[libcalamares] Avoid warnings / errors on both gcc and clang

- Clang 8 can detect that there is no need for a return if all
   previous paths already return. GCC 8 does not. Clang warns if
   the unreachable return is there, GCC errors out if it isn't.
 - Introduce a hack NOTREACHED that comments-out on Clang, and
   marks as unreachable (but still present) on GCC.
 - This might go away with an [[unreachable]] annotation or
   similar.

CMakeLists.txt

@@ -193,6 +193,7 @@ if( CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
     )
         string( APPEND CMAKE_CXX_FLAGS " ${CLANG_WARNINGS}" )
     endforeach()
+    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='//'" )
 
     # Third-party code where we don't care so much about compiler warnings
     # (because it's uncomfortable to patch) get different flags; use
@@ -218,6 +219,8 @@ else()
 
     set( SUPPRESS_3RDPARTY_WARNINGS "" )
     set( SUPPRESS_BOOST_WARNINGS "" )
+
+    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='__builtin_unreachable();'" )
 endif()
 
 # Use mark_thirdparty_code() to reduce warnings from the compiler

src/libcalamares/geoip/Handler.cpp

@@ -113,6 +113,7 @@ create_interface( Handler::Type t, const QString& selector )
             return nullptr;
 #endif
     }
+    NOTREACHED return nullptr;
 }
 
 static RegionZonePair

src/libcalamares/partition/PartitionSize.cpp

@@ -166,6 +166,7 @@ PartitionSize::toBytes() const
     case SizeUnit::GiB:
         return CalamaresUtils::GiBtoBytes( static_cast<unsigned long long>( value() ) );
     }
+    NOTREACHED return -1;
 }
 
 bool
@@ -186,6 +187,7 @@ PartitionSize::operator< ( const PartitionSize& other ) const
     case SizeUnit::GiB:
         return ( toBytes() < other.toBytes () );
     }
+    NOTREACHED return false;
 }
 
 bool
@@ -206,6 +208,7 @@ PartitionSize::operator> ( const PartitionSize& other ) const
     case SizeUnit::GiB:
         return ( toBytes() > other.toBytes () );
     }
+    NOTREACHED return false;
 }
 
 bool
@@ -226,6 +229,7 @@ PartitionSize::operator== ( const PartitionSize& other ) const
     case SizeUnit::GiB:
         return ( toBytes() == other.toBytes () );
     }
+    NOTREACHED return false;
 }
 
 } // namespace Calamares