Defer tail call ret ty equality to check_tail_calls

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1931,7 +1931,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     self.tcx(),
                     self.infcx.typing_env(self.infcx.param_env),
                 ) {
-                    span_mirbug!(self, term, "call to converging function {:?} w/o dest", sig);
+                    span_mirbug!(self, term, "call to diverging function {:?} w/o dest", sig);
                 }
             }
         }

compiler/rustc_mir_build/src/check_tail_calls.rs

@@ -60,7 +60,7 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
         let BodyTy::Fn(caller_sig) = self.thir.body_type else {
             span_bug!(
                 call.span,
-                "`become` outside of functions should have been disallowed by hit_typeck"
+                "`become` outside of functions should have been disallowed by hir_typeck"
             )
         };
 
@@ -118,21 +118,7 @@ impl<'tcx> TailCallCkVisitor<'_, 'tcx> {
         }
 
         if caller_sig.inputs_and_output != callee_sig.inputs_and_output {
-            if caller_sig.inputs() != callee_sig.inputs() {
-                self.report_arguments_mismatch(expr.span, caller_sig, callee_sig);
-            }
-
-            // FIXME(explicit_tail_calls): this currently fails for cases where opaques are used.
-            // e.g.
-            // ```
-            // fn a() -> impl Sized { become b() } // ICE
-            // fn b() -> u8 { 0 }
-            // ```
-            // we should think what is the expected behavior here.
-            // (we should probably just accept this by revealing opaques?)
-            if caller_sig.output() != callee_sig.output() {
-                span_bug!(expr.span, "hir typeck should have checked the return type already");
-            }
+            self.report_arguments_mismatch(expr.span, caller_sig, callee_sig);
         }
 
         {

tests/ui/explicit-tail-calls/ret-ty-hr-mismatch.rs

@@ -0,0 +1,15 @@
+#![feature(explicit_tail_calls)]
+#![allow(incomplete_features)]
+
+fn foo() -> for<'a> fn(&'a i32) {
+    become bar();
+    //~^ ERROR mismatched signatures
+}
+
+fn bar() -> fn(&'static i32) {
+    dummy
+}
+
+fn dummy(_: &i32) {}
+
+fn main() {}

tests/ui/explicit-tail-calls/ret-ty-hr-mismatch.stderr

@@ -0,0 +1,12 @@
+error: mismatched signatures
+  --> $DIR/ret-ty-hr-mismatch.rs:5:5
+   |
+LL |     become bar();
+   |     ^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have matching signatures
+   = note: caller signature: `fn() -> for<'a> fn(&'a i32)`
+   = note: callee signature: `fn() -> fn(&i32)`
+
+error: aborting due to 1 previous error
+