[HLSL 2021] Allow dependent non-type parameters (#4100)

Non-type template parameters need more complicated constant resolution
because they need to look through layers of templates. This change
makes it so that when evaluating integer constants we use C++ 11
constant expression evaluation to look through template instantiations.

'beanz/cbieneman/dependent-constant-int'.
../tools/clang/test/HLSLFileCheck/hlsl/template/DependentNonTypeParam.hl
sl

tools/clang/lib/AST/ExprConstant.cpp

@@ -9393,7 +9393,8 @@ static bool EvaluateCPlusPlus11IntegralConstantExpr(const ASTContext &Ctx,
 
 bool Expr::isIntegerConstantExpr(const ASTContext &Ctx,
                                  SourceLocation *Loc) const {
-  if (Ctx.getLangOpts().CPlusPlus11)
+  // HLSL Change - if templates are enabled we need to act like C++11 here
+  if (Ctx.getLangOpts().CPlusPlus11 || Ctx.getLangOpts().EnableTemplates)
     return EvaluateCPlusPlus11IntegralConstantExpr(Ctx, this, nullptr, Loc);
 
   ICEDiag D = CheckICE(this, Ctx);
@@ -9406,7 +9407,8 @@ bool Expr::isIntegerConstantExpr(const ASTContext &Ctx,
 
 bool Expr::isIntegerConstantExpr(llvm::APSInt &Value, const ASTContext &Ctx,
                                  SourceLocation *Loc, bool isEvaluated) const {
-  if (Ctx.getLangOpts().CPlusPlus11)
+  // HLSL Change - if templates are enabled we need to act like C++11 here
+  if (Ctx.getLangOpts().CPlusPlus11 || Ctx.getLangOpts().EnableTemplates)
     return EvaluateCPlusPlus11IntegralConstantExpr(Ctx, this, &Value, Loc);
 
   if (!isIntegerConstantExpr(Ctx, Loc))

tools/clang/test/HLSLFileCheck/hlsl/template/DependentNonTypeParam.hlsl

@@ -0,0 +1,40 @@
+// RUN: %dxc -E main -T ps_6_0 -ast-dump -enable-templates %s | FileCheck -check-prefix=AST %s
+// RUN: %dxc -E main -T ps_6_0 -ast-dump -HV 2021 %s | FileCheck -check-prefix=AST %s
+// RUN: %dxc -E main -T ps_6_0 -enable-templates %s | FileCheck %s
+// RUN: %dxc -E main -T ps_6_0 -HV 2021 %s | FileCheck %s
+
+template<uint VSize, typename T>
+vector<T, VSize> make_vec(T X) {
+  return (vector<T, VSize>)X;
+}
+
+// Just verify that we got to codegen, generated main, and load the two used
+// components from the input. The actual code-gen is less interesting to this
+// test case.
+
+// CHECK: define void @main()
+// CHECK: @dx.op.loadInput.f32
+// CHECK: @dx.op.loadInput.f32
+float2 main(float4 a:A) : SV_Target {
+  float3 M4 = make_vec<4>(a.x).xyz;
+  return M4.xy * a.y;
+}
+
+// AST:      | |-NonTypeTemplateParmDecl 0x{{[0-9a-zA-Z]+}} <line:6:10, col:15> col:15 referenced 'uint':'unsigned int' VSize
+// AST-NEXT: | |-TemplateTypeParmDecl 0x{{[0-9a-zA-Z]+}} <col:22, col:31> col:31 referenced typename T
+// AST-NEXT: | |-FunctionDecl 0x{{[0-9a-zA-Z]+}} <line:7:1, line:9:1> line:7:18 make_vec 'vector<T, VSize> (T)'
+// AST-NEXT: | | |-ParmVarDecl 0x{{[0-9a-zA-Z]+}} <col:27, col:29> col:29 referenced X 'T'
+// AST-NEXT: | | `-CompoundStmt 0x{{[0-9a-zA-Z]+}} <col:32, line:9:1>
+// AST-NEXT: | |   `-ReturnStmt 0x{{[0-9a-zA-Z]+}} <line:8:3, col:28>
+// AST-NEXT: | |     `-CStyleCastExpr 0x{{[0-9a-zA-Z]+}} <col:10, col:28> 'vector<T, VSize>' <Dependent>
+// AST-NEXT: | |       `-DeclRefExpr 0x{{[0-9a-zA-Z]+}} <col:28> 'T' lvalue ParmVar 0x{{[0-9a-zA-Z]+}} 'X' 'T'
+// AST-NEXT: | `-FunctionDecl 0x{{[0-9a-zA-Z]+}} <line:7:1, line:9:1> line:7:18 used make_vec 'vector<float, 4U> (float)'
+// AST-NEXT: |   |-TemplateArgument integral 4
+// AST-NEXT: |   |-TemplateArgument type 'float'
+// AST-NEXT: |   |-ParmVarDecl 0x{{[0-9a-zA-Z]+}} <col:27, col:29> col:29 used X 'float':'float'
+// AST-NEXT: |   `-CompoundStmt 0x{{[0-9a-zA-Z]+}} <col:32, line:9:1>
+// AST-NEXT: |     `-ReturnStmt 0x{{[0-9a-zA-Z]+}} <line:8:3, col:28>
+// AST-NEXT: |       `-CStyleCastExpr 0x{{[0-9a-zA-Z]+}} <col:10, col:28> 'vector<float, 4U>':'vector<float, 4>' <NoOp>
+// AST-NEXT: |         `-ImplicitCastExpr 0x{{[0-9a-zA-Z]+}} <col:28> 'vector<float, 4>':'vector<float, 4>' <HLSLVectorSplat>
+// AST-NEXT: |           `-ImplicitCastExpr 0x{{[0-9a-zA-Z]+}} <col:28> 'float':'float' <LValueToRValue>
+// AST-NEXT: |             `-DeclRefExpr 0x{{[0-9a-zA-Z]+}} <col:28> 'float':'float' lvalue ParmVar 0x{{[0-9a-zA-Z]+}} 'X' 'float':'float'