Updates release notes for PR #4005

ponylang · Feb 15, 2022 · c575186 · c575186
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diff --git a/.release-notes/fix-3655.md b/.release-notes/fix-3655.md
diff --git a/.release-notes/next-release.md b/.release-notes/next-release.md
@@ -40,3 +40,16 @@ actor Main
 
 Field initialization is now done in a later pass after exhaustiveness checking has been done.
 
+## Fix compiler crash related to using tuples as a generic constraint
+
+Tuple types aren't legal constraints on generics in Pony and we have a check in an early compiler pass to detect and report that error. However, it was previously possible to "sneak" a tuple type as a generic constraint past the earlier check in the compiler by "smuggling" it in a type alias.
+
+The type aliases aren't flattened into their various components until after the code that disallows tuples as generic constraints which would result in the following code causing ponyc to assert:
+
+```ponyc
+type Blocksize is (U8, U32)
+class Block[T: Blocksize]
+```
+
+We've added an additional check to the compiler in a later pass to report an error on the "smuggled" tuple type as a constraint.
+