[CSSimplify] Deplay member lookup until single-element tuple with pack expansion is sufficiently resolved

Such tuples should be treated specially because once pack expansion
is sufficiently resolved they'd get exploded and the resulting type
is what member lookup should use as a base.

Resolves: rdar://110721928

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -9378,6 +9378,17 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
     return result;
   }
 
+  // Delay member lookup until single-element tuple with pack expansion
+  // is sufficiently resolved.
+  if (isSingleUnlabeledPackExpansionTuple(instanceTy)) {
+    auto elementTy = instanceTy->castTo<TupleType>()->getElementType(0);
+    if (elementTy->is<TypeVariableType>()) {
+      MemberLookupResult result;
+      result.OverallResult = MemberLookupResult::Unsolved;
+      return result;
+    }
+  }
+
   // Okay, start building up the result list.
   MemberLookupResult result;
   result.OverallResult = MemberLookupResult::HasResults;

test/Constraints/pack-expansion-expressions.swift

@@ -707,3 +707,18 @@ do {
     test3(str: "", a, a)  // expected-error {{ambiguous use of 'test3'}}
   }
 }
+
+// rdar://112095973 - single-element tuples are not unwrapped in member references
+do {
+  struct V<Value> {
+    let key: Int
+  }
+
+  struct S<each T> {
+    let data: (repeat V<each T>)
+  }
+
+  func test<U>(_ value: S<U>) {
+    _ = value.data.key // Ok
+  }
+}