[Diagnostics] Extend missing argument(s) diagnostic to support closure returns

If return type is a function, it's possible to return a closure
which can have some of its arguments unused in the body e.g.

`let _: () -> ((Int) -> Void) = { return { } }`

In this case resulting closure has to use its only parameter or
explictly ignore it by declaring `_ in`.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -3767,12 +3767,11 @@ bool ImplicitInitOnNonConstMetatypeFailure::diagnoseAsError() {
 bool MissingArgumentsFailure::diagnoseAsError() {
   auto &cs = getConstraintSystem();
   auto *locator = getLocator();
-  auto path = locator->getPath();
 
-  if (path.empty() ||
-      !(path.back().getKind() == ConstraintLocator::ApplyArgToParam ||
-        path.back().getKind() == ConstraintLocator::ContextualType ||
-        path.back().getKind() == ConstraintLocator::ApplyArgument))
+  if (!(locator->isLastElement<LocatorPathElt::ApplyArgToParam>() ||
+        locator->isLastElement<LocatorPathElt::ContextualType>() ||
+        locator->isLastElement<LocatorPathElt::ApplyArgument>() ||
+        locator->isLastElement<LocatorPathElt::ClosureResult>()))
     return false;
 
   // If this is a misplaced `missng argument` situation, it would be
@@ -4015,6 +4014,13 @@ bool MissingArgumentsFailure::diagnoseClosure(ClosureExpr *closure) {
     funcType = cs.getContextualType()->getAs<FunctionType>();
   } else if (auto info = getFunctionArgApplyInfo(locator)) {
     funcType = info->getParamType()->getAs<FunctionType>();
+  } else if (locator->isLastElement<LocatorPathElt::ClosureResult>()) {
+    // Based on the locator we know this this is something like this:
+    // `let _: () -> ((Int) -> Void) = { return {} }`.
+    funcType = getType(getRawAnchor())
+                   ->castTo<FunctionType>()
+                   ->getResult()
+                   ->castTo<FunctionType>();
   }
 
   if (!funcType)

test/Constraints/closures.swift

@@ -484,7 +484,7 @@ let _ = { $0 = $0 = 42 } // expected-error {{assigning a variable to itself}}
 let mismatchInClosureResultType : (String) -> ((Int) -> Void) = {
   (String) -> ((Int) -> Void) in
     return { }
-    // expected-error@-1 {{contextual type for closure argument list expects 1 argument, which cannot be implicitly ignored}}
+    // expected-error@-1 {{contextual type for closure argument list expects 1 argument, which cannot be implicitly ignored}} {{13-13= _ in}}
 }
 
 // SR-3520: Generic function taking closure with inout parameter can result in a variety of compiler errors or EXC_BAD_ACCESS

test/Constraints/diagnostics.swift

@@ -288,7 +288,7 @@ func r18800223(_ i : Int) {
 
 
   var buttonTextColor: String?
-  _ = (buttonTextColor != nil) ? 42 : {$0}; // expected-error {{type of expression is ambiguous without more context}}
+  _ = (buttonTextColor != nil) ? 42 : {$0}; // expected-error {{unable to infer closure type in the current context}}
 }
 
 // <rdar://problem/21883806> Bogus "'_' can only appear in a pattern or on the left side of an assignment" is back

test/expr/closure/closures.swift

@@ -12,6 +12,7 @@ var closure1 : () -> Int = {4}  // Function producing 4 whenever it is called.
 var closure2 : (Int,Int) -> Int = { 4 } // expected-error{{contextual type for closure argument list expects 2 arguments, which cannot be implicitly ignored}} {{36-36= _,_ in}}
 var closure3a : () -> () -> (Int,Int) = {{ (4, 2) }} // multi-level closing.
 var closure3b : (Int,Int) -> (Int) -> (Int,Int) = {{ (4, 2) }} // expected-error{{contextual type for closure argument list expects 2 arguments, which cannot be implicitly ignored}}  {{52-52=_,_ in }}
+// expected-error@-1 {{contextual type for closure argument list expects 1 argument, which cannot be implicitly ignored}} {{53-53= _ in}}
 var closure4 : (Int,Int) -> Int = { $0 + $1 }
 var closure5 : (Double) -> Int = {
        $0 + 1.0