[ConstraintSystem] Correct the first type of the ShapeOf constraint when

generating constraints for PackExpansionExpr.

The result pattern type of a pack expansion doesn't always contain pack
references. Instead, use the type of the first pack reference binding.

Author: hborla

lib/Sema/CSGen.cpp

@@ -2932,7 +2932,10 @@ namespace {
       auto *shapeTypeVar = CS.createTypeVariable(shapeLoc,
                                                  TVO_CanBindToPack |
                                                  TVO_CanBindToHole);
-      CS.addConstraint(ConstraintKind::ShapeOf, patternTy, shapeTypeVar,
+      auto packReference = expr->getBindings().front();
+      auto packType = CS.simplifyType(CS.getType(packReference))
+          ->castTo<PackExpansionType>()->getPatternType();
+      CS.addConstraint(ConstraintKind::ShapeOf, packType, shapeTypeVar,
                        CS.getConstraintLocator(expr));
 
       return PackExpansionType::get(patternTy, shapeTypeVar);

test/Constraints/pack-expansion-expressions.swift

@@ -58,3 +58,8 @@ func outerArchetype<T..., U>(t: T..., u: U) where T: P {
 func sameElement<T..., U>(t: T..., u: U) where T: P, T == U {
   let _: T... = t.f(u)...
 }
+
+func forEachEach<C..., U>(c: C..., function: (U) -> Void)
+    where C: Collection, C.Element == U {
+  _ = c.forEach(function)...
+}