unpack static assert on unsupported types

Unsupported types might not cause compile time errors but can result
in SIGILL errors at runtime when compiler warnings are ignored.

This was found when compiling an intel-ipmi-oem handler that attempted
to unpack an enum class type. The code compiles down to an empty
function (no return statement or value), which can result in all sorts
of undefined behavior. This change forces the unsupported types to emit
a static assert and fail to compile.

Tested: Created a handler that requests an enum class as an input and
        saw that the build fails with a static assert.

Change-Id: I123da15cb001756f07761cf7a60b799469926a2a
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>

1 files changed, 20 insertions, 17 deletions

diff --git a/include/ipmid/message/unpack.hpp b/include/ipmid/message/unpack.hpp
index fb2b066..5be92c9 100644
--- a/include/ipmid/message/unpack.hpp
+++ b/include/ipmid/message/unpack.hpp
@@ -99,26 +99,29 @@ struct UnpackSingle
             }
             return 0;
         }
-        else
+        else if constexpr (utility::is_tuple<T>::value)
         {
-            if constexpr (utility::is_tuple<T>::value)
+            bool priorError = p.unpackError;
+            size_t priorIndex = p.rawIndex;
+            // more stuff to unroll if partial bytes are out
+            size_t priorBitCount = p.bitCount;
+            fixed_uint_t<details::bitStreamSize> priorBits = p.bitStream;
+            int ret = p.unpack(t);
+            if (ret != 0)
             {
-                bool priorError = p.unpackError;
-                size_t priorIndex = p.rawIndex;
-                // more stuff to unroll if partial bytes are out
-                size_t priorBitCount = p.bitCount;
-                fixed_uint_t<details::bitStreamSize> priorBits = p.bitStream;
-                int ret = p.unpack(t);
-                if (ret != 0)
-                {
-                    t = T();
-                    p.rawIndex = priorIndex;
-                    p.bitStream = priorBits;
-                    p.bitCount = priorBitCount;
-                    p.unpackError = priorError;
-                }
-                return 0;
+                t = T();
+                p.rawIndex = priorIndex;
+                p.bitStream = priorBits;
+                p.bitCount = priorBitCount;
+                p.unpackError = priorError;
             }
+            return 0;
+        }
+        else
+        {
+            static_assert(
+                utility::dependent_false<T>::value,
+                "Attempt to unpack a type that has no IPMI unpack operation");
         }
     }
 };