[WIP] Fix unreachable warnings in constant_folding.cpp (#3146)

* [WIP] Fix unreachable warnings in constant_folding.cpp

* Introduce NGRAPH_UNREACHABLE macro and silent unreachable warnings

* Fix typo and add missing NGRAPH_UNREACHABLE

src/ngraph/check.hpp

@@ -156,3 +156,9 @@ namespace ngraph
 /// \throws ::ngraph::CheckFailure if `cond` is false.
 #define NGRAPH_CHECK(cond, ...)                                                                    \
     NGRAPH_CHECK_HELPER(::ngraph::CheckFailure, "", (cond), ##__VA_ARGS__)
+
+/// \brief Macro to signal a code path that is unreachable in a successful execution. It's
+/// implemented with NGRAPH_CHECK macro.
+/// \param ... Additional error message that should describe why that execution path is unreachable.
+/// \throws ::ngrap::CheckFailure if the macro is executed.
+#define NGRAPH_UNREACHABLE(...) NGRAPH_CHECK(false, "Unreachable: ", ##__VA_ARGS__)

src/ngraph/pass/constant_folding.cpp

@@ -845,6 +845,9 @@ shared_ptr<op::Constant> fold_constant_convert_helper0(shared_ptr<op::Constant>
     case element::Type_t::u64:
         return fold_constant_convert_helper1<TI, uint64_t>(constant, output_element_type);
     }
+
+    NGRAPH_UNREACHABLE("Unexpected switch case");
+
 #if !(defined(__GNUC__) && (__GNUC__ == 4 && __GNUC_MINOR__ == 8))
 #pragma GCC diagnostic pop
 #endif
@@ -900,6 +903,9 @@ static shared_ptr<op::Constant> fold_constant_convert(shared_ptr<op::Constant> c
     case element::Type_t::u64:
         return fold_constant_convert_helper0<uint64_t>(constant, output_element_type);
     }
+
+    NGRAPH_UNREACHABLE("Unexpected switch case");
+
 #if !(defined(__GNUC__) && (__GNUC__ == 4 && __GNUC_MINOR__ == 8))
 #pragma GCC diagnostic pop
 #endif
@@ -1021,6 +1027,9 @@ static shared_ptr<op::Constant> fold_constant_reverse(shared_ptr<op::Constant> c
     case element::Type_t::u64:
         return fold_constant_reverse_helper<uint64_t>(constant, reversed_axes);
     }
+
+    NGRAPH_UNREACHABLE("Unexpected switch case");
+
 #if !(defined(__GNUC__) && (__GNUC__ == 4 && __GNUC_MINOR__ == 8))
 #pragma GCC diagnostic pop
 #endif
@@ -1113,6 +1122,9 @@ static shared_ptr<op::Constant> fold_constant_product(shared_ptr<op::Constant> c
     case element::Type_t::u64:
         return fold_constant_product_helper<uint64_t>(constant, reduction_axes, result_shape);
     }
+
+    NGRAPH_UNREACHABLE("Unexpected switch case");
+
 #if !(defined(__GNUC__) && (__GNUC__ == 4 && __GNUC_MINOR__ == 8))
 #pragma GCC diagnostic pop
 #endif