Future-proof against loose bounds if default variant is non-exhaustive.

Co-Authored-By: Mark Rousskov <mark.simulacrum@gmail.com>

src/test/ui/macros/macros-nonfatal-errors.rs

@@ -116,3 +116,24 @@ fn main() {
 
     trace_macros!(invalid); //~ ERROR
 }
+
+/// Check that `#[derive(Default)]` does use a `T : Default` bound when the
+/// `#[default]` variant is `#[non_exhaustive]` (should this end up allowed).
+const _: () = {
+    #[derive(Default)]
+    enum NonExhaustiveDefaultGeneric<T> {
+        #[default]
+        #[non_exhaustive]
+        Foo, //~ ERROR default variant must be exhaustive
+        Bar(T),
+    }
+
+    fn assert_impls_default<T: Default>() {}
+
+    enum NotDefault {}
+
+    // Note: the `derive(Default)` currently bails early enough for trait-checking
+    // not to happen. Should it bail late enough, or even pass, make sure to
+    // assert that the following line fails.
+    let _ = assert_impls_default::<NonExhaustiveDefaultGeneric<NotDefault>>;
+};

src/test/ui/macros/macros-nonfatal-errors.stderr

@@ -215,11 +215,21 @@ error: trace_macros! accepts only `true` or `false`
 LL |     trace_macros!(invalid);
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
+error: default variant must be exhaustive
+  --> $DIR/macros-nonfatal-errors.rs:127:9
+   |
+LL |         #[non_exhaustive]
+   |         ----------------- declared `#[non_exhaustive]` here
+LL |         Foo,
+   |         ^^^
+   |
+   = help: consider a manual implementation of `Default`
+
 error: cannot find macro `llvm_asm` in this scope
   --> $DIR/macros-nonfatal-errors.rs:99:5
    |
 LL |     llvm_asm!(invalid);
    |     ^^^^^^^^
 
-error: aborting due to 27 previous errors
+error: aborting due to 28 previous errors