[SILGen] NFC: Add test-cases for rdar://problem/37790062

Port multiple test-cases from `test/Constraints/closures.swift`
to SILGen to verify that closures are inferred as having `Void`
result type, and there is no function conversion thunk emitted
to convert from `() -> T` to `() -> Void` for literal closures.

Author: xedin

lib/Sema/CSBindings.cpp

@@ -381,7 +381,7 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) {
 
         if (auto *locator = typeVar->getImpl().getLocator()) {
           auto path = locator->getPath();
-          auto voidType = TupleType::getEmpty(getASTContext());
+          auto voidType = getASTContext().TheEmptyTupleType;
 
           // If this is a type variable representing closure result,
           // which is on the right-side of some relational constraint
@@ -391,7 +391,7 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) {
           if (!path.empty() &&
               path.back().getKind() == ConstraintLocator::ClosureResult &&
               binding->Kind == AllowedBindingKind::Supertypes &&
-              exactTypes.insert(voidType->getCanonicalType()).second) {
+              exactTypes.insert(voidType).second) {
             result.addPotentialBinding(
                 {voidType, binding->Kind, constraint->getKind()},
                 /*allowJoinMeet=*/false);

test/SILGen/closures.swift

@@ -791,3 +791,67 @@ struct r29810997 {
 
 //   DI will turn this into a direct capture of the specific stored property.
 // CHECK-LABEL: sil hidden @$S8closures16r29810997_helperyS3iXEF : $@convention(thin) (@noescape @callee_guaranteed (Int) -> Int) -> Int
+
+// rdar://problem/37790062
+
+protocol P_37790062 {
+  associatedtype T
+  var elt: T { get }
+}
+
+func rdar37790062() {
+  struct S<T> {
+    init(_ a: () -> T, _ b: () -> T) {}
+  }
+
+  class C1 : P_37790062 {
+    typealias T = Int
+    var elt: T { return 42 }
+  }
+
+  class C2 : P_37790062 {
+    typealias T = (String, Int, Void)
+    var elt: T { return ("question", 42, ()) }
+  }
+
+  func foo() -> Int { return 42 }
+  func bar() -> Void {}
+  func baz() -> (String, Int) { return ("question", 42) }
+  func bzz<T>(_ a: T) -> T { return a }
+  func faz<T: P_37790062>(_ a: T) -> T.T { return a.elt }
+
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU_
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU0_
+  // CHECK: function_ref @$S8closures12rdar37790062yyF1SL_VyADyxGxyXE_xyXEtcfC
+  _ = S({ foo() }, { bar() })
+
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU1_
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU2_
+  // CHECK: function_ref @$S8closures12rdar37790062yyF1SL_VyADyxGxyXE_xyXEtcfC
+  _ = S({ baz() }, { bar() })
+
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU3_
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU4_
+  // CHECK: function_ref @$S8closures12rdar37790062yyF1SL_VyADyxGxyXE_xyXEtcfC
+  _ = S({ bzz(("question", 42)) }, { bar() })
+
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU5_
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU6_
+  // CHECK: function_ref @$S8closures12rdar37790062yyF1SL_VyADyxGxyXE_xyXEtcfC
+  _ = S({ bzz(String.self) }, { bar() })
+
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU7_
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU8_
+  // CHECK: function_ref @$S8closures12rdar37790062yyF1SL_VyADyxGxyXE_xyXEtcfC
+  _ = S({ bzz(((), (()))) }, { bar() })
+
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU9_
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU10_
+  // CHECK: function_ref @$S8closures12rdar37790062yyF1SL_VyADyxGxyXE_xyXEtcfC
+  _ = S({ bzz(C1()) }, { bar() })
+
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU11_
+  // CHECK: function_ref @$S8closures12rdar37790062yyFyyXEfU12_
+  // CHECK: function_ref @$S8closures12rdar37790062yyF1SL_VyADyxGxyXE_xyXEtcfC
+  _ = S({ faz(C2()) }, { bar() })
+}