diff --git a/src/solve/trait-solving.md b/src/solve/trait-solving.md
index 71f6581c2..b0cd9af6c 100644
--- a/src/solve/trait-solving.md
+++ b/src/solve/trait-solving.md
@@ -96,6 +96,10 @@ Trait solving during codegen should have the same result as during typeck. As we
 all free regions during codegen we must not rely on them during typeck. A noteworthy example
 is special behavior for `'static`.
 
+We also have to be careful with relying on equality of regions in the trait solver.
+This is fine for codegen, as we treat all erased regions are equal. We can however
+lose equality information from HIR to MIR typeck.
+
 ### 7. Removing ambiguity makes strictly more things compile
 
 We *should* not rely on ambiguity for things to compile.