Avoid follow up errors

Author: oli-obk

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

@@ -134,6 +134,21 @@ impl TaitConstraintLocator<'_> {
             debug!("no constraint: no typeck results");
             return;
         }
+
+        if let Some(hir_sig) = self.tcx.hir_node_by_def_id(item_def_id).fn_decl() {
+            if hir_sig.output.get_infer_ret_ty().is_some() {
+                let guar = self.tcx.dcx().span_delayed_bug(
+                    hir_sig.output.span(),
+                    "inferring return types and opaque types do not mix well",
+                );
+                self.found = Some(ty::OpaqueHiddenType {
+                    span: DUMMY_SP,
+                    ty: Ty::new_error(self.tcx, guar),
+                });
+                return;
+            }
+        }
+
         // Calling `mir_borrowck` can lead to cycle errors through
         // const-checking, avoid calling it if we don't have to.
         // ```rust

tests/ui/type-alias-impl-trait/issue-77179.rs

@@ -6,9 +6,7 @@ type Pointer<T> = impl std::ops::Deref<Target = T>;
 
 fn test() -> Pointer<_> {
     //~^ ERROR: the placeholder `_` is not allowed within types
-    //~| ERROR: non-defining opaque type use in defining scope
     Box::new(1)
-    //~^ ERROR expected generic type parameter, found `i32`
 }
 
 fn main() {

tests/ui/type-alias-impl-trait/issue-77179.stderr

@@ -7,28 +7,6 @@ LL | fn test() -> Pointer<_> {
    |              |       not allowed in type signatures
    |              help: replace with the correct return type: `Pointer<i32>`
 
-error[E0792]: non-defining opaque type use in defining scope
-  --> $DIR/issue-77179.rs:7:14
-   |
-LL | fn test() -> Pointer<_> {
-   |              ^^^^^^^^^^ argument `i32` is not a generic parameter
-   |
-note: for this opaque type
-  --> $DIR/issue-77179.rs:5:19
-   |
-LL | type Pointer<T> = impl std::ops::Deref<Target = T>;
-   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error[E0792]: expected generic type parameter, found `i32`
-  --> $DIR/issue-77179.rs:10:5
-   |
-LL | type Pointer<T> = impl std::ops::Deref<Target = T>;
-   |              - this generic parameter must be used with a generic type parameter
-...
-LL |     Box::new(1)
-   |     ^^^^^^^^^^^
-
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0121, E0792.
-For more information about an error, try `rustc --explain E0121`.
+For more information about this error, try `rustc --explain E0121`.