Check that all hidden types are the same and then deduplicate them.

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -55,9 +55,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         infcx: &InferCtxt<'_, 'tcx>,
         opaque_ty_decls: VecMap<OpaqueTypeKey<'tcx>, (OpaqueHiddenType<'tcx>, OpaqueTyOrigin)>,
     ) -> VecMap<OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>> {
-        opaque_ty_decls
-            .into_iter()
-            .map(|(opaque_type_key, (concrete_type, origin))| {
+        let mut result: VecMap<OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>> = VecMap::new();
+        for (opaque_type_key, (concrete_type, origin)) in opaque_ty_decls {
             let substs = opaque_type_key.substs;
             debug!(?concrete_type, ?substs);
 
@@ -121,9 +120,28 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             } else {
                 infcx.tcx.ty_error()
             };
-                (opaque_type_key, OpaqueHiddenType { ty, span: concrete_type.span })
-            })
-            .collect()
+            // Sometimes two opaque types are the same only after we remap the generic parameters
+            // back to the opaque type definition. E.g. we may have `OpaqueType<X, Y>` mapped to `(X, Y)`
+            // and `OpaqueType<Y, X>` mapped to `(Y, X)`, and those are the same, but we only know that
+            // once we convert the generic parameters to those of the opaque type.
+            if let Some(prev) = result.get_mut(&opaque_type_key) {
+                if prev.ty != ty {
+                    let mut err = infcx.tcx.sess.struct_span_err(
+                        concrete_type.span,
+                        &format!("hidden type `{}` differed from previous `{}`", ty, prev.ty),
+                    );
+                    err.span_note(prev.span, "previous hidden type bound here");
+                    err.emit();
+                    prev.ty = infcx.tcx.ty_error();
+                }
+                // Pick a better span if there is one.
+                // FIXME(oli-obk): collect multiple spans for better diagnostics down the road.
+                prev.span = prev.span.substitute_dummy(concrete_type.span);
+            } else {
+                result.insert(opaque_type_key, OpaqueHiddenType { ty, span: concrete_type.span });
+            }
+        }
+        result
     }
 
     /// Map the regions in the type to named regions. This is similar to what

src/test/ui/type-alias-impl-trait/multiple_definitions.rs

@@ -0,0 +1,30 @@
+// check-pass
+
+use std::marker::PhantomData;
+
+pub struct ConcreteError {}
+pub trait IoBase {}
+struct X {}
+impl IoBase for X {}
+
+pub struct ClusterIterator<B, E, S = B> {
+    pub fat: B,
+    phantom_s: PhantomData<S>,
+    phantom_e: PhantomData<E>,
+}
+
+pub struct FileSystem<IO: IoBase> {
+    pub disk: IO,
+}
+
+impl<IO: IoBase> FileSystem<IO> {
+    pub fn cluster_iter(&self) -> ClusterIterator<impl IoBase + '_, ConcreteError> {
+        ClusterIterator {
+            fat: X {},
+            phantom_s: PhantomData::default(),
+            phantom_e: PhantomData::default(),
+        }
+    }
+}
+
+fn main() {}