Fix etl::mem_cast cross-size copy constructor and assignment (#1488)

The templated cross-size copy constructor and assignment operator were
broken in two ways:

- They accessed other.buffer / rhs.buffer, which is private in a
  different mem_cast instantiation, so the cross-size overloads failed
  to compile whenever they were actually instantiated.
- They copied Size_ (the destination size) bytes from a source buffer
  that is only Other_Size bytes large, reading past the end of the
  source when the destination was larger.

Use the public data() accessor and copy Other_Size bytes. The existing
static_assert(Size >= Other_Size) guarantees the destination is big
enough.

Add test_mem_cast_copy_and_assign_from_smaller regression test.

Co-authored-by: John Wellbelove <jwellbelove@users.noreply.github.com>

include/etl/mem_cast.h

@@ -115,7 +115,7 @@ namespace etl
     {
       ETL_STATIC_ASSERT(Size >= Other_Size, "Other size is too large");
 
-      memcpy(buffer, other.buffer, Size_);
+      memcpy(buffer, other.data(), Other_Size);
     }
 
     //***********************************
@@ -126,7 +126,7 @@ namespace etl
     {
       ETL_STATIC_ASSERT(Size >= Other_Size, "RHS size is too large");
 
-      memcpy(buffer, rhs.buffer, Size_);
+      memcpy(buffer, rhs.data(), Other_Size);
 
       return *this;
     }

test/test_mem_cast.cpp

@@ -152,6 +152,24 @@ namespace
       CHECK_EQUAL(3, memCast.ref<Data>().a[2]);
     }
 
+    //*************************************************************************
+    // Cross-size copy/assignment must copy only the source's bytes and must
+    // not read past the end of the (smaller) source buffer.
+    TEST(test_mem_cast_copy_and_assign_from_smaller)
+    {
+      etl::mem_cast<sizeof(uint32_t), alignof(uint32_t)> smaller;
+      smaller.assign<uint32_t>(0x12345678UL);
+
+      // Cross-size copy construct (destination larger than source).
+      etl::mem_cast<sizeof(uint32_t) * 2U, alignof(uint32_t)> larger_copy(smaller);
+      CHECK_EQUAL(0x12345678UL, larger_copy.ref<uint32_t>());
+
+      // Cross-size assignment (destination larger than source).
+      etl::mem_cast<sizeof(uint32_t) * 2U, alignof(uint32_t)> larger_assign;
+      larger_assign = smaller;
+      CHECK_EQUAL(0x12345678UL, larger_assign.ref<uint32_t>());
+    }
+
     //*************************************************************************
     TEST(test_mem_cast_assign_type_at_dynamic_offset)
     {