[AST] PackMacher: Match pack expansion elements pair-wise if pack arity matches

This covers situations like `Pack{repeat each T1, repeat each T2}`
vs. `Pack{repeat $T1, repeat $T2}` where type variables are allowed
to bind to packs.

Resolves: rdar://109539394

Author: xedin

include/swift/AST/PackExpansionMatcher.h

@@ -186,6 +186,23 @@ class TypeListPackMatcher {
       }
     }
 
+    // Both sides have equal number of pack expansion type.
+    if (lhsElts.size() == rhsElts.size()) {
+      for (unsigned i = 0, n = lhsElts.size(); i != n; ++i) {
+        auto lhsType = getElementType(lhsElts[i]);
+        auto rhsType = getElementType(rhsElts[i]);
+
+        if (IsPackExpansionType(lhsType) && IsPackExpansionType(rhsType)) {
+          pairs.emplace_back(lhsType, rhsType, i, i);
+        } else {
+          pairs.clear();
+          return true;
+        }
+      }
+
+      return false;
+    }
+
     // Otherwise, all remaining possibilities are invalid:
     // - Neither side has any pack expansions, and they have different lengths.
     // - One side has a pack expansion but the other side is too short, eg

test/Constraints/pack-expansion-expressions.swift

@@ -528,3 +528,12 @@ func test_pack_expansion_to_void_conv_for_closure_result<each T>(x: repeat each
   let _: (Int) -> Void = { repeat ($0, print(each x)) } // expected-warning {{'repeat (Int, ())' is unused}}
   let _: (Int, String) -> Void = { ($0, repeat ($1, print(each x))) } // expected-warning {{'(Int, repeat (String, ()))' is unused}}
 }
+
+// rdar://109539394 - crash on passing multiple variadic lists to singly variadic callee
+do {
+  func test1<each T>(_: repeat each T) {}
+
+  func caller<each T1, each T2>(t1: repeat each T1, t2: repeat each T2) {
+    test1(repeat each t1, repeat each t2) // Ok
+  }
+}