[ConstraintSystem] Fix conversion of variadic/inout closure parameters for internal use

Delayed constraint generation for the body of the single-statement
closures regressed variadic parameter handling. Fix it by using
`FunctionType::Param::getParameterType` for "internal" version of
the parameter type which translates variadic/inout correctly.

Resolved: rdar://problem/58647769

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -6569,25 +6569,29 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
 
   auto *paramList = closure->getParameters();
   for (unsigned i = 0, n = paramList->size(); i != n; ++i) {
-    const auto *param = paramList->get(i);
+    const auto &param = closureType->getParams()[i];
 
-    Type externalType = closureType->getParams()[i].getOldType();
     Type internalType;
 
-    if (param->getTypeRepr()) {
-      internalType = externalType;
+    if (paramList->get(i)->getTypeRepr()) {
+      // Internal type is the type used in the body of the closure,
+      // so "external" type translates to it as follows:
+      //  - `Int...` -> `[Int]`,
+      //  - `inout Int` -> `@lvalue Int`.
+      internalType = param.getParameterType();
     } else {
       auto *paramLoc =
           getConstraintLocator(closure, LocatorPathElt::TupleElement(i));
 
       internalType = createTypeVariable(paramLoc, TVO_CanBindToLValue |
                                                       TVO_CanBindToNoEscape);
 
+      auto externalType = param.getOldType();
       addConstraint(ConstraintKind::BindParam, externalType, internalType,
                     paramLoc);
     }
 
-    setType(param, internalType);
+    setType(paramList->get(i), internalType);
   }
 
   assignFixedType(typeVar, closureType, closureLocator);

test/Constraints/closures.swift

@@ -942,3 +942,8 @@ class Foo<State: StateType> {
                        })
   }
 }
+
+// Make sure that `String...` is translated into `[String]` in the body
+func test_explicit_variadic_is_interpreted_correctly() {
+  _ = { (T: String...) -> String in T[0] + "" } // Ok
+}