[CSSyntacticElement] Bring type variables from outer pack expansion into scope

If closure appears inside of a pack expansion, the elements
that reference pack elements have to bring expansion's shape
type in scope to make sure that the shapes match.

lib/Sema/CSSyntacticElement.cpp

@@ -153,6 +153,17 @@ public:
       }
     }
 
+    // If closure appears inside of a pack expansion, the elements
+    // that reference pack elements have to bring expansion's shape
+    // type in scope to make sure that the shapes match.
+    if (auto *packElement = getAsExpr<PackElementExpr>(expr)) {
+      if (auto *outerEnvironment = CS.getPackEnvironment(packElement)) {
+        auto *expansionTy = CS.simplifyType(CS.getType(outerEnvironment))
+                                ->castTo<PackExpansionType>();
+        expansionTy->getCountType()->getTypeVariables(ReferencedVars);
+      }
+    }
+
     return Action::Continue(expr);
   }
 

test/Constraints/pack-expansion-expressions.swift

@@ -748,13 +748,11 @@ do {
     (repeat takesClosure { each t }) // Ok
   }
 
-  // FIXME: multi-statement closures should type-check.
   func testMultiStmtClosure<each T>(_ t: repeat each T) -> (repeat each T) {
      (repeat takesClosure {
-       // expected-error@-1 {{pack expansion requires that '_' and 'each T' have the same shape}}
        let v = each t
        return v
-    })
+    }) // Ok
   }
 
   func takesAutoclosure<T>(_ fn: @autoclosure () -> T) -> T { return fn() }