[Build] Working around broken name mangling in MSVC 16.5.5 + CUDA 11.3 (#3790)

* fix

* explain

* oops

src/graph/transform/cuda/cuda_to_block.cu

@@ -381,9 +381,12 @@ ToBlockGPU(
 
 }  // namespace
 
-template<>
+// Use explicit names to get around MSVC's broken mangling that thinks the following two
+// functions are the same.
+// Using template<> fails to export the symbols.
 std::tuple<HeteroGraphPtr, std::vector<IdArray>>
-ToBlock<kDLGPU, int32_t>(
+// ToBlock<kDLGPU, int32_t>
+ToBlockGPU32(
     HeteroGraphPtr graph,
     const std::vector<IdArray> &rhs_nodes,
     bool include_rhs_in_lhs,
@@ -391,9 +394,9 @@ ToBlock<kDLGPU, int32_t>(
   return ToBlockGPU<int32_t>(graph, rhs_nodes, include_rhs_in_lhs, lhs_nodes);
 }
 
-template<>
 std::tuple<HeteroGraphPtr, std::vector<IdArray>>
-ToBlock<kDLGPU, int64_t>(
+// ToBlock<kDLGPU, int64_t>
+ToBlockGPU64(
     HeteroGraphPtr graph,
     const std::vector<IdArray> &rhs_nodes,
     bool include_rhs_in_lhs,

src/graph/transform/to_bipartite.cc

@@ -159,6 +159,37 @@ ToBlock<kDLCPU, int64_t>(HeteroGraphPtr graph,
   return ToBlockCPU<int64_t>(graph, rhs_nodes, include_rhs_in_lhs, lhs_nodes);
 }
 
+#ifdef DGL_USE_CUDA
+
+// Forward declaration of GPU ToBlock implementations - actual implementation is in
+// ./cuda/cuda_to_block.cu
+// This is to get around the broken name mangling in VS2019 CL 16.5.5 + CUDA 11.3
+// which complains that the two template specializations have the same signature.
+std::tuple<HeteroGraphPtr, std::vector<IdArray>>
+ToBlockGPU32(HeteroGraphPtr, const std::vector<IdArray>&, bool, std::vector<IdArray>* const);
+std::tuple<HeteroGraphPtr, std::vector<IdArray>>
+ToBlockGPU64(HeteroGraphPtr, const std::vector<IdArray>&, bool, std::vector<IdArray>* const);
+
+template<>
+std::tuple<HeteroGraphPtr, std::vector<IdArray>>
+ToBlock<kDLGPU, int32_t>(HeteroGraphPtr graph,
+                         const std::vector<IdArray> &rhs_nodes,
+                         bool include_rhs_in_lhs,
+                         std::vector<IdArray>* const lhs_nodes) {
+  return ToBlockGPU32(graph, rhs_nodes, include_rhs_in_lhs, lhs_nodes);
+}
+
+template<>
+std::tuple<HeteroGraphPtr, std::vector<IdArray>>
+ToBlock<kDLGPU, int64_t>(HeteroGraphPtr graph,
+                         const std::vector<IdArray> &rhs_nodes,
+                         bool include_rhs_in_lhs,
+                         std::vector<IdArray>* const lhs_nodes) {
+  return ToBlockGPU64(graph, rhs_nodes, include_rhs_in_lhs, lhs_nodes);
+}
+
+#endif  // DGL_USE_CUDA
+
 DGL_REGISTER_GLOBAL("transform._CAPI_DGLToBlock")
 .set_body([] (DGLArgs args, DGLRetValue *rv) {
     const HeteroGraphRef graph_ref = args[0];