Containers: make ArrayView -> StringView construction constexpr.

So it's possible to semi-easily create constexpr StringView instances
without having to supply explicit sizes. I.e., like this:

    constexpr StringView a = arrayView("hello").exceptSuffix(1);

Author: mosra

doc/corrade-changelog.dox

@@ -86,7 +86,8 @@ namespace Corrade {
 -   New @ref Containers::String class as a lightweight but more flexible
     alternative to @ref std::string
 -   New @ref Containers::BasicStringView "Containers::StringView" class as a
-    lightweight but more flexible alternative to C++17 @ref std::string_view
+    lightweight but more flexible alternative to C++17 @ref std::string_view.
+    See also [mosra/corrade#123](https://github.com/mosra/corrade/issues/123).
 -   New @ref Containers::MoveReference and @ref Containers::AnyReference
     counterparts to @ref Containers::Reference for exclusively r-value
     references and both l-value and r-value references

src/Corrade/Containers/StringView.cpp

@@ -63,8 +63,6 @@ template<class T> BasicStringView<T>::BasicStringView(String& string) noexcept:
    needs an explicit export otherwise the symbol doesn't get exported. */
 template<> template<> CORRADE_UTILITY_EXPORT BasicStringView<const char>::BasicStringView(const String& string) noexcept: BasicStringView{string.data(), string.size(), StringViewFlag::NullTerminated} {}
 
-template<class T> BasicStringView<T>::BasicStringView(const StringViewFlags flags, const ArrayView<T> other) noexcept: BasicStringView{other.data(), other.size(), flags} {}
-
 template<class T> Array<BasicStringView<T>> BasicStringView<T>::split(const char delimiter) const {
     Array<BasicStringView<T>> parts;
     T* const end = this->end();

src/Corrade/Containers/StringView.h

@@ -1164,9 +1164,9 @@ BasicStringView {
 
         /* Called from BasicStringView(U&&, StringViewFlags), see its comment
            for details; arguments in a flipped order to avoid accidental
-           ambiguity. Not constexpr/inline to avoid header dependency on
-           ArrayView.h. */
-        explicit BasicStringView(StringViewFlags flags, ArrayView<T> data) noexcept;
+           ambiguity. The ArrayView type is a template to avoid having to
+           include ArrayView.h. */
+        template<class U, class = typename std::enable_if<std::is_same<T, U>::value>::type> constexpr explicit BasicStringView(StringViewFlags flags, ArrayView<U> data) noexcept: BasicStringView{data.data(), data.size(), flags} {}
 
         /* Used by the char* constructor, delinlined because it calls into
            std::strlen() */

src/Corrade/Containers/Test/StringViewTest.cpp

@@ -24,12 +24,15 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+/* Deliberately including first to make sure it works without ArrayView being
+   included first */
+#include "Corrade/Containers/StringView.h"
+
 #include <sstream>
 
 #include "Corrade/Cpu.h"
 #include "Corrade/Containers/Array.h"
 #include "Corrade/Containers/StaticArray.h"
-#include "Corrade/Containers/StringView.h"
 #include "Corrade/TestSuite/Tester.h"
 #include "Corrade/TestSuite/Compare/Container.h"
 #include "Corrade/TestSuite/Compare/Numeric.h"
@@ -122,6 +125,7 @@ struct StringViewTest: TestSuite::Tester {
     void constructNullptrNullTerminated();
 
     template<class T> void convertArrayView();
+    void convertArrayViewConstexpr();
     template<class T> void convertVoidArrayView();
     void convertConstFromArrayView();
     void convertToConstArrayView();
@@ -263,6 +267,7 @@ StringViewTest::StringViewTest() {
 
               &StringViewTest::convertArrayView<const char>,
               &StringViewTest::convertArrayView<char>,
+              &StringViewTest::convertArrayViewConstexpr,
               &StringViewTest::convertConstFromArrayView,
               &StringViewTest::convertToConstArrayView,
               &StringViewTest::convertConstFromArray,
@@ -664,6 +669,18 @@ template<class T> void StringViewTest::convertArrayView() {
     CORRADE_COMPARE(static_cast<const void*>(array2.data()), &data[0]);
 }
 
+constexpr const char String[] = "hell\0!!"; /* 7 chars + \0 at the end */
+
+void StringViewTest::convertArrayViewConstexpr() {
+    constexpr StringView view = {arrayView(String).exceptSuffix(1), StringViewFlag::Global|StringViewFlag::NullTerminated};
+    constexpr std::size_t size = view.size();
+    constexpr StringViewFlags flags = view.flags();
+    constexpr const void* data = view.data();
+    CORRADE_COMPARE(size, 7);
+    CORRADE_COMPARE(flags, StringViewFlag::Global|StringViewFlag::NullTerminated);
+    CORRADE_COMPARE(data, String);
+}
+
 void StringViewTest::convertConstFromArrayView() {
     char data[] = "hello!";
     ArrayView<char> array = data;