When evaluating candidates for closeness, make sure to consider UnresolvedType to be

a match, since it *could* be, and typically conforms to whatever the expression is.

Fixing this improves the diagnostic in Constraints/closures.swift significantly, and
fixes these bugs:
<rdar://problem/21718970> QoI: [uninferred generic param] cannot invoke 'foo' with an argument list of type '(Int)'
<rdar://problem/21718955> Swift useless error: cannot invoke 'foo' with no arguments

where before we produced:
error: cannot invoke 'foo' with an argument list of type '(Int)'

and now produce:

x.swift:5:10: error: generic parameter 'A' could not be inferred
Whatever.foo(a: 23)
         ^
x.swift:1:7: note: 'A' declared as parameter to type 'Whatever'
class Whatever<A: IntegerArithmetic, B: IntegerArithmetic> {
      ^

Author: lattner

lib/Sema/CSDiag.cpp

@@ -1174,7 +1174,9 @@ CalleeCandidateInfo::evaluateCloseness(DeclContext *dc, Type candArgListType,
 
       TypeSubstitutionMap archetypesMap;
       bool matched;
-      if (paramType->hasUnresolvedType() || rArgType->hasTypeVariable())
+      if (paramType->hasUnresolvedType())
+        matched = true;
+      else if (rArgType->hasTypeVariable())
         matched = false;
       else {
         auto matchType = paramType;
@@ -4452,7 +4454,6 @@ bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
   if (calleeInfo.diagnoseSimpleErrors(callExpr))
     return true;
 
-  
   // A common error is to apply an operator that only has inout forms (e.g. +=)
   // to non-lvalues (e.g. a local let).  Produce a nice diagnostic for this
   // case.

test/Constraints/closures.swift

@@ -192,7 +192,7 @@ struct S<T> {
 
   func subscribe<Object: AnyObject where Object: Hashable>(object: Object?, method: (Object, T) -> ()) {
     let wrappedMethod = { (object: AnyObject, value: T) in }
-    // expected-error @+1 {{cannot convert value of type '(AnyObject, T) -> ()' to expected argument type '(AnyObject, _) -> ()'}}
+    // expected-error @+1 {{value of optional type 'Object?' not unwrapped; did you mean to use '!' or '?'?}}
     cs.forEach { $0.w.append(value: wrappedMethod, forKey: object) }
   }
 }

test/Constraints/generics.swift

@@ -226,3 +226,15 @@ func test9215114<T: P19215114, U: Q19215114>(_ t: T) -> (U) -> () {
   return f
 }
 
+// <rdar://problem/21718970> QoI: [uninferred generic param] cannot invoke 'foo' with an argument list of type '(Int)'
+class Whatever<A: IntegerArithmetic, B: IntegerArithmetic> {  // expected-note 2 {{'A' declared as parameter to type 'Whatever'}}
+  static func foo(a: B) {}
+  
+  static func bar() {}
+
+}
+Whatever.foo(a: 23) // expected-error {{generic parameter 'A' could not be inferred}}
+
+// <rdar://problem/21718955> Swift useless error: cannot invoke 'foo' with no arguments
+Whatever.bar()  // expected-error {{generic parameter 'A' could not be inferred}}
+