit.first + i);
+      qubits.emplace_back(static_cast<qc::Qubit>(qubit.first + i));
     }
   }