Sema: Fix another type variable leak with closures

When we type check a closure expression with inferred parameter
types, we assign type variables to them. If type checking fails,
we end up in FailureDiagnosis::diagnoseClosureExpr().

If there was no contextual type, we would skip the code path
which nukes type variables in the closure's ParamDecls before
proceeding to type check the body. Type checking of the body
creates a new constraint system which would pick up the outer
constraint system's type variables and blow up.

Fixes <rdar://problem/39489003>.

Author: slavapestov

lib/Sema/CSDiag.cpp

@@ -6532,18 +6532,18 @@ bool FailureDiagnosis::diagnoseClosureExpr(
     }
 
     expectedResultType = fnType->getResult();
-  } else {
-    // Defend against type variables from our constraint system leaking into
-    // recursive constraints systems formed when checking the body of the
-    // closure.  These typevars come into them when the body does name
-    // lookups against the parameter decls.
-    //
-    // Handle this by rewriting the arguments to UnresolvedType().
-    for (auto VD : *CE->getParameters()) {
-      if (VD->getType()->hasTypeVariable() || VD->getType()->hasError()) {
-        VD->setType(CS.getASTContext().TheUnresolvedType);
-        VD->setInterfaceType(VD->getType()->getInOutObjectType());
-      }
+  }
+
+  // Defend against type variables from our constraint system leaking into
+  // recursive constraints systems formed when checking the body of the
+  // closure.  These typevars come into them when the body does name
+  // lookups against the parameter decls.
+  //
+  // Handle this by rewriting the arguments to UnresolvedType().
+  for (auto VD : *CE->getParameters()) {
+    if (VD->getType()->hasTypeVariable() || VD->getType()->hasError()) {
+      VD->setType(CS.getASTContext().TheUnresolvedType);
+      VD->setInterfaceType(VD->getType()->getInOutObjectType());
     }
   }
 

test/Constraints/closures.swift

@@ -690,3 +690,24 @@ func rdar37790062() {
   _ = S({ bzz(C1()) }, { bar() }) // expected-warning {{result of call to 'bzz' is unused}}
   _ = S({ faz(C2()) }, { bar() }) // expected-warning {{result of call to 'faz' is unused}}
 }
+
+// <rdar://problem/39489003>
+typealias KeyedItem<K, T> = (key: K, value: T)
+
+protocol Node {
+  associatedtype T
+  associatedtype E
+  associatedtype K
+  var item: E {get set}
+  var children: [(key: K, value: T)] {get set}
+}
+
+extension Node {
+  func getChild(for key:K)->(key: K, value: T) {
+    return children.first(where: { (item:KeyedItem) -> Bool in
+        return item.key == key
+        // expected-error@-1 {{binary operator '==' cannot be applied to operands of type '_' and 'Self.K'}}
+        // expected-note@-2 {{overloads for '==' exist with these partially matching parameter lists:}}
+      })
+  }
+}