Re: [lmi] Fwd: How did this unit test fail?

[Greg Chicares]

On 2018-03-27 11:50, Vadim Zeitlin wrote:
[...]
>  In the meanwhile, one workaround I may report is that removing the use of
> "constexpr" from bourn_cast.hpp definitely avoids the problem. Considering
> that this is required for building with clang anyhow because it (correctly,
> AFAICS) disallows the use of non-constexpr lmi::TypeInfo and
> std::ostringstream classes in a constexpr function, perhaps we should do
> this just to make sure to avoid the problem?

Excellent: std::ostringstream has no constexpr ctor; nasal demons follow.

As currently written, 'constexpr' is wrong in floating-to-integral function:

  template<typename To, typename From>
  constexpr inline To bourn_cast(From from, std::true_type, std::false_type)
    ...std::ostringstream...

and therefore it's wrong on the general version that calls that function:

  template<typename To, typename From>
  constexpr inline To bourn_cast(From from)

so we can either...

 - remove constexpr rather globally from bourn_cast(), which really, really
wants to be constexpr; or

 - remove diagnostic code that has never been found useful in practice.

Could you please test the Visigothic proof of concept that I hacked out
below? It works perfectly here, both of these ways:

  /opt/lmi/src/lmi[0]$make $coefficiency unit_tests 
unit_test_targets=bourn_cast_test.exe 2>&1 |less -S                             
                                              

  /opt/lmi/src/lmi[0]$make $coefficiency unit_tests 
unit_test_targets=bourn_cast_test.exe build_type=safestdlib 2>&1 |less -S

and if all your compilers like it, I'll lay my axe aside and rework it
carefully for committal.   

---------8<--------8<--------8<--------8<--------8<--------8<--------8<-------
diff --git a/bourn_cast.hpp b/bourn_cast.hpp
index 50d1714a..728a0c05 100644
--- a/bourn_cast.hpp
+++ b/bourn_cast.hpp
@@ -185,6 +185,9 @@ constexpr inline To bourn_cast(From from, std::true_type, 
std::false_type)
     To const r = static_cast<To>(from);
     if(r != from)
         {
+#if 1
+        throw std::runtime_error("Cast would not preserve value.");
+#else // 0
         lmi::TypeInfo from_type(typeid(From));
         lmi::TypeInfo   to_type(typeid(To  ));
         std::ostringstream oss;
@@ -195,6 +198,7 @@ constexpr inline To bourn_cast(From from, std::true_type, 
std::false_type)
             << " would not preserve value."
             ;
         throw std::runtime_error(oss.str());
+#endif // 0
         }
     return r;
 }
diff --git a/bourn_cast_test.cpp b/bourn_cast_test.cpp
index 96687e61..ed360bbb 100644
--- a/bourn_cast_test.cpp
+++ b/bourn_cast_test.cpp
@@ -653,7 +653,7 @@ void mete_static()
         {
         z = static_cast<To>(j);
         }
-    (void)&z;
+    stifle_warning_for_unused_variable(z);
 }
 
 /// Speed test: convert one million times, using bourn_cast.
@@ -670,7 +670,7 @@ void mete_bourn()
         {
         z = bourn_cast<To>(j);
         }
-    (void)&z;
+    stifle_warning_for_unused_variable(z);
 }
 
 void assay_speed()
--------->8-------->8-------->8-------->8-------->8-------->8-------->8-------