diff --git a/clang/docs/Toolchain.rst b/clang/docs/Toolchain.rst
--- a/clang/docs/Toolchain.rst
+++ b/clang/docs/Toolchain.rst
@@ -276,6 +276,19 @@
 `C standard library <https://en.cppreference.com/w/c>`_
 implementations.
 
+Clang and LLVM make some assumptions about the behavior of the C standard
+library beyond those required by the C standard:
+
+* ``memcpy(p, q, 0)`` and ``memmove(p, q, 0)`` are expected to have no effect,
+  even if ``p`` or ``q`` is a null or otherwise-invalid (but correctly-aligned)
+  pointer.
+* ``memcpy(p, p, n)`` is expected to leave the contents of the memory pointed
+  to by ``p`` unchanged, but may perform (redundant) loads and stores through
+  the ``n`` bytes pointed to by ``p``.
+
+C standard library implementations that do not guarantee these properties
+incompatible with Clang and LLVM (and with several other major compilers).
+
 C++ ABI library
 ---------------