[tests] Adding literal downcast tests

Author: LucianoPAlmeida

test/expr/cast/literals_downcast.swift

@@ -0,0 +1,30 @@
+// RUN: %target-typecheck-verify-swift -enable-objc-interop
+
+import Foundation
+
+let ok = "A" as Character // OK
+let succeed = "A" as? String // expected-warning {{always succeeds}}
+let bad = "A" as? Character // expected-warning {{conditional downcast from literal to 'Character' always fails; consider using 'as' coercion}} {{none}}
+let bad2 = "Aa" as? Character // expected-warning {{cast from 'String' to unrelated type 'Character' always fails}}
+let bad1 = 1 as? Character // expected-warning {{cast from 'Int' to unrelated type 'Character' always fails}}
+
+let okInt = 1 as Int // OK
+let badInt = 1 as? Int // expected-warning {{always succeeds}}
+let badInt1 = 1.0 as? Int // expected-warning {{cast from 'Double' to unrelated type 'Int' always fails}}
+let badInt2 = 1 as? Double // expected-warning {{conditional downcast from literal to 'Double' always fails; consider using 'as' coercion}} {{none}}
+
+let okUInt = 1 as UInt // OK
+let badUInt = 1 as? UInt // expected-warning {{conditional downcast from literal to 'UInt' always fails; consider using 'as' coercion}} {{none}}
+let badUInt1 = 1.0 as? UInt // expected-warning {{cast from 'Double' to unrelated type 'UInt' always fails}}
+
+// Can downcast by bridging
+let bridge = "A" as? NSString // expected-warning {{always succeeds}}
+let bridge1 = 1 as? NSNumber // expected-warning {{always succeeds}}
+
+// Custom protocol adoption
+struct S: ExpressibleByStringLiteral {
+  typealias StringLiteralType = String
+  init(stringLiteral value: Self.StringLiteralType) {}
+}
+
+let a = "A" as? S // expected-warning {{conditional downcast from literal to 'S' always fails; consider using 'as' coercion}} {{none}}