Fix debug ICE for extern type with where clauses

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -364,7 +364,12 @@ impl<'a> AstValidator<'a> {
         self.err_handler().emit_err(errors::BoundInContext { span, ctx });
     }
 
-    fn check_foreign_ty_genericless(&self, generics: &Generics, where_span: Span) {
+    fn check_foreign_ty_genericless(
+        &self,
+        generics: &Generics,
+        before_where_clause: &TyAliasWhereClause,
+        after_where_clause: &TyAliasWhereClause,
+    ) {
         let cannot_have = |span, descr, remove_descr| {
             self.err_handler().emit_err(errors::ExternTypesCannotHave {
                 span,
@@ -378,9 +383,14 @@ impl<'a> AstValidator<'a> {
             cannot_have(generics.span, "generic parameters", "generic parameters");
         }
 
-        if !generics.where_clause.predicates.is_empty() {
-            cannot_have(where_span, "`where` clauses", "`where` clause");
-        }
+        let check_where_clause = |where_clause: &TyAliasWhereClause| {
+            if let TyAliasWhereClause(true, where_clause_span) = where_clause {
+                cannot_have(*where_clause_span, "`where` clauses", "`where` clause");
+            }
+        };
+
+        check_where_clause(before_where_clause);
+        check_where_clause(after_where_clause);
     }
 
     fn check_foreign_kind_bodyless(&self, ident: Ident, kind: &str, body: Option<Span>) {
@@ -1039,7 +1049,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 self.check_defaultness(fi.span, *defaultness);
                 self.check_foreign_kind_bodyless(fi.ident, "type", ty.as_ref().map(|b| b.span));
                 self.check_type_no_bounds(bounds, "`extern` blocks");
-                self.check_foreign_ty_genericless(generics, where_clauses.0.1);
+                self.check_foreign_ty_genericless(generics, &where_clauses.0, &where_clauses.1);
                 self.check_foreign_item_ascii_only(fi.ident);
             }
             ForeignItemKind::Static(_, _, body) => {

tests/ui/extern/issue-112363-extern-item-where-clauses-debug-ice.rs

@@ -0,0 +1,10 @@
+extern "C" {
+    type Item = [T] where [T]: Sized;
+    //~^ incorrect `type` inside `extern` block
+    //~| `type`s inside `extern` blocks cannot have `where` clauses
+    //~| cannot find type `T` in this scope
+    //~| cannot find type `T` in this scope
+    //~| extern types are experimental
+}
+
+fn main() {}

tests/ui/extern/issue-112363-extern-item-where-clauses-debug-ice.stderr

@@ -0,0 +1,47 @@
+error: incorrect `type` inside `extern` block
+  --> $DIR/issue-112363-extern-item-where-clauses-debug-ice.rs:2:10
+   |
+LL | extern "C" {
+   | ---------- `extern` blocks define existing foreign types and types inside of them cannot have a body
+LL |     type Item = [T] where [T]: Sized;
+   |          ^^^^   --- the invalid body
+   |          |
+   |          cannot have a body
+   |
+   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
+
+error: `type`s inside `extern` blocks cannot have `where` clauses
+  --> $DIR/issue-112363-extern-item-where-clauses-debug-ice.rs:2:21
+   |
+LL | extern "C" {
+   | ---------- `extern` block begins here
+LL |     type Item = [T] where [T]: Sized;
+   |                     ^^^^^^^^^^^^^^^^ help: remove the `where` clause
+   |
+   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
+
+error[E0412]: cannot find type `T` in this scope
+  --> $DIR/issue-112363-extern-item-where-clauses-debug-ice.rs:2:28
+   |
+LL |     type Item = [T] where [T]: Sized;
+   |                            ^ not found in this scope
+
+error[E0412]: cannot find type `T` in this scope
+  --> $DIR/issue-112363-extern-item-where-clauses-debug-ice.rs:2:18
+   |
+LL |     type Item = [T] where [T]: Sized;
+   |                  ^ not found in this scope
+
+error[E0658]: extern types are experimental
+  --> $DIR/issue-112363-extern-item-where-clauses-debug-ice.rs:2:5
+   |
+LL |     type Item = [T] where [T]: Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #43467 <https://github.com/rust-lang/rust/issues/43467> for more information
+   = help: add `#![feature(extern_types)]` to the crate attributes to enable
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0412, E0658.
+For more information about an error, try `rustc --explain E0412`.

tests/ui/parser/foreign-ty-semantic-fail.rs

@@ -15,4 +15,5 @@ extern "C" {
     //~^ ERROR incorrect `type` inside `extern` block
 
     type E: where;
+    //~^ ERROR `type`s inside `extern` blocks cannot have `where` clauses
 }

tests/ui/parser/foreign-ty-semantic-fail.stderr

@@ -61,5 +61,16 @@ LL |     type D = u8;
    |
    = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
 
-error: aborting due to 6 previous errors
+error: `type`s inside `extern` blocks cannot have `where` clauses
+  --> $DIR/foreign-ty-semantic-fail.rs:17:13
+   |
+LL | extern "C" {
+   | ---------- `extern` block begins here
+...
+LL |     type E: where;
+   |             ^^^^^ help: remove the `where` clause
+   |
+   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
+
+error: aborting due to 7 previous errors