Fix bug in reflection template arguments.

Rather than representing a reflection non-type template argument as a
CXXReflectExpr, represent it using a ConstantExpr. Some reflections
aren't representable as a CXXReflectExpr (e.g., results of
reflect_value), leading to errant diagnostics for otherwise legal
substitutions.

Closes issue llvm#67.

Author: katzdm

clang/lib/Sema/SemaTemplate.cpp

@@ -8844,25 +8844,13 @@ static Expr *BuildExpressionFromIntegralTemplateArgumentValue(
 static ExprResult
 BuildExpressionFromReflection(Sema &S, const ReflectionValue &R,
                               SourceLocation Loc) {
-  switch (R.getKind()) {
-  case ReflectionValue::RK_null:
-    return CXXReflectExpr::Create(S.Context, Loc, Loc);
-  case ReflectionValue::RK_type:
-    return CXXReflectExpr::Create(S.Context, Loc, Loc, R.getAsType());
-  case ReflectionValue::RK_expr_result:
-    return CXXReflectExpr::Create(S.Context, Loc, R.getAsExprResult());
-  case ReflectionValue::RK_declaration:
-    return CXXReflectExpr::Create(S.Context, Loc, Loc, R.getAsDecl());
-  case ReflectionValue::RK_template:
-    return CXXReflectExpr::Create(S.Context, Loc, Loc, R.getAsTemplate());
-  case ReflectionValue::RK_namespace:
-    return CXXReflectExpr::Create(S.Context, Loc, Loc, R.getAsNamespace());
-  case ReflectionValue::RK_base_specifier:
-    return CXXReflectExpr::Create(S.Context, Loc, Loc, R.getAsBaseSpecifier());
-  case ReflectionValue::RK_data_member_spec:
-    return CXXReflectExpr::Create(S.Context, Loc, Loc, R.getAsDataMemberSpec());
-  }
-  llvm_unreachable("unknown reflection kind");
+  ConstantExpr *CE =
+      ConstantExpr::CreateEmpty(S.Context, ConstantResultStorageKind::APValue);
+  CE->setType(S.Context.MetaInfoTy);
+  CE->setValueKind(VK_PRValue);
+  CE->SetResult(APValue(R.getKind(), R.getOpaqueValue()), S.Context);
+
+  return CE;
 }
 
 /// Construct a new expression that refers to the given reflection template

libcxx/test/std/experimental/reflection/to-and-from-values.pass.cpp

@@ -277,6 +277,31 @@ static_assert(rvfirst == std::meta::reflect_value(std::make_pair(1, true)));
 static_assert([:rvfirst:].first == 1);
 }  // namespace values_from_objects
 
+                   // =======================================
+                   // bb_clang_p2996_issue_67_regression_test
+                   // =======================================
+
+namespace bb_clang_p2996_issue_67_regression_test {
+template<class T> struct TCls {};
+
+template <std::size_t Count, std::meta::info... Rs>
+struct Cls
+{
+    static void fn()
+    {
+        [] <auto... Idxs> (std::index_sequence<Idxs...>) consteval {
+            (void) (Rs...[Idxs], ...);
+        }(std::make_index_sequence<Count>());
+    }
+};
+
+void odr_use()
+{
+    Cls<1, std::meta::reflect_value(0)>::fn();
+}
+}  // namespace bb_clang_p2996_issue_67_regression_test
+
+
 int main() {
   // RUN: grep "call-lambda-value: 1" %t.stdout
   extract<void(*)(int)>(