[Portability][MSVC] Replace compound-literal-style dependent-type initialization in dense MLA decode

Background

I encountered this issue while building the vllm-project/FlashMLA fork through SystemPanic/vllm-windows for a deployment serving DeepSeek-V4-Flash-0731 on 4 x H200 NVL GPUs.

Problem

csrc/sm90/decode/dense/splitkv_mla.cuh contains 16 dependent-type initializations using the following form:

(typename T::X){}

Examples include:

TiledMMA tiled_mma_sQ =
    (typename T::TiledMMA_QK_sQ){};

Tensor sV = make_tensor(
    sK.data(),
    (typename T::SmemLayoutV){}
);

Tensor sQ = make_tensor(
    make_smem_ptr(plan.smem_sQ.data()),
    (typename T::SmemLayoutQ){}
);

This parenthesized type followed by a braced initializer is compound-literal-style syntax accepted as an extension by GNU-compatible toolchains. It is not portable to the NVCC and MSVC build used for my Windows deployment.

I understand that native Windows may not be part of the supported FlashMLA build matrix. However, the equivalent standard C++ form works across toolchains and does not change the existing Linux behavior.

Tested fix

The working downstream build removes the parentheses around the dependent type:

- TiledMMA tiled_mma_sQ =
-     (typename T::TiledMMA_QK_sQ){};
+ TiledMMA tiled_mma_sQ =
+     typename T::TiledMMA_QK_sQ{};

- Tensor sV = make_tensor(
-     sK.data(),
-     (typename T::SmemLayoutV){}
- );
+ Tensor sV = make_tensor(
+     sK.data(),
+     typename T::SmemLayoutV{}
+ );

- Tensor sQ = make_tensor(
-     make_smem_ptr(plan.smem_sQ.data()),
-     (typename T::SmemLayoutQ){}
- );
+ Tensor sQ = make_tensor(
+     make_smem_ptr(plan.smem_sQ.data()),
+     typename T::SmemLayoutQ{}
+ );

The same mechanical conversion was applied to all 16 affected brace-initialization sites.

This allowed the dense MLA source to build successfully with NVCC and MSVC.

Semantics

Both forms value-initialize a temporary of the same dependent member type.

The affected expressions use empty initializers, and the tested change does not alter:

• initializer arguments;
• narrowing behavior;
• object lifetime;
• overload resolution;
• the resulting type;
• expected Linux code generation.

The replacement form:

typename T::X{}

is standard C++ and does not require an MSVC-specific conditional.

Environment

• Windows Server 2022
• Visual Studio Build Tools with MSVC
• CUDA/NVCC 13.2
• Python 3.12
• PyTorch 2.11.0+cu130
• SystemPanic/vllm-windows 0.25-based deployment
• DeepSeek-V4-Flash-0731
• 4 x H200 NVL GPUs, SM90a

Suggested resolution

Convert the 16 compound-literal-style brace-initialization expressions:

(typename T::X){}

to the standard C++ form:

typename T::X{}

Other occurrences such as C-style casts and sizeof(typename T::...) are separate, valid constructs and should remain unchanged.