Merge pull request #41824 from xedin/rdar-85516390

[CSDiagnostics] Contextual failure could result in optional chain hav…

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -2351,7 +2351,9 @@ bool ContextualFailure::diagnoseAsError() {
     if (isExpr<OptionalTryExpr>(anchor) ||
         isExpr<OptionalEvaluationExpr>(anchor)) {
       auto objectType = fromType->getOptionalObjectType();
-      if (objectType->isEqual(toType)) {
+      // Cannot assume that `fromType` is always optional here since
+      // it could assume a type from context.
+      if (objectType && objectType->isEqual(toType)) {
         MissingOptionalUnwrapFailure failure(getSolution(), getType(anchor),
                                              toType,
                                              getConstraintLocator(anchor));

validation-test/Sema/type_checker_crashers_fixed/rdar85516390.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {
+}
+
+struct MyThing : P {
+  var myVar: String? { "" }
+}
+
+struct Test {
+  func test(thing: MyThing?) -> P {
+    return thing?.myVar
+    // expected-error@-1 {{return expression of type 'String' does not conform to 'P'}}
+    // expected-error@-2 {{value of optional type 'String?' must be unwrapped to a value of type 'String'}}
+    // expected-note@-3 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
+    // expected-note@-4 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}
+  }
+}