From d89c096af75524dd5e1cf7bc2d1d0fb43997db9c Mon Sep 17 00:00:00 2001
From: Pavel Yaskevich <pyaskevich@apple.com>
Date: Mon, 3 Aug 2020 16:24:19 -0700
Subject: [PATCH 1/2] [Diagnostics] Do more checking before recording `force
 downcast` fix

Solver should do more checking upfront before recording
`force downcast` fix, to make sure that it's indeed always
applicable when recorded, otherwise it would be possible
to misdiagnose or omit diagnostics in certain situations.

Resolves: rdar://problem/65254452
---
 lib/Sema/CSDiagnostics.cpp          |  2 --
 lib/Sema/CSSimplify.cpp             |  6 ++++++
 test/Constraints/rdar65254452.swift | 17 +++++++++++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 test/Constraints/rdar65254452.swift

diff --git a/lib/Sema/CSDiagnostics.cpp b/lib/Sema/CSDiagnostics.cpp
index 9409da96097cb..433c0e9d64733 100644
--- a/lib/Sema/CSDiagnostics.cpp
+++ b/lib/Sema/CSDiagnostics.cpp
@@ -988,8 +988,6 @@ bool MissingExplicitConversionFailure::diagnoseAsError() {
     return false;
 
   bool useAs = TypeChecker::isExplicitlyConvertibleTo(fromType, toType, DC);
-  if (!useAs && !TypeChecker::checkedCastMaySucceed(fromType, toType, DC))
-    return false;
 
   auto *expr = findParentExpr(anchor);
   if (!expr)
diff --git a/lib/Sema/CSSimplify.cpp b/lib/Sema/CSSimplify.cpp
index fbc473622529d..6e882b6c38e98 100644
--- a/lib/Sema/CSSimplify.cpp
+++ b/lib/Sema/CSSimplify.cpp
@@ -2968,6 +2968,9 @@ static bool
 repairViaBridgingCast(ConstraintSystem &cs, Type fromType, Type toType,
                       SmallVectorImpl<RestrictionOrFix> &conversionsOrFixes,
                       ConstraintLocatorBuilder locator) {
+  if (fromType->hasTypeVariable() || toType->hasTypeVariable())
+    return false;
+
   auto objectType1 = fromType->getOptionalObjectType();
   auto objectType2 = toType->getOptionalObjectType();
 
@@ -2986,6 +2989,9 @@ repairViaBridgingCast(ConstraintSystem &cs, Type fromType, Type toType,
   if (!canBridgeThroughCast(cs, fromType, toType))
     return false;
 
+  if (!TypeChecker::checkedCastMaySucceed(fromType, toType, cs.DC))
+    return false;
+
   conversionsOrFixes.push_back(ForceDowncast::create(
       cs, fromType, toType, cs.getConstraintLocator(locator)));
   return true;
diff --git a/test/Constraints/rdar65254452.swift b/test/Constraints/rdar65254452.swift
new file mode 100644
index 0000000000000..7e49b45f3c716
--- /dev/null
+++ b/test/Constraints/rdar65254452.swift
@@ -0,0 +1,17 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify %s
+// REQUIRES: objc_interop
+
+import Foundation
+
+class Obj: NSObject {
+}
+
+class Container {
+  var objects: [Obj]
+  init(objects: [Obj]) {}
+}
+
+func test(other: Container) {
+  _ = Container(objects: other)
+  // expected-error@-1 {{cannot convert value of type 'Container' to expected argument type '[Obj]'}}
+}

From b47698bd544af93e0e55736a97f77828bcd4deb1 Mon Sep 17 00:00:00 2001
From: Pavel Yaskevich <pyaskevich@apple.com>
Date: Mon, 3 Aug 2020 16:44:32 -0700
Subject: [PATCH 2/2] [TypeChecker] NFC: Add debug logging before applying
 solution in `applyFunctionBuilderBodyTransform`

---
 lib/Sema/BuilderTransform.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/Sema/BuilderTransform.cpp b/lib/Sema/BuilderTransform.cpp
index a0ecaae819918..b00deec91536f 100644
--- a/lib/Sema/BuilderTransform.cpp
+++ b/lib/Sema/BuilderTransform.cpp
@@ -1564,6 +1564,13 @@ Optional<BraceStmt *> TypeChecker::applyFunctionBuilderBodyTransform(
     // The system was salvaged; continue on as if nothing happened.
   }
 
+  if (cs.isDebugMode()) {
+    auto &log = llvm::errs();
+    log << "--- Applying Solution ---\n";
+    solutions.front().dump(log);
+    log << '\n';
+  }
+
   // FIXME: Shouldn't need to do this.
   cs.applySolution(solutions.front());