[CSFix] Add a skeleton of a fix for argument destructuring to match pack expansion

Author: xedin

include/swift/Sema/CSFix.h

@@ -434,6 +434,10 @@ enum class FixKind : uint8_t {
 
   /// Allow pack expansion expressions in a context that does not support them.
   AllowInvalidPackExpansion,
+
+  /// Allow a pack expansion parameter of N elements to be matched
+  /// with a single tuple literal argument of the same arity.
+  DestructureTupleToMatchPackExpansionParameter,
 };
 
 class ConstraintFix {
@@ -3416,6 +3420,39 @@ class AllowGlobalActorMismatch final : public ContextualMismatch {
   }
 };
 
+/// Passing an argument of tuple type to a value pack expansion parameter
+/// that expected N distinct elements.
+class DestructureTupleToMatchPackExpansionParameter final
+    : public ConstraintFix {
+  PackType *ParamShape;
+
+  DestructureTupleToMatchPackExpansionParameter(ConstraintSystem &cs,
+                                                PackType *paramShapeTy,
+                                                ConstraintLocator *locator)
+      : ConstraintFix(cs,
+                      FixKind::DestructureTupleToMatchPackExpansionParameter,
+                      locator),
+        ParamShape(paramShapeTy) {
+    assert(locator->isLastElement<LocatorPathElt::ApplyArgToParam>());
+  }
+
+public:
+  std::string getName() const override {
+    return "allow pack expansion to match tuple argument";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  static DestructureTupleToMatchPackExpansionParameter *
+  create(ConstraintSystem &cs, PackType *paramShapeTy,
+         ConstraintLocator *locator);
+
+  static bool classof(const ConstraintFix *fix) {
+    return fix->getKind() ==
+           FixKind::DestructureTupleToMatchPackExpansionParameter;
+  }
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -2742,3 +2742,15 @@ AllowGlobalActorMismatch::create(ConstraintSystem &cs, Type fromType,
   return new (cs.getAllocator())
       AllowGlobalActorMismatch(cs, fromType, toType, locator);
 }
+
+bool DestructureTupleToMatchPackExpansionParameter::diagnose(
+    const Solution &solution, bool asNote) const {
+  return false;
+}
+
+DestructureTupleToMatchPackExpansionParameter *
+DestructureTupleToMatchPackExpansionParameter::create(
+    ConstraintSystem &cs, PackType *paramShapeTy, ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      DestructureTupleToMatchPackExpansionParameter(cs, paramShapeTy, locator);
+}

lib/Sema/CSSimplify.cpp

@@ -14773,6 +14773,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowSwiftToCPointerConversion:
   case FixKind::AllowTupleLabelMismatch:
   case FixKind::AddExplicitExistentialCoercion:
+  case FixKind::DestructureTupleToMatchPackExpansionParameter:
     llvm_unreachable("handled elsewhere");
   }