Math: fix batch min()/max()/minmax() to work with const input views.

Apart from returning const T instead of T it kinda worked, but in case
of floating-point vectors it tried to operate with
`std::pair<std::size_t, const T>` internally and failed miserably.

src/Magnum/Math/FunctionsBatch.h

@@ -40,9 +40,9 @@ namespace Implementation {
 
 /** @todo Utility/Algorithms.h has a similar (but different) variant of this,
     maybe turn that into some public utility once we have one more use case? */
-template<class T, class View = decltype(Corrade::Containers::Implementation::ErasedArrayViewConverter<typename std::remove_reference<T&&>::type>::from(std::declval<T&&>()))> static auto stridedArrayViewTypeFor(T&&) -> typename View::Type;
-template<class T> static T stridedArrayViewTypeFor(const Corrade::Containers::ArrayView<T>&);
-template<class T> static T stridedArrayViewTypeFor(const Corrade::Containers::StridedArrayView1D<T>&);
+template<class T, class View = decltype(Corrade::Containers::Implementation::ErasedArrayViewConverter<typename std::remove_reference<T&&>::type>::from(std::declval<T&&>()))> static auto stridedArrayViewTypeFor(T&&) -> typename std::remove_const<typename View::Type>::type;
+template<class T> static typename std::remove_const<T>::type stridedArrayViewTypeFor(const Corrade::Containers::ArrayView<T>&);
+template<class T> static typename std::remove_const<T>::type stridedArrayViewTypeFor(const Corrade::Containers::StridedArrayView1D<T>&);
 
 }
 

src/Magnum/Math/Test/FunctionsBatchTest.cpp

@@ -24,6 +24,7 @@
 */
 
 #include <vector>
+#include <Corrade/Containers/Array.h>
 #include <Corrade/Containers/ArrayViewStl.h>
 #include <Corrade/TestSuite/Tester.h>
 
@@ -44,6 +45,8 @@ struct FunctionsBatchTest: Corrade::TestSuite::Tester {
 
     void nanIgnoring();
     void nanIgnoringVector();
+
+    void constIterable();
 };
 
 using namespace Literals;
@@ -62,7 +65,9 @@ FunctionsBatchTest::FunctionsBatchTest() {
               &FunctionsBatchTest::minmax,
 
               &FunctionsBatchTest::nanIgnoring,
-              &FunctionsBatchTest::nanIgnoringVector});
+              &FunctionsBatchTest::nanIgnoringVector,
+
+              &FunctionsBatchTest::constIterable});
 }
 
 void FunctionsBatchTest::isInf() {
@@ -284,6 +289,20 @@ void FunctionsBatchTest::nanIgnoringVector() {
     CORRADE_COMPARE(Math::minmax(allNan).second[1], Constants::nan());
 }
 
+void FunctionsBatchTest::constIterable() {
+    const Vector2 data[]{{5, -3}, {-2, 14}, {9, -5}};
+
+    /* It shouldn't try to operate with a const type (such as trying to to
+       assign to `std::pair<std::size_t, const Vector2>`) internally, instead
+       it should remove the const */
+    CORRADE_COMPARE(Math::min(Corrade::Containers::arrayView(data)),
+        (Vector2{-2, -5}));
+    CORRADE_COMPARE(Math::max(Corrade::Containers::stridedArrayView(data)),
+        (Vector2{9, 14}));
+    CORRADE_COMPARE(Math::minmax(Corrade::Containers::Array<const Vector2>{data, 3, [](const Vector2*, std::size_t){}}),
+        std::make_pair(Vector2{-2, -5}, Vector2{9, 14}));
+}
+
 }}}}
 
 CORRADE_TEST_MAIN(Magnum::Math::Test::FunctionsBatchTest)