Check the base type of pattern types for validty first

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -4,7 +4,6 @@
 //! That's useful because it means other passes (e.g. promotion) can rely on `const`s
 //! to be const-safe.
 
-use std::assert_matches::assert_matches;
 use std::borrow::Cow;
 use std::fmt::Write;
 use std::hash::Hash;
@@ -1241,15 +1240,15 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValueVisitor<'tcx, M> for ValidityVisitor<'rt,
                     self.visit_field(val, 0, &self.ecx.project_index(val, 0)?)?;
                 }
             }
-            ty::Pat(_base, pat) => {
+            ty::Pat(base, pat) => {
+                // First check that the base type is valid
+                self.visit_value(&val.transmute(self.ecx.layout_of(*base)?, self.ecx)?)?;
                 // When you extend this match, make sure to also add tests to
                 // tests/ui/type/pattern_types/validity.rs((
                 match **pat {
                     // Range patterns are precisely reflected into `valid_range` and thus
                     // handled fully by `visit_scalar` (called below).
-                    ty::PatternKind::Range { .. } => {
-                        assert_matches!(val.layout.backend_repr, BackendRepr::Scalar(_));
-                    },
+                    ty::PatternKind::Range { .. } => {},
                 }
             }
             _ => {