[libcxx] Optimize vectors construction of trivial types from an iterator range with const-ness mismatch.

We already have a specialization that will use memcpy for construction
of trivial types from an iterator range like

    std::vector<int>(int *, int *);

But if we have const-ness mismatch like

    std::vector<int>(const int *, const int *);

we would use a slow path that copies each element individually. This change
enables the optimal specialization for const-ness mismatch. Fixes PR37574.

Contributions to the patch are made by Arthur O'Dwyer, Louis Dionne.

rdar://problem/40485845

Reviewers: mclow.lists, EricWF, ldionne, scanon

Reviewed By: ldionne

Subscribers: christof, ldionne, howard.hinnant, cfe-commits

Differential Revision: https://reviews.llvm.org/D48342

1 files changed, 31 insertions, 0 deletions

diff --git a/libcxx/test/std/containers/sequences/vector/vector.cons/construct_iter_iter.pass.cpp b/libcxx/test/std/containers/sequences/vector/vector.cons/construct_iter_iter.pass.cpp
index 21a7df4a7f0..60fe2899ac2 100644
--- a/libcxx/test/std/containers/sequences/vector/vector.cons/construct_iter_iter.pass.cpp
+++ b/libcxx/test/std/containers/sequences/vector/vector.cons/construct_iter_iter.pass.cpp
@@ -146,9 +146,40 @@ void test_ctor_under_alloc() {
 #endif
 }
 
+// Initialize a vector with a different value type.
+void test_ctor_with_different_value_type() {
+  {
+    // Make sure initialization is performed with each element value, not with
+    // a memory blob.
+    float array[3] = {0.0f, 1.0f, 2.0f};
+    std::vector<int> v(array, array + 3);
+    assert(v[0] == 0);
+    assert(v[1] == 1);
+    assert(v[2] == 2);
+  }
+  struct X { int x; };
+  struct Y { int y; };
+  struct Z : X, Y { int z; };
+  {
+    Z z;
+    Z *array[1] = { &z };
+    // Though the types Z* and Y* are very similar, initialization still cannot
+    // be done with `memcpy`.
+    std::vector<Y*> v(array, array + 1);
+    assert(v[0] == &z);
+  }
+  {
+    // Though the types are different, initialization can be done with `memcpy`.
+    int32_t array[1] = { -1 };
+    std::vector<uint32_t> v(array, array + 1);
+    assert(v[0] == 4294967295);
+  }
+}
+
 
 int main() {
   basic_test_cases();
   emplaceable_concept_tests(); // See PR34898
   test_ctor_under_alloc();
+  test_ctor_with_different_value_type();
 }