Trade: assertions for string data being too far apart on 64 bits.

I'm seeing an assert for null string data not being correctly thrown in
a SceneTools API on ARM64 and an overflow of this 48-bit offset seems to
be a culprit. So better have that covered in the constructor already.

src/Magnum/Trade/SceneData.cpp

@@ -647,6 +647,11 @@ SceneFieldData::SceneFieldData(const SceneField name, const SceneMappingType map
         "Trade::SceneFieldData: expected mapping view stride to fit into 16 bits but got" << mappingData.stride(), );
     CORRADE_ASSERT(fieldData.stride() >= -32768 && fieldData.stride() <= 32767,
         "Trade::SceneFieldData: expected field view stride to fit into 16 bits but got" << fieldData.stride(), );
+    #ifndef CORRADE_TARGET_32BIT
+    /* 47 because the distance is signed */
+    CORRADE_ASSERT(Utility::abs(stringData - static_cast<const char*>(fieldData.data())) < (1ll << 47),
+        "Trade::SceneFieldData: (signed) distance between string data and field data expected to fit into 48 bits but got" << static_cast<const void*>(stringData) << "and" << fieldData.data(), );
+    #endif
 }
 
 SceneFieldData::SceneFieldData(const SceneField name, const Containers::StridedArrayView2D<const char>& mappingData, const char* const stringData, const SceneFieldType fieldType, const Containers::StridedArrayView2D<const char>& fieldData, const SceneFieldFlags flags) noexcept: SceneFieldData{name, {}, Containers::StridedArrayView1D<const void>{{mappingData.data(), ~std::size_t{}}, mappingData.size()[0], mappingData.stride()[0]}, stringData, fieldType, Containers::StridedArrayView1D<const void>{{fieldData.data(), ~std::size_t{}}, fieldData.size()[0], fieldData.stride()[0]}, flags} {

src/Magnum/Trade/SceneData.h

@@ -4158,7 +4158,13 @@ constexpr SceneFieldData::SceneFieldData(const SceneField name, const std::size_
     _field{
         (CORRADE_CONSTEXPR_ASSERT(fieldStride >= -32768 && fieldStride <= 32767,
             "Trade::SceneFieldData: expected field view stride to fit into 16 bits but got" << fieldStride), Short(fieldStride)),
-        Long(stringOffset)},
+        (
+            #ifndef CORRADE_TARGET_32BIT
+            /* 47 because the distance is signed */
+            CORRADE_CONSTEXPR_ASSERT(stringOffset < (1ull << 47),
+                "Trade::SceneFieldData: expected string data offset to fit into 48 bits but got" << stringOffset),
+            #endif
+        Long(stringOffset))},
     _fieldData{fieldOffset} {}
 
 template<class T> Containers::StridedArrayView1D<const T> SceneData::mapping(const UnsignedInt fieldId) const {

src/Magnum/Trade/Test/SceneDataTest.cpp

@@ -127,6 +127,9 @@ struct SceneDataTest: TestSuite::Tester {
     void constructFieldArrayTypeErased2DNonContiguous();
     void constructFieldBitTooLargeBitOffset();
     void constructFieldBitTooLargeSize();
+    #ifndef CORRADE_TARGET_32BIT
+    void constructFieldStringDataTooFarApart();
+    #endif
 
     void construct();
     void constructZeroFields();
@@ -471,6 +474,9 @@ SceneDataTest::SceneDataTest() {
               &SceneDataTest::constructFieldArrayTypeErased2DNonContiguous,
               &SceneDataTest::constructFieldBitTooLargeBitOffset,
               &SceneDataTest::constructFieldBitTooLargeSize,
+              #ifndef CORRADE_TARGET_32BIT
+              &SceneDataTest::constructFieldStringDataTooFarApart,
+              #endif
 
               &SceneDataTest::construct,
               &SceneDataTest::constructZeroFields,
@@ -2434,6 +2440,32 @@ void SceneDataTest::constructFieldBitTooLargeSize() {
     #endif
 }
 
+#ifndef CORRADE_TARGET_32BIT
+void SceneDataTest::constructFieldStringDataTooFarApart() {
+    CORRADE_SKIP_IF_NO_ASSERT();
+
+    UnsignedShort mappingData[3]{};
+
+    std::ostringstream out;
+    Error redirectError{&out};
+    SceneFieldData{sceneFieldCustom(166),
+        Containers::arrayView(mappingData),
+        reinterpret_cast<const char*>(0xfeedbeefull), SceneFieldType::StringOffset8,
+        Containers::arrayView(reinterpret_cast<const UnsignedByte*>(0x8000feedbeefull), 3)};
+    SceneFieldData{sceneFieldCustom(166),
+        Containers::arrayView(mappingData),
+        reinterpret_cast<const char*>(0x8000feedbeefull), SceneFieldType::StringOffset8,
+        Containers::arrayView(reinterpret_cast<const UnsignedByte*>(0xfeedbeefull), 3)};
+    SceneFieldData{sceneFieldCustom(661), 3,
+        SceneMappingType::UnsignedShort, 1725676, 2,
+        0x800000000000ull, SceneFieldType::StringOffset8, 72567654, 8};
+    CORRADE_COMPARE(out.str(),
+        "Trade::SceneFieldData: (signed) distance between string data and field data expected to fit into 48 bits but got 0xfeedbeef and 0x8000feedbeef\n"
+        "Trade::SceneFieldData: (signed) distance between string data and field data expected to fit into 48 bits but got 0x8000feedbeef and 0xfeedbeef\n"
+        "Trade::SceneFieldData: expected string data offset to fit into 48 bits but got 140737488355328\n");
+}
+#endif
+
 void SceneDataTest::construct() {
     struct TransformParent {
         UnsignedShort object;