[CS] Don't retrieve an overload choice for x[](y)

Previously we returned a subscript member locator for an apply of a
subscript expr, which is incorrect because the callee is the function
returned from the subscript rather than the subscript itself.

(cherry picked from commit 40d2f51)

Author: hamishknight

lib/Sema/ConstraintSystem.cpp

@@ -409,6 +409,12 @@ ConstraintLocator *ConstraintSystem::getConstraintLocator(
 }
 
 ConstraintLocator *ConstraintSystem::getCalleeLocator(Expr *expr) {
+  // Make sure we handle subscripts before looking at apply exprs. We don't
+  // want to return a subscript member locator for an expression such as x[](y),
+  // as its callee is not the subscript, but rather the function it returns.
+  if (isa<SubscriptExpr>(expr))
+    return getConstraintLocator(expr, ConstraintLocator::SubscriptMember);
+
   if (auto *applyExpr = dyn_cast<ApplyExpr>(expr)) {
     auto *fnExpr = applyExpr->getFn();
     // For an apply of a metatype, we have a short-form constructor. Unlike
@@ -437,9 +443,6 @@ ConstraintLocator *ConstraintSystem::getCalleeLocator(Expr *expr) {
   if (isa<UnresolvedMemberExpr>(expr))
     return getConstraintLocator(locator, ConstraintLocator::UnresolvedMember);
 
-  if (isa<SubscriptExpr>(expr))
-    return getConstraintLocator(locator, ConstraintLocator::SubscriptMember);
-
   if (isa<MemberRefExpr>(expr))
     return getConstraintLocator(locator, ConstraintLocator::Member);
 

test/Constraints/function.swift

@@ -109,8 +109,11 @@ func foo(block: () -> (), other: () -> Int) {
 
 struct S {
   init<T>(_ x: T, _ y: T) {} // expected-note {{generic parameters are always considered '@escaping'}}
-  init(fn: () -> Int) {
+  subscript<T>() -> (T, T) -> Void { { _, _ in } }
+
+  init(fn: () -> Int) { // expected-note {{parameter 'fn' is implicitly non-escaping}}
     self.init({ 0 }, fn) // expected-error {{converting non-escaping parameter 'fn' to generic parameter 'T' may allow it to escape}}
+    _ = self[]({ 0 }, fn) // expected-error {{passing non-escaping parameter 'fn' to function expecting an @escaping closure}}
   }
 }