Also consider TAIT to be uncomputable if the MIR body is tainted

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -183,9 +183,17 @@ impl TaitConstraintLocator<'_> {
  };
 
  // Use borrowck to get the type with unerased regions.
- let concrete_opaque_types = &self.tcx.mir_borrowck(item_def_id).concrete_opaque_types;
- debug!(?concrete_opaque_types);
- if let Some(&concrete_type) = concrete_opaque_types.get(&self.def_id) {
+ let borrowck_results = &self.tcx.mir_borrowck(item_def_id);
+
+ // If the body was tainted, then assume the opaque may have been constrained and just set it to error.
+ if let Some(guar) = borrowck_results.tainted_by_errors {
+ self.found =
+ Some(ty::OpaqueHiddenType { span: DUMMY_SP, ty: Ty::new_error(self.tcx, guar) });
+ return;
+ }
+
+ debug!(?borrowck_results.concrete_opaque_types);
+ if let Some(&concrete_type) = borrowck_results.concrete_opaque_types.get(&self.def_id) {
  debug!(?concrete_type, "found constraint");
  if let Some(prev) = &mut self.found {
  if concrete_type.ty != prev.ty && !(concrete_type, prev.ty).references_error() {

tests/ui/type-alias-impl-trait/unconstrained-due-to-bad-pattern.rs

@@ -0,0 +1,14 @@
+#![feature(type_alias_impl_trait)]
+
+type Tait = impl Copy;
+// Make sure that this TAIT isn't considered unconstrained...
+
+fn empty_opaque() -> Tait {
+ if false {
+ match empty_opaque() {}
+ //~^ ERROR non-empty
+ }
+ 0u8
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/unconstrained-due-to-bad-pattern.stderr

@@ -0,0 +1,17 @@
+error[E0004]: non-exhaustive patterns: type `Tait` is non-empty
+ --> $DIR/unconstrained-due-to-bad-pattern.rs:8:15
+ |
+LL | match empty_opaque() {}
+ | ^^^^^^^^^^^^^^
+ |
+ = note: the matched value is of type `Tait`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
+ |
+LL ~ match empty_opaque() {
+LL + _ => todo!(),
+LL + }
+ |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0004`.