Fix ICE for never pattern as closure parameters

compiler/rustc_hir_typeck/src/expr.rs

@@ -1817,12 +1817,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &'tcx hir::Expr<'tcx>,
     ) -> Ty<'tcx> {
         let element_ty = if !args.is_empty() {
+            // This shouldn't happen unless there's another error
+            // (e.g., never patterns in inappropriate contexts).
+            if self.diverges.get() != Diverges::Maybe {
+                self.dcx()
+                    .struct_span_err(expr.span, "unexpected divergence state in checking array")
+                    .delay_as_bug();
+            }
+
             let coerce_to = expected
                 .to_option(self)
                 .and_then(|uty| self.try_structurally_resolve_type(expr.span, uty).builtin_index())
                 .unwrap_or_else(|| self.next_ty_var(expr.span));
             let mut coerce = CoerceMany::with_coercion_sites(coerce_to, args);
-            assert_eq!(self.diverges.get(), Diverges::Maybe);
+
             for e in args {
                 let e_ty = self.check_expr_with_hint(e, coerce_to);
                 let cause = self.misc(e.span);

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -104,8 +104,8 @@ pub(crate) struct FnCtxt<'a, 'tcx> {
     /// the diverges flag is set to something other than `Maybe`.
     pub(super) diverges: Cell<Diverges>,
 
-    /// If one of the function arguments is a never pattern, this counts as diverging code. This
-    /// affect typechecking of the function body.
+    /// If one of the function arguments is a never pattern, this counts as diverging code.
+    /// This affect typechecking of the function body.
     pub(super) function_diverges_because_of_empty_arguments: Cell<Diverges>,
 
     /// Whether the currently checked node is the whole body of the function.

tests/ui/never_type/never-pattern-as-closure-param-141592.rs

@@ -0,0 +1,24 @@
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+enum Never {}
+
+fn example(x: Never) -> [i32; 1] {
+    let ! = x;
+    [1]
+}
+
+fn function_param_never(!: Never) -> [i32; 1] {
+    [1]
+}
+
+fn generic_never<T>(!: T) -> [i32; 1] //~ ERROR mismatched types
+where
+    T: Copy,
+{
+    [1]
+}
+
+fn main() {
+    let _ = "12".lines().map(|!| [1]); //~ ERROR mismatched types
+}

tests/ui/never_type/never-pattern-as-closure-param-141592.stderr

@@ -0,0 +1,18 @@
+error: mismatched types
+  --> $DIR/never-pattern-as-closure-param-141592.rs:15:21
+   |
+LL | fn generic_never<T>(!: T) -> [i32; 1]
+   |                     ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `T`
+
+error: mismatched types
+  --> $DIR/never-pattern-as-closure-param-141592.rs:23:31
+   |
+LL |     let _ = "12".lines().map(|!| [1]);
+   |                               ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `str`
+
+error: aborting due to 2 previous errors
+