[ConstraintSystem] Detect and fix use of static members in a key path in the solver

Previously it was possible to create an invalid solution where
static members would be referenced in a key path, which is not
currently supported and would only be diagnosed while applying
such solution to AST e.g.

```swift
struct S {
  static var foo: Int = 42
}

_ = \S.Type.foo
```

Author: xedin

lib/Sema/CSApply.cpp

@@ -4572,10 +4572,8 @@ namespace {
           }
 
           // Key paths don't currently support static members.
-          if (varDecl->isStatic()) {
-            cs.TC.diagnose(componentLoc, diag::expr_keypath_static_member,
-                           property->getFullName());
-          }
+          // There is a fix which diagnoses such situation already.
+          assert(!varDecl->isStatic());
         }
 
         cs.TC.requestMemberLayout(property);

lib/Sema/CSFix.cpp

@@ -422,3 +422,14 @@ TreatKeyPathSubscriptIndexAsHashable::create(ConstraintSystem &cs, Type type,
   return new (cs.getAllocator())
       TreatKeyPathSubscriptIndexAsHashable(cs, type, locator);
 }
+
+bool AllowStaticMemberRefInKeyPath::diagnose(Expr *root, bool asNote) const {
+  return false;
+}
+
+AllowStaticMemberRefInKeyPath *
+AllowStaticMemberRefInKeyPath::create(ConstraintSystem &cs, ValueDecl *member,
+                                      ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      AllowStaticMemberRefInKeyPath(cs, member, locator);
+}

lib/Sema/CSFix.h

@@ -139,10 +139,13 @@ enum class FixKind : uint8_t {
 
   /// Allow KeyPaths to use AnyObject as root type
   AllowAnyObjectKeyPathRoot,
-  
+
   /// Using subscript references in the keypath requires that each
   /// of the index arguments to be Hashable.
   TreatKeyPathSubscriptIndexAsHashable,
+
+  /// Allow a reference to a static member as a key path component.
+  AllowStaticMemberRefInKeyPath,
 };
 
 class ConstraintFix {
@@ -780,6 +783,25 @@ class TreatKeyPathSubscriptIndexAsHashable final : public ConstraintFix {
   create(ConstraintSystem &cs, Type type, ConstraintLocator *locator);
 };
 
+class AllowStaticMemberRefInKeyPath final : public ConstraintFix {
+  ValueDecl *Member;
+
+  AllowStaticMemberRefInKeyPath(ConstraintSystem &cs, ValueDecl *member,
+                                ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::AllowStaticMemberRefInKeyPath, locator),
+        Member(member) {}
+
+public:
+  std::string getName() const override {
+    return "allow reference to a static member as a key path component";
+  }
+
+  bool diagnose(Expr *root, bool asNote = false) const override;
+
+  static AllowStaticMemberRefInKeyPath *
+  create(ConstraintSystem &cs, ValueDecl *member, ConstraintLocator *locator);
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSSimplify.cpp

@@ -5015,11 +5015,26 @@ ConstraintSystem::simplifyKeyPathConstraint(Type keyPathTy,
       if (!choices[i].isDecl()) {
         return SolutionKind::Error;
       }
+
       auto storage = dyn_cast<AbstractStorageDecl>(choices[i].getDecl());
       if (!storage) {
         return SolutionKind::Error;
       }
 
+      // Referencing static members in key path is not currently allowed.
+      if (storage->isStatic()) {
+        if (!shouldAttemptFixes())
+          return SolutionKind::Error;
+
+        auto componentLoc =
+            locator.withPathElement(LocatorPathElt::getKeyPathComponent(i));
+        auto *fix = AllowStaticMemberRefInKeyPath::create(
+            *this, choices[i].getDecl(), getConstraintLocator(componentLoc));
+
+        if (recordFix(fix))
+          return SolutionKind::Error;
+      }
+
       if (isReadOnlyKeyPathComponent(storage)) {
         capability = ReadOnly;
         continue;
@@ -6314,6 +6329,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowInaccessibleMember:
   case FixKind::AllowAnyObjectKeyPathRoot:
   case FixKind::TreatKeyPathSubscriptIndexAsHashable:
+  case FixKind::AllowStaticMemberRefInKeyPath:
     llvm_unreachable("handled elsewhere");
   }