[Tests] Add initial tests for placeholder types

Author: Jumhyn

include/swift/AST/DiagnosticsSema.def

@@ -3676,7 +3676,7 @@ NOTE(descriptive_generic_type_declared_here,none,
      "%0 declared here", (StringRef))
 
 ERROR(placeholder_type_not_allowed,none,
-      "you cannot use a placeholder type here", ())
+      "type placeholder not allowed here", ())
 
 WARNING(use_of_void_pointer,none,
 "Unsafe%0Pointer<Void> has been replaced by Unsafe%0RawPointer", (StringRef))

test/Sema/placeholder_type.swift

@@ -0,0 +1,51 @@
+// RUN: %target-typecheck-verify-swift
+
+let x: _ = 0
+let dict1: [_: Int] = ["hi": 0]
+let dict2: [Character: _] = ["h": 0]
+
+let arr = [_](repeating: "hi", count: 3)
+
+func foo(_ arr: [_] = [0]) {} // expected-error {{placeholder type not allowed here}}
+
+let foo = _.foo // expected-error {{could not infer type for placeholder}}
+let zero: _ = .zero // expected-error {{cannot infer contextual base in reference to member 'zero'}}
+
+struct S<T> {
+    var x: T
+}
+
+var s1: S<_> = .init(x: 0)
+var s2 = S<_>(x: 0)
+
+let losslessStringConverter = Double.init as (String) -> _?
+
+let optInt: _? = 0
+let implicitOptInt: _! = 0
+
+let func1: (_) -> Double = { (x: Int) in 0.0 }
+let func2: (Int) -> _ = { x in 0.0 }
+let func3: (_) -> _ = { (x: Int) in 0.0 }
+let func4: (_, String) -> _ = { (x: Int, y: String) in 0.0 }
+let func5: (_, String) -> _ = { (x: Int, y: Double) in 0.0 } // expected-error {{cannot convert value of type '(Int, Double) -> _' to specified type '(_, String) -> _'}}
+
+let type: _.Type = Int.self
+let type2: Int.Type.Type = _.Type.self
+
+struct MyType1<T, U> {
+    init(t: T, mt2: MyType2<T>) where U == MyType2<T> {}
+}
+
+struct MyType2<T> {
+    init(t: T) {}
+}
+
+let _: MyType2<_> = .init(t: "c" as Character)
+let _: MyType1<_, MyType2<_>> = .init(t: "s" as Character, mt2: .init(t: "c" as Character))
+
+func dictionary<K, V>(ofType: [K: V].Type) -> [K: V] { [:] }
+
+let _: [String: _] = dictionary(ofType: [_: Int].self)
+let _: [_: _] = dictionary(ofType: [String: Int].self)
+let _: [String: Int] = dictionary(ofType: _.self)
+