doc/go1.15: mention vet warning for impossible type assertions
For #4483
Change-Id: Iab76baf50b79eda1e3acfd662d0e7830c7962f5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/234518
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/doc/go1.15.html b/doc/go1.15.html
index 7271755..43fc14f 100644
--- a/doc/go1.15.html
+++ b/doc/go1.15.html
@@ -157,6 +157,32 @@
the language.
</p>
+<h4 id="vet-impossible-interface">New warning for impossible interface conversions</h4>
+
+<p><!-- CL 218779, 232660 -->
+ The vet tool now warns about type assertions from one interface type
+ to another interface type when the type assertion will always fail.
+ This will happen if both interface types implement a method with the
+ same name but with a different type signature.
+</p>
+
+<p>
+ There is no reason to write a type assertion that always fails, so
+ any code that triggers this vet check should be rewritten.
+</p>
+
+<p>
+ This new vet check is enabled by default when using <code>go test</code>.
+</p>
+
+<p>
+ We are considering prohibiting impossible interface type assertions
+ in a future release of Go.
+ Such a language change would not be backward compatible.
+ We are using this vet check as a first trial step toward changing
+ the language.
+</p>
+
<h2 id="runtime">Runtime</h2>
<p>
