[HLSL] Clang is missing a warning for implicit truncation of vector types in HLSL

[bogner]

HLSL allows implicit truncation of vector types, but DXC warns about it. Clang needs to learn this warning.

// RUN: %dxc -T cs_6_5 -verify
StructuredBuffer<int4> In : register(t0);
RWStructuredBuffer<int> Out : register(u0);

[numthreads(4,2,1)]
void main(uint GI : SV_GroupIndex) {
  Out[GI] = In[GI]; // expected-warning{{implicit truncation of vector type}}
}

See https://hlsl.godbolt.org/z/o7j6d7r88

[damyanp]

Isn't this just because -Wconversion isn't enabled by default? With -Wconversion:

<source>:7:13: warning: implicit conversion turns vector to scalar: 'vector<int, 4>' (vector of 4 'int' values) to 'int' [-Wconversion]
    7 |   Out[GI] = In[GI];
      |           ~ ^~~~~~