Build failure with llvm-23.1.2 and clang-23.1.2 [error: call to 'swap' is ambiguous]

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This was found when building libvmaf version 3.2.1 on Gentoo with llvm-23.1.2 and clang-23.1.2 (i.e. on an LLVM profile). I reproduced this bug even with master.

A snippet from the build log (a full build log can be found in the downstream bug):

FAILED: [code=1] src/liblibsvm.a.p/svm.cpp.o 
clang++ -Isrc/liblibsvm.a.p -Isrc -I../vmaf-3.2.1/libvmaf/src -Iinclude -I../vmaf-3.2.1/libvmaf/include -I../vmaf-3.2.1/libvmaf/src/feature -I../vmaf-3.2.1/libvmaf/src/feature/common -fdiagnostics-color=always -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=c++11 -D_GNU_SOURCE -O2 -pipe -march=native -fPIC -pthread -pedantic -DOC_NEW_STYLE_INCLUDES -U_LIBCPP_ENABLE_ASSERTIONS -MD -MQ src/liblibsvm.a.p/svm.cpp.o -MF src/liblibsvm.a.p/svm.cpp.o.d -o src/liblibsvm.a.p/svm.cpp.o -c ../vmaf-3.2.1/libvmaf/src/svm.cpp
In file included from ../vmaf-3.2.1/libvmaf/src/svm.cpp:13:
In file included from /usr/include/c++/v1/vector:314:
In file included from /usr/include/c++/v1/__vector/vector.h:49:
/usr/include/c++/v1/__split_buffer:195:5: error: call to 'swap' is ambiguous
  195 |     swap(__begin_, __begin);
      |     ^~~~

I'm guessing this is newer Clang is more stricter in terms of imports and the swap function in libvmaf/src/svm.cpp is conflicting with std::swap. As a test/workaround I re-named the inline swap function to custom_swap and it did help in building.

diff --git a/libvmaf/src/svm.cpp b/libvmaf/src/svm.cpp
index 94d3379d80d7..de31e6822557 100644
--- a/libvmaf/src/svm.cpp
+++ b/libvmaf/src/svm.cpp
@@ -21,7 +21,7 @@ template <class T> static inline T min(T x,T y) { return (x<y)?x:y; }
 #ifndef max
 template <class T> static inline T max(T x,T y) { return (x>y)?x:y; }
 #endif
-template <class T> static inline void swap(T& x, T& y) { T t=x; x=y; y=t; }
+template <class T> static inline void custom_swap(T& x, T& y) { T t=x; x=y; y=t; }
 template <class S, class T> static inline void clone(T*& dst, S* src, int n)
 {
 	dst = new T[n];
@@ -149,7 +149,7 @@ int Cache::get_data(const int index, Qfloat **data, int len)
 		// allocate new space
 		h->data = (Qfloat *)realloc(h->data,sizeof(Qfloat)*len);
 		size -= more;
-		swap(h->len,len);
+		custom_swap(h->len,len);
 	}
 
 	lru_insert(h);
@@ -163,18 +163,18 @@ void Cache::swap_index(int i, int j)
 
 	if(head[i].len) lru_delete(&head[i]);
 	if(head[j].len) lru_delete(&head[j]);
-	swap(head[i].data,head[j].data);
-	swap(head[i].len,head[j].len);
+	custom_swap(head[i].data,head[j].data);
+	custom_swap(head[i].len,head[j].len);
 	if(head[i].len) lru_insert(&head[i]);
 	if(head[j].len) lru_insert(&head[j]);
 
-	if(i>j) swap(i,j);
+	if(i>j) custom_swap(i,j);
 	for(head_t *h = lru_head.next; h!=&lru_head; h=h->next)
 	{
 		if(h->len > i)
 		{
 			if(h->len > j)
-				swap(h->data[i],h->data[j]);
+				custom_swap(h->data[i],h->data[j]);
 			else
 			{
 				// give up
@@ -214,8 +214,8 @@ public:
 	virtual double *get_QD() const = 0;
 	virtual void swap_index(int i, int j) const	// no so const...
 	{
-		swap(x[i],x[j]);
-		if(x_square) swap(x_square[i],x_square[j]);
+		custom_swap(x[i],x[j]);
+		if(x_square) custom_swap(x_square[i],x_square[j]);
 	}
 protected:
 
@@ -454,13 +454,13 @@ private:
 void Solver::swap_index(int i, int j)
 {
 	Q->swap_index(i,j);
-	swap(y[i],y[j]);
-	swap(G[i],G[j]);
-	swap(alpha_status[i],alpha_status[j]);
-	swap(alpha[i],alpha[j]);
-	swap(p[i],p[j]);
-	swap(active_set[i],active_set[j]);
-	swap(G_bar[i],G_bar[j]);
+	custom_swap(y[i],y[j]);
+	custom_swap(G[i],G[j]);
+	custom_swap(alpha_status[i],alpha_status[j]);
+	custom_swap(alpha[i],alpha[j]);
+	custom_swap(p[i],p[j]);
+	custom_swap(active_set[i],active_set[j]);
+	custom_swap(G_bar[i],G_bar[j]);
 }
 
 void Solver::reconstruct_gradient()
@@ -1301,8 +1301,8 @@ public:
 	{
 		cache->swap_index(i,j);
 		Kernel::swap_index(i,j);
-		swap(y[i],y[j]);
-		swap(QD[i],QD[j]);
+		custom_swap(y[i],y[j]);
+		custom_swap(QD[i],QD[j]);
 	}
 
 	~SVC_Q()
@@ -1350,7 +1350,7 @@ public:
 	{
 		cache->swap_index(i,j);
 		Kernel::swap_index(i,j);
-		swap(QD[i],QD[j]);
+		custom_swap(QD[i],QD[j]);
 	}
 
 	~ONE_CLASS_Q()
@@ -1390,9 +1390,9 @@ public:
 
 	void swap_index(int i, int j) const
 	{
-		swap(sign[i],sign[j]);
-		swap(index[i],index[j]);
-		swap(QD[i],QD[j]);
+		custom_swap(sign[i],sign[j]);
+		custom_swap(index[i],index[j]);
+		custom_swap(QD[i],QD[j]);
 	}
 
 	Qfloat *get_Q(int i, int len) const
@@ -1908,7 +1908,7 @@ static void svm_binary_svc_probability(
 	for(i=0;i<prob->l;i++)
 	{
 		int j = i+rand()%(prob->l-i);
-		swap(perm[i],perm[j]);
+		custom_swap(perm[i],perm[j]);
 	}
 	for(i=0;i<nr_fold;i++)
 	{
@@ -2059,8 +2059,8 @@ static void svm_group_classes(const svm_problem *prob, int *nr_class_ret, int **
 	//
 	if (nr_class == 2 && label[0] == -1 && label[1] == 1)
 	{
-		swap(label[0],label[1]);
-		swap(count[0],count[1]);
+		custom_swap(label[0],label[1]);
+		custom_swap(count[0],count[1]);
 		for(i=0;i<l;i++)
 		{
 			if(data_label[i] == 0)
@@ -2373,7 +2373,7 @@ void svm_cross_validation(const svm_problem *prob, const svm_parameter *param, i
 			for(i=0;i<count[c];i++)
 			{
 				int j = i+rand()%(count[c]-i);
-				swap(index[start[c]+j],index[start[c]+i]);
+				custom_swap(index[start[c]+j],index[start[c]+i]);
 			}
 		for(i=0;i<nr_fold;i++)
 		{
@@ -2410,7 +2410,7 @@ void svm_cross_validation(const svm_problem *prob, const svm_parameter *param, i
 		for(i=0;i<l;i++)
 		{
 			int j = i+rand()%(l-i);
-			swap(perm[i],perm[j]);
+			custom_swap(perm[i],perm[j]);
 		}
 		for(i=0;i<=nr_fold;i++)
 			fold_start[i]=i*l/nr_fold;

Downstream bug: https://bugs.gentoo.org/983392

[lusoris]

This matches a name clash in svm.cpp: it defines a global template <class T> static inline void swap(T&, T&) (line 24, next to the min/max templates at lines 19 and 22). The log shows libc++ 23's __split_buffer (line 195) calling swap unqualified on svm_node* members; ADL for a pointer to a global-namespace type finds both the global template and std::swap.

I only have clang 22.1.8 with libstdc++ here, no libc++ 23, so I could not reproduce the libc++ error itself. The mechanism reproduces with a stand-in (a struct svm_node in the global namespace, the same swap template, then using std::swap; swap(a, b); on two svm_node*): both clang 22 and gcc 16 report call to 'swap' is ambiguous, and replacing the template with using std::swap; makes it compile.

#1617 removes the three templates and uses using std::min; using std::max; using std::swap;, which is the same fix. On master with #1617 applied, svm.cpp compiles with libstdc++ (full build) and with libc++ 21.1 (zig's bundled copy) at -std=c++11 and -std=c++23, and the Netflix src01 pair gives identical pooled vmaf (76.667831 mean) before and after. Building with libc++ 23 is untested. The renaming workaround from the report would also work.