From 739ba4bb3908f9dfb02ad3530744c608b6c5f49f Mon Sep 17 00:00:00 2001
From: Pavel Yaskevich <pyaskevich@apple.com>
Date: Mon, 6 Jul 2020 11:46:03 -0700
Subject: [PATCH] [CSBindings] Open collection binding associated with
 @autoclosure argument

To preserve subtype relationship between element types of a collection
passed as an argument to a parameter represented by another collection
type let's extend opening of the collection types to be done for arguments
to @autoclosure parameters as well because implicit closure is transparent
to the caller.

Resolves: [SR-13135](https://bugs.swift.org/browse/SR-13135)
Resolves: rdar://problem/65088975
---
 lib/Sema/CSBindings.cpp                  |  3 ++-
 test/Constraints/argument_matching.swift | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/lib/Sema/CSBindings.cpp b/lib/Sema/CSBindings.cpp
index a3e7ce416b79e..76febfc314d99 100644
--- a/lib/Sema/CSBindings.cpp
+++ b/lib/Sema/CSBindings.cpp
@@ -1034,7 +1034,8 @@ bool TypeVarBindingProducer::computeNext() {
 
     auto srcLocator = binding.getLocator();
     if (srcLocator &&
-        srcLocator->isLastElement<LocatorPathElt::ApplyArgToParam>() &&
+        (srcLocator->isLastElement<LocatorPathElt::ApplyArgToParam>() ||
+         srcLocator->isLastElement<LocatorPathElt::AutoclosureResult>()) &&
         !type->hasTypeVariable() && type->isKnownStdlibCollectionType()) {
       // If the type binding comes from the argument conversion, let's
       // instead of binding collection types directly, try to bind
diff --git a/test/Constraints/argument_matching.swift b/test/Constraints/argument_matching.swift
index 1a8a1984faf1a..0a9f9ecb50ef0 100644
--- a/test/Constraints/argument_matching.swift
+++ b/test/Constraints/argument_matching.swift
@@ -1607,3 +1607,19 @@ struct DiagnoseAllLabels {
     f(aax: 0, bbx: 1, dd: 3, ff: 5) // expected-error {{incorrect argument labels in call (have 'aax:bbx:dd:ff:', expected 'aa:bb:dd:ff:')}} {{7-10=aa}} {{15-18=bb}} {{none}}
   }
 }
+
+// SR-13135: Type inference regression in Swift 5.3 - can't infer a type of @autoclosure result.
+func sr13135() {
+  struct Foo {
+    var bar: [Int] = []
+  }
+
+  let baz: Int? = nil
+
+  func foo<T: Equatable>(
+    _ a: @autoclosure () throws -> T,
+    _ b: @autoclosure () throws -> T
+  ) {}
+
+  foo(Foo().bar, [baz])
+}