Register hidden types when equating function definitions in coercion

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1139,7 +1139,7 @@ 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

@@ -1,5 +1,7 @@
 //! Test that coercing between function items of the same function,
-//! but with different args works.
+//! but with different generic args works.
+
+//@check-pass
 
 #![feature(type_alias_impl_trait)]
 
@@ -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(());
@@ -59,7 +61,7 @@ fn k() -> impl Sized {
             let f = foo;
             bind(k(), f)
         }
-        false => foo::<()>, //~ ERROR: incompatible types
+        false => foo::<()>,
     };
     todo!()
 }