[CSGen] Don't increase optionality of weak var patterns

If the externally-imposed type is already optional,
let's not add optionality to the pattern.

Author: xedin

lib/Sema/CSGen.cpp

@@ -2209,12 +2209,17 @@ namespace {
 
         switch (optionality) {
         case ReferenceOwnershipOptionality::Required:
-          varType = TypeChecker::getOptionalType(var->getLoc(), varType);
-          assert(!varType->hasError());
-
-          if (oneWayVarType) {
-            oneWayVarType =
-                TypeChecker::getOptionalType(var->getLoc(), oneWayVarType);
+          // If the externally-imposed type is already optional,
+          // let's not add optionality to the pattern.
+          if (!externalPatternType ||
+              !externalPatternType->getOptionalObjectType()) {
+            varType = TypeChecker::getOptionalType(var->getLoc(), varType);
+            assert(!varType->hasError());
+
+            if (oneWayVarType) {
+              oneWayVarType =
+                  TypeChecker::getOptionalType(var->getLoc(), oneWayVarType);
+            }
           }
           break;
 

test/Constraints/result_builder.swift

@@ -788,3 +788,19 @@ let ts1 = MyTupleStruct {
 
 // CHECK: MyTupleStruct<(Int, String, Optional<String>), (Double, String)>(first: (Function), second: (3.14159, "blah"))
 print(ts1)
+
+// Make sure that `weakV` is `Test?` and not `Test??`
+func test_weak_optionality_stays_the_same() {
+  class Test {
+    func fn() -> Int { 42 }
+  }
+
+  tuplify(true) { c in
+    weak var weakV: Test? = Test()
+
+    0
+    if let v = weakV {
+      v.fn()
+    }
+  }
+}