Add test

Author: compiler-errors

tests/ui/transmute/ambiguity-in-closure-arg.rs

@@ -0,0 +1,11 @@
+// Minimized test for <https://github.com/rust-lang/rust/issues/123461>.
+
+struct Unconstrained<T>(T);
+
+fn main() {
+    unsafe { std::mem::transmute::<_, ()>(|o_b: Unconstrained<_>| {}) };
+    //~^ ERROR type annotations needed
+    // We unfortunately don't check `Wf(Unconstrained<_>)`, so we won't
+    // hit an ambiguity error before checking the transmute. That means
+    // we still may have inference variables in our transmute src.
+}

tests/ui/transmute/ambiguity-in-closure-arg.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/ambiguity-in-closure-arg.rs:6:44
+   |
+LL |     unsafe { std::mem::transmute::<_, ()>(|o_b: Unconstrained<_>| {}) };
+   |                                            ^^^^^^^^^^^^^^^^^^^^^ cannot infer type
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.