Fix accidental type inference in array coercion

compiler/rustc_hir_typeck/src/expr.rs

@@ -1760,7 +1760,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let coerce_to = expected
                 .to_option(self)
                 .and_then(|uty| match *self.try_structurally_resolve_type(expr.span, uty).kind() {
-                    ty::Array(ty, _) | ty::Slice(ty) => Some(ty),
+                    // Avoid using a type variable as the coerce_to type, as it may resolve
+                    // during the first coercion instead of being based on the common type.
+                    ty::Array(ty, _) | ty::Slice(ty) if !ty.is_ty_var() => Some(ty),
                     _ => None,
                 })
                 .unwrap_or_else(|| self.next_ty_var(expr.span));

tests/ui/coercion/coerce-many-with-ty-var.rs

@@ -0,0 +1,25 @@
+//@ run-pass
+// Check that least upper bound coercions don't resolve type variable merely based on the first
+// coercion. Check issue #136420.
+
+fn foo() {}
+fn bar() {}
+
+fn infer<T>(_: T) {}
+
+fn infer_array_element<T>(_: [T; 2]) {}
+
+fn main() {
+    infer(if false {
+        foo
+    } else {
+        bar
+    });
+
+    infer(match false {
+        true => foo,
+        false => bar,
+    });
+
+    infer_array_element([foo, bar]);
+}