Add test for @_requiresConcreteImplementation

Author: xwu

lib/Sema/TypeCheckProtocol.cpp

@@ -3456,7 +3456,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
       break;
 
     case CheckKind::RequiresConcreteImplementation:
-      diagnoseOrDefer(requirement, false,
+      diagnoseOrDefer(requirement, /*isError=*/true,
         [witness, requirement](NormalProtocolConformance *conformance) {
           auto &diags = witness->getASTContext().Diags;
           SourceLoc diagLoc = getLocForDiagnosingWitness(conformance, witness);

test/attr/attr_requires_concrete_implementation.swift

@@ -0,0 +1,48 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {
+  @_requiresConcreteImplementation
+  func f(_: Int) -> Bool // expected-note {{requirement 'f' declared here}}
+  // expected-note@-1 {{overridden declaration is here}}
+  func f<T: SignedInteger>(_: T) -> Bool
+
+  func g(_: Int) -> Bool
+}
+
+extension P {
+  func f<T: SignedInteger>(_ x: T) -> Bool { // expected-note {{'f' declared here}}
+    return f(Int(x))
+  }
+
+  func g(_ x: Int) -> Bool { // expected-note {{'g' declared here}}
+    return f(x)
+  }
+}
+
+protocol Q: P {
+  override func f(_: Int) -> Bool // expected-error {{override of protocol requirement marked '@_requiresConcreteImplementation' should also be marked }}
+}
+
+protocol R: P {
+  @_requiresConcreteImplementation
+  override func g(_: Int) -> Bool // expected-note {{requirement 'g' declared here}}
+}
+
+struct S: P { // expected-error {{type 'S' does not conform to protocol 'P'}}
+  // expected-error@-1 {{protocol requirement marked '@_requiresConcreteImplementation' cannot be satisfied }}
+}
+
+struct T: P, R { // expected-error {{type 'T' does not conform to protocol 'R'}}
+  // expected-error@-1 {{protocol requirement marked '@_requiresConcreteImplementation' cannot be satisfied }}
+  func f(_: Int) -> Bool { true }
+}
+
+struct U: R {
+  func f(_: Int) -> Bool { true }
+  func g(_: Int) -> Bool { false }
+}
+
+class C {
+  @_requiresConcreteImplementation // expected-error {{'@_requiresConcreteImplementation' can only be specified on protocol requirements}}
+  var x: Int = 42
+}