Sema: Fix applying solution for OptionalTryExpr

In Swift 5 mode, CSGen generates a conversion constraint from
the type of OptionalTryExpr's subexpression, call it T, and
Optional<$tv> for a new type variable $tv.

When applying the solution, we would coerce the sub-expression
to T? if T was not optional, or T otherwise. This was wrong
because there's no reason that $tv is actually equal to T.

Instead, we must coerce the sub-expression to Optional<$tv>,
which is the type of the overall OptionalTryExpr.

Fixes <rdar://problem/46742002>.

Author: slavapestov

lib/Sema/CSApply.cpp

@@ -2824,23 +2824,14 @@ namespace {
         return simplifyExprType(expr);
       }
 
-      Type subExprType = cs.getType(expr->getSubExpr());
-      Type targetType = simplifyType(subExprType);
-      
-      // If the subexpression is not optional, wrap it in
-      // an InjectIntoOptionalExpr. Then use the type of the
-      // subexpression as the type of the 'try?' expr
-      bool subExprIsOptional = (bool) subExprType->getOptionalObjectType();
-      
-      if (!subExprIsOptional) {
-        targetType = OptionalType::get(targetType);
-        auto subExpr = coerceToType(expr->getSubExpr(), targetType,
-                                    cs.getConstraintLocator(expr));
-        if (!subExpr) return nullptr;
-        expr->setSubExpr(subExpr);
-      }
-      
-      cs.setType(expr, targetType);
+      Type exprType = simplifyType(cs.getType(expr));
+
+      auto subExpr = coerceToType(expr->getSubExpr(), exprType,
+                                  cs.getConstraintLocator(expr));
+      if (!subExpr) return nullptr;
+      expr->setSubExpr(subExpr);
+
+      cs.setType(expr, exprType);
       return expr;
     }
 

test/Parse/try.swift

@@ -258,3 +258,14 @@ let _: Int? = try? producer.produceDoubleOptionalInt() // expected-error {{value
 let _: Int?? = try? producer.produceDoubleOptionalInt() // expected-error {{value of optional type 'Int???' not unwrapped; did you mean to use 'try!' or chain with '?'?}}
 let _: Int??? = try? producer.produceDoubleOptionalInt() // good
 let _: String = try? producer.produceDoubleOptionalInt() // expected-error {{cannot convert value of type 'Int???' to specified type 'String'}}
+
+// rdar://problem/46742002
+protocol Dummy : class {}
+
+class F<T> {
+  func wait() throws -> T { fatalError() }
+}
+
+func bar(_ a: F<Dummy>, _ b: F<Dummy>) {
+  _ = (try? a.wait()) === (try? b.wait())
+}

test/Parse/try_swift5.swift

@@ -261,3 +261,14 @@ let _: Int? = try? producer.produceDoubleOptionalInt() // expected-error {{canno
 let _: Int?? = try? producer.produceDoubleOptionalInt() // good
 let _: Int??? = try? producer.produceDoubleOptionalInt() // good
 let _: String = try? producer.produceDoubleOptionalInt() // expected-error {{cannot convert value of type 'Int??' to specified type 'String'}}
+
+// rdar://problem/46742002
+protocol Dummy : class {}
+
+class F<T> {
+  func wait() throws -> T { fatalError() }
+}
+
+func bar(_ a: F<Dummy>, _ b: F<Dummy>) {
+  _ = (try? a.wait()) === (try? b.wait())
+}