PEP 808: backwards compatibility example update (#4710)

Author: henryiii

peps/pep-0808.rst

@@ -273,6 +273,9 @@ metadata.
 Backwards Compatibility
 =======================
 
+Using metadata from SDists or wheels is unaffected. The METADATA version does
+not need to be incremented.
+
 This does not affect any existing ``pyproject.toml``'s, since this was strictly
 not allowed before this PEP.
 
@@ -283,8 +286,27 @@ frontends may need to be updated to benefit from the partially static metadata.
 Some frontends and other tooling may need updating, such as schema
 validation, just like other ``pyproject.toml`` PEPs.
 
-Using metadata from SDists or wheels is unaffected. The METADATA version does
-not need to be incremented.
+Static analysis tools may require updating to handle this change. Tools should
+check the dynamic table first, like this:
+
+.. code-block::
+
+   match pyproject["project"]:
+       # New in PEP 808
+       case {my.key: value, "dynamic": dyn} if my.key in dyn:
+           print(f"Partial {my.key}: {value}")
+       case {"dynamic": dyn} if my.key in dyn:
+           print(f"Fully dynamic {my.key}")
+       case {my.key: value}:
+           print(f"Fully static {my.key}: {value}")
+       case _:
+           print(f"No metadata for {my.key}")
+
+Before this PEP, tools could reverse the order of the dynamic and static
+blocks, assuming that an entry in the project table meant it could not be
+dynamic. If they do this, they will now incorrectly assume they have all the
+metadata for a field, when they in fact only have part of it.
+
 
 Security Implications
 =====================