trait_selection: Resolve inference variables after deep normalization

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -245,6 +245,12 @@ where
         return Err(errors);
     }
 
+    // The fallback in `normalize_with_universes` builds `value` using fresh inference
+    // variables and returns their obligations separately. Fulfillment may constrain
+    // these variables, but it does not fold `value` again, so resolve them before
+    // returning.
+    let value = at.infcx.resolve_vars_if_possible(value);
+
     Ok((value, stalled_coroutine_goals))
 }
 

tests/ui/traits/next-solver/normalize/deeply-normalize-fallback-issue-160875.rs

@@ -0,0 +1,45 @@
+//@ needs-rustc-debug-assertions
+//@ compile-flags: -Znext-solver
+
+// Regression test for #160875.
+//
+// This needs a rustc with debug assertions: the only symptom of the bug is the
+// `debug_assert_eq!` in `try_normalize_after_erasing_regions`. That function
+// resolves inference variables itself before returning, so without the
+// assertion the pre-fix and post-fix output are identical and there is nothing
+// for a test to observe.
+//
+// The first normalization attempt folds the inner alias before trying the outer
+// one. The outer goal then returns `NoSolution`, so `normalize_with_universes`
+// discards the partial result and retries the original value with
+// `ReplaceAliasWithInfer`. The nested alias stays in place because it has escaping
+// bound vars, while the outer alias becomes a fresh inference variable.
+//
+// Fulfillment later proves the fallback obligations and constrains that variable,
+// but it does not rebuild the value produced by the fold. Failing to resolve the
+// value before returning made `normalize_erasing_regions` see `fn(?3t)` even though
+// the inference context had already resolved it to `fn(&'?0 ())`.
+//
+// `bar` is a `const fn` so that drop elaboration -- and with it the
+// `PostAnalysisNormalize` pass which normalizes its signature -- runs even
+// though this test only emits metadata. The body error makes this test reach the
+// MIR assertion; a valid body takes the same normalization fallback but does not
+// reach that assertion here.
+
+#![feature(inherent_associated_types)]
+#![allow(incomplete_features)]
+
+struct Foo<T>(T);
+
+impl<'a> Foo<fn(&'a ())> {
+    type Assoc = &'a ();
+}
+
+const fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {
+    //~^ ERROR lifetime bound not satisfied
+    //~| ERROR lifetime bound not satisfied
+    x
+    //~^ ERROR cannot find value `x` in this scope
+}
+
+fn main() {}

tests/ui/traits/next-solver/normalize/deeply-normalize-fallback-issue-160875.stderr

@@ -0,0 +1,24 @@
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/deeply-normalize-fallback-issue-160875.rs:41:5
+   |
+LL |     x
+   |     ^ not found in this scope
+
+error[E0478]: lifetime bound not satisfied
+  --> $DIR/deeply-normalize-fallback-issue-160875.rs:38:17
+   |
+LL | const fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0478]: lifetime bound not satisfied
+  --> $DIR/deeply-normalize-fallback-issue-160875.rs:38:17
+   |
+LL | const fn bar(_: fn(Foo<for<'b> fn(Foo<fn(&'b ())>::Assoc)>::Assoc)) {
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0425, E0478.
+For more information about an error, try `rustc --explain E0425`.