[SPIR-V] Fix crash with out-of-line template decl (#8079)

The SPIR-V backend made no differentence between VarDecl from template
declaration and VarDecl from template instantiation. Fixing this by not
creating a SPIR-V variable for template declaration (and waiting for the
final instantiation).

Fixes #5823

Author: Keenuts

tools/clang/lib/SPIRV/SpirvEmitter.cpp

@@ -2034,6 +2034,28 @@ void SpirvEmitter::doVarDecl(const VarDecl *decl) {
     return;
   }
 
+  // Considering the following example:
+  //
+  // ```cpp
+  //  template<typename T> const static T MyClass<T>::myVar[2] = { 1, 2 };
+  //  [...]
+  //  int use = MyClass<int>::myVar[0];
+  // ```
+  //
+  // The AST will contain 2 variable declarations:
+  //  - VarDecl for the template declaration
+  //  - VarDecl for the template instantiation
+  // One of them is not yet defined (InitExpr will have the type void), the
+  // other is the actual instantiation, hence will have the proper type. They
+  // can be differentiated by looking at the declaration context:
+  //  - the undefined ones will be in the template declaration context.
+  // We must not create a variable for the template declaration but wait
+  // for the instantiation (if any).
+  auto *RC = dyn_cast<clang::CXXRecordDecl>(decl->getDeclContext());
+  auto *TC = RC ? RC->getDescribedClassTemplate() : nullptr;
+  if (decl->getInit() && TC)
+    return;
+
   // We cannot handle external initialization of column-major matrices now.
   if (isExternalVar(decl) &&
       isOrContainsNonFpColMajorMatrix(astContext, spirvOptions, decl->getType(),

tools/clang/test/CodeGenSPIRV/template.static.var.split.hlsl

@@ -0,0 +1,14 @@
+// RUN: %dxc -T cs_6_8 -E main -fcgl %s -spirv | FileCheck %s
+
+// CHECK-NOT: OpVariable
+
+template<typename T>
+struct MyClass {
+  const static T array[2];
+};
+
+template<typename T> const static T MyClass<T>::array[2] = { 1, 2 };
+
+[numthreads(1, 1, 1)]
+void main() {
+}

tools/clang/test/CodeGenSPIRV/template.static.var.split.instantiate.hlsl

@@ -0,0 +1,21 @@
+// RUN: %dxc -T cs_6_8 -E main -fcgl %s -spirv | FileCheck %s
+
+
+template<typename T>
+struct MyClass {
+  const static T array[2];
+};
+
+// CHECK: %array = OpVariable %_ptr_Private__arr_int_uint_2 Private
+// CHECK: [[init_var:%[a-zA-Z0-9_]+]] = OpCompositeConstruct %_arr_int_uint_2 %int_1 %int_2
+// CHECK: OpStore %array [[init_var]]
+template<typename T> const static T MyClass<T>::array[2] = { 1, 2 };
+
+[numthreads(1, 1, 1)]
+void main() {
+// CHECK: [[ptr:%[a-zA-Z0-9_]+]] = OpAccessChain %_ptr_Private_int %array %int_1
+// CHECK: [[tmp:%[a-zA-Z0-9_]+]] = OpLoad %int [[ptr]]
+// CHECK:                          OpStore %tmp [[tmp]]
+  int tmp = MyClass<int>::array[1];
+}
+