MeshTools: fix generateIndices() to work with attribute-less meshes.

The vertex count has to be explicitly propagated to the output in
that case because it can't be figured out from any attributes in this
case.

doc/changelog.dox

@@ -841,6 +841,8 @@ See also:
     usually just didn't remove any duplicate whatsoever while
     `--remove-duplicates-fuzzy` did, no matter how ridiculously low epsilon
     was used.
+-   Fixed @ref MeshTools::generateIndices() to work correctly with
+    attribute-less @ref Trade::MeshData instances
 -   @ref Platform::EmscriptenApplication randomly created antialiased contexts
     due to an uninitialized variable in its
     @ref Platform::EmscriptenApplication::GLConfiguration "GLConfiguration"

src/Magnum/MeshTools/GenerateIndices.cpp

@@ -321,7 +321,7 @@ Trade::MeshData generateIndices(Trade::MeshData&& data) {
 
     Trade::MeshIndexData indices{MeshIndexType::UnsignedInt, indexData};
     return Trade::MeshData{primitive, std::move(indexData), indices,
-        std::move(vertexData), std::move(attributeData)};
+        std::move(vertexData), std::move(attributeData), vertexCount};
 }
 
 Trade::MeshData generateIndices(const Trade::MeshData& data) {

src/Magnum/MeshTools/Test/GenerateIndicesTest.cpp

@@ -67,6 +67,7 @@ struct GenerateIndicesTest: TestSuite::Tester {
 
     void generateIndicesMeshData();
     void generateIndicesMeshDataMove();
+    void generateIndicesMeshDataNoAttributes();
     void generateIndicesMeshDataIndexed();
     void generateIndicesMeshDataInvalidPrimitive();
 };
@@ -196,6 +197,7 @@ GenerateIndicesTest::GenerateIndicesTest() {
         Containers::arraySize(MeshDataData));
 
     addTests({&GenerateIndicesTest::generateIndicesMeshDataMove,
+              &GenerateIndicesTest::generateIndicesMeshDataNoAttributes,
               &GenerateIndicesTest::generateIndicesMeshDataIndexed,
               &GenerateIndicesTest::generateIndicesMeshDataInvalidPrimitive});
 }
@@ -688,6 +690,19 @@ void GenerateIndicesTest::generateIndicesMeshDataMove() {
     CORRADE_COMPARE(out.vertexData().data(), static_cast<void*>(vertices.data()));
 }
 
+void GenerateIndicesTest::generateIndicesMeshDataNoAttributes() {
+    Trade::MeshData out = generateIndices(Trade::MeshData{MeshPrimitive::TriangleStrip, 4});
+    CORRADE_VERIFY(out.isIndexed());
+    CORRADE_COMPARE(out.indexType(), MeshIndexType::UnsignedInt);
+    CORRADE_COMPARE_AS(out.indices<UnsignedInt>(),
+        Containers::arrayView<UnsignedInt>({
+            0, 1, 2,
+            2, 1, 3, /* Reversed */
+        }), TestSuite::Compare::Container);
+    CORRADE_COMPARE(out.vertexCount(), 4);
+    CORRADE_COMPARE(out.attributeCount(), 0);
+}
+
 void GenerateIndicesTest::generateIndicesMeshDataIndexed() {
     CORRADE_SKIP_IF_NO_ASSERT();