[SPIRV] Handle a cast to void (#7227)

Fixes #7134

Author: s-perron

tools/clang/lib/SPIRV/SpirvEmitter.cpp

@@ -3657,14 +3657,16 @@ SpirvInstruction *SpirvEmitter::doCastExpr(const CastExpr *expr,
       emitError("implicit cast kind '%0' unimplemented", expr->getExprLoc())
           << expr->getCastKindName() << expr->getSourceRange();
       expr->dump();
-      return 0;
+      return nullptr;
     }
   }
+  case CastKind::CK_ToVoid:
+    return nullptr;
   default:
     emitError("implicit cast kind '%0' unimplemented", expr->getExprLoc())
         << expr->getCastKindName() << expr->getSourceRange();
     expr->dump();
-    return 0;
+    return nullptr;
   }
 }
 

tools/clang/test/CodeGenSPIRV/cast.to.void.hlsl

@@ -0,0 +1,18 @@
+// RUN: %dxc dxc -T cs_6_6 -E Main -spirv %s -fcgl | FileCheck %s
+
+
+// Make sure no code is generated for the cast to void.
+
+// CHECK: %src_Main = OpFunction %void None
+// CHECK-NEXT: OpLabel
+// CHECK-NEXT: %x = OpVariable
+// CHECK-NEXT: OpStore %x %false
+// CHECK-NEXT: OpReturn
+// CHECK-NEXT: OpFunctionEnd
+
+[numthreads(1, 1, 1)]
+void Main()
+{
+    bool x = false;
+    (void)x;
+}