Don't report E0740 for type error

compiler/rustc_hir_analysis/src/check/check.rs

@@ -114,9 +114,11 @@ fn check_union_fields(tcx: TyCtxt<'_>, span: Span, item_def_id: LocalDefId) -> b
                     allowed_union_field(*elem, tcx, param_env)
                 }
                 _ => {
-                    // Fallback case: allow `ManuallyDrop` and things that are `Copy`.
+                    // Fallback case: allow `ManuallyDrop` and things that are `Copy`,
+                    // also no need to report an error if the type is unresolved.
                     ty.ty_adt_def().is_some_and(|adt_def| adt_def.is_manually_drop())
                         || ty.is_copy_modulo_regions(tcx, param_env)
+                        || ty.references_error()
                 }
             }
         }

tests/ui/union/unresolved-field-isnt-copy.rs

@@ -0,0 +1,8 @@
+// Unresolved fields are not copy, but also shouldn't report an extra E0740.
+
+pub union Foo {
+    x: *const Missing,
+    //~^ ERROR cannot find type `Missing` in this scope
+}
+
+fn main() {}

tests/ui/union/unresolved-field-isnt-copy.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `Missing` in this scope
+  --> $DIR/unresolved-field-isnt-copy.rs:4:15
+   |
+LL |     x: *const Missing,
+   |               ^^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.