Francisco Casas (@fcasas) commented about libs/vkd3d-shader/hlsl.y:
struct hlsl_ir_var *var; struct hlsl_type *type;
/* Functions are unconditionally inlined. */
modifiers &= ~HLSL_MODIFIER_INLINE;
if (modifiers & ~HLSL_MODIFIERS_MAJORITY_MASK) hlsl_error(ctx, &@1, VKD3D_SHADER_ERROR_HLSL_INVALID_MODIFIER, "Only majority modifiers are allowed on functions.");
This error could be updated, since the "inline" modifier is also allowed. Still, as for now it doesn't seem to be a real reason to use it.