fix gcc build (gcc 7) (#690)

simple solution, disable the specific warning

include/gsl/multi_span

@@ -51,6 +51,16 @@
 #endif                          // _MSC_VER < 1910
 #endif                          // _MSC_VER
 
+// GCC 7 does not like the signed unsigned missmatch (size_t ptrdiff_t)
+// While there is a conversion from signed to unsigned, it happens at 
+// compiletime, so the compiler wouldn't have to warn indiscriminently, but 
+// could check if the source value actually doesn't fit into the target type 
+// and only warn in those cases.
+#if __GNUC__ > 6
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wsign-conversion"
+#endif
+
 #ifdef GSL_THROW_ON_CONTRACT_VIOLATION
 #define GSL_NOEXCEPT /*noexcept*/
 #else
@@ -2225,4 +2235,8 @@ general_span_iterator<Span> operator+(typename general_span_iterator<Span>::diff
 
 #endif // _MSC_VER
 
+#if __GNUC__ > 6 
+#pragma GCC diagnostic pop
+#endif // __GNUC__ > 6
+
 #endif // GSL_MULTI_SPAN_H

include/gsl/span

@@ -58,6 +58,16 @@
 
 #endif                          // _MSC_VER
 
+// GCC 7 does not like the signed unsigned missmatch (size_t ptrdiff_t)
+// While there is a conversion from signed to unsigned, it happens at 
+// compiletime, so the compiler wouldn't have to warn indiscriminently, but 
+// could check if the source value actually doesn't fit into the target type 
+// and only warn in those cases.
+#if __GNUC__ > 6
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wsign-conversion"
+#endif
+
 namespace gsl
 {
 
@@ -749,4 +759,8 @@ constexpr ElementType& at(span<ElementType, Extent> s, index i)
 #pragma warning(pop)
 #endif // _MSC_VER
 
+#if __GNUC__ > 6 
+#pragma GCC diagnostic pop
+#endif // __GNUC__ > 6
+
 #endif // GSL_SPAN_H