[CSDiagnostics] Contextual failure could result in optional chain having non-optional type

Fixes a crash during diagnostics by not assuming that optional chain
would always produce an optional type, which is not true because in
error scenarios it could get assigned an invalid type from context.

Resolves: rdar://85516390

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'}}
+  }
+}