Register hidden types when equating function definitions in coercion

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1145,7 +1145,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         // are the same function and their parameters have a LUB.
                         match self.commit_if_ok(|_| {
                             self.at(cause, self.param_env).lub(
-                                DefineOpaqueTypes::No,
+                                DefineOpaqueTypes::Yes,
                                 prev_ty,
                                 new_ty,
                             )

tests/ui/fn/fn_def_opaque_coercion.rs

@@ -1,6 +1,8 @@
 //! Test that coercing between function items of the same function,
 //! but with different args works.
 
+//@check-pass
+
 #![feature(type_alias_impl_trait)]
 
 fn foo<T>(t: T) -> T {
@@ -44,7 +46,7 @@ type J = impl Sized;
 fn j(a: J) {
     let x = match true {
         true => foo::<J>,
-        false => foo::<()>, //~ ERROR: incompatible types
+        false => foo::<()>,
     };
     x(a);
     x(());