I should be using + instead of += apparently.

Yay for ##c++ for pointing this out.

src/Command.cpp

@@ -81,7 +81,7 @@ static void SplitWithQuotes( const RString sSource, const char Delimitor, vector
 
 RString Commands::GetOriginalCommandString() const
 {
-	return std::accumulate(v.begin(), v.end(), RString(), [](RString &res, Command const &c) { return res += c.GetOriginalCommandString(); });
+	return std::accumulate(v.begin(), v.end(), RString(), [](RString &res, Command const &c) { return res + c.GetOriginalCommandString(); });
 }
 
 void ParseCommands( const RString &sCommands, Commands &vCommandsOut )

src/ModelTypes.cpp

@@ -150,7 +150,7 @@ void AnimatedTexture::SetState( int iState )
 float AnimatedTexture::GetAnimationLengthSeconds() const
 {
 	return std::accumulate(vFrames.begin(), vFrames.end(), 0.f,
-		[](float total, AnimatedTextureState const &state) { return total += state.fDelaySecs; });
+		[](float total, AnimatedTextureState const &state) { return total + state.fDelaySecs; });
 }
 
 void AnimatedTexture::SetSecondsIntoAnimation( float fSeconds )

src/PlayerStageStats.cpp

@@ -315,7 +315,7 @@ int PlayerStageStats::GetLessonScoreActual() const
 int PlayerStageStats::GetLessonScoreNeeded() const
 {
 	float score = std::accumulate(m_vpPossibleSteps.begin(), m_vpPossibleSteps.end(), 0.f,
-		[](float total, Steps const *steps) { return total += steps->GetRadarValues(PLAYER_1).m_Values.v.fNumTapsAndHolds; });
+		[](float total, Steps const *steps) { return total + steps->GetRadarValues(PLAYER_1).m_Values.v.fNumTapsAndHolds; });
 	return lrintf( score * LESSON_PASS_THRESHOLD );
 }
 

src/Sprite.cpp

@@ -743,7 +743,7 @@ void Sprite::SetState( int iNewState )
 
 float Sprite::GetAnimationLengthSeconds() const
 {
-	return std::accumulate(m_States.begin(), m_States.end(), 0.f, [](float total, State const &s) { return total += s.fDelay; });
+	return std::accumulate(m_States.begin(), m_States.end(), 0.f, [](float total, State const &s) { return total + s.fDelay; });
 }
 
 void Sprite::SetSecondsIntoAnimation( float fSeconds )

src/Trail.cpp

@@ -159,14 +159,14 @@ int Trail::GetMeter() const
 int Trail::GetTotalMeter() const
 {
 	return std::accumulate(m_vEntries.begin(), m_vEntries.end(), 0, [](int total, TrailEntry const &e) {
-		return total += e.pSteps->GetMeter();
+		return total + e.pSteps->GetMeter();
 	});
 }
 
 float Trail::GetLengthSeconds() const
 {
 	return std::accumulate(m_vEntries.begin(), m_vEntries.end(), 0.f, [](float total, TrailEntry const &e) {
-		return total += e.pSong->m_fMusicLengthSeconds;
+		return total + e.pSong->m_fMusicLengthSeconds;
 	});
 }
 

src/TrailUtil.cpp

@@ -15,7 +15,7 @@ int TrailUtil::GetNumSongs( const Trail *pTrail )
 float TrailUtil::GetTotalSeconds( const Trail *pTrail )
 {
 	auto const &entries = pTrail->m_vEntries;
-	return std::accumulate(entries.begin(), entries.end(), 0.f, [](float total, TrailEntry const &e) { return total += e.pSong->m_fMusicLengthSeconds; });
+	return std::accumulate(entries.begin(), entries.end(), 0.f, [](float total, TrailEntry const &e) { return total + e.pSong->m_fMusicLengthSeconds; });
 }
 
 ///////////////////////////////////////////////////////////////////////