[CSFix] Attempt to replace .subscript(...) with subscript operator

If there are no members named "subscript" available, let's try to
replace it with subscript operator with might be what was intended.

lib/Sema/CSFix.cpp

@@ -235,3 +235,12 @@ InsertExplicitCall *InsertExplicitCall::create(ConstraintSystem &cs,
                                                ConstraintLocator *locator) {
   return new (cs.getAllocator()) InsertExplicitCall(cs, locator);
 }
+
+bool UseSubscriptOperator::diagnose(Expr *root, bool asNote) const {
+  return false;
+}
+
+UseSubscriptOperator *UseSubscriptOperator::create(ConstraintSystem &cs,
+                                                   ConstraintLocator *locator) {
+  return new (cs.getAllocator()) UseSubscriptOperator(cs, locator);
+}

lib/Sema/CSFix.h

@@ -96,6 +96,9 @@ enum class FixKind : uint8_t {
 
   /// Add explicit `()` at the end of function or member to call it.
   InsertCall,
+
+  /// Instead of spelling out `subscript` directly, use subscript operator.
+  UseSubscriptOperator,
 };
 
 class ConstraintFix {
@@ -447,6 +450,21 @@ public:
                                     ConstraintLocator *locator);
 };
 
+class UseSubscriptOperator final : public ConstraintFix {
+public:
+  UseSubscriptOperator(ConstraintSystem &cs, ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::UseSubscriptOperator, locator) {}
+
+  std::string getName() const override {
+    return "replace '.subscript(...)' with subscript operator";
+  }
+
+  bool diagnose(Expr *root, bool asNote = false) const override;
+
+  static UseSubscriptOperator *create(ConstraintSystem &cs,
+                                      ConstraintLocator *locator);
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSSimplify.cpp

@@ -3765,10 +3765,6 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
       // function takes no arguments, otherwise it
       // would make sense to report this a missing member.
       if (funcType->getNumParams() == 0) {
-        auto *fix = InsertExplicitCall::create(*this, locator);
-        if (recordFix(fix))
-          return SolutionKind::Error;
-
         auto result = simplifyMemberConstraint(
             kind, funcType->getResult(), member, memberTy, useDC,
             functionRefKind, outerAlternatives, flags, locatorB);
@@ -3777,9 +3773,24 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
         // let's return, otherwise let's fall-through and report
         // this problem as a missing member.
         if (result == SolutionKind::Solved)
-          return result;
+          return recordFix(InsertExplicitCall::create(*this, locator))
+                     ? SolutionKind::Error
+                     : SolutionKind::Solved;
       }
     }
+
+    // Instead of using subscript operator spelled out `subscript` directly.
+    if (member.getBaseName() == getTokenText(tok::kw_subscript)) {
+      auto result = simplifyMemberConstraint(
+          kind, baseTy, DeclBaseName::createSubscript(), memberTy, useDC,
+          functionRefKind, {}, flags, locatorB);
+
+      // Looks like it was indeed meant to be a subscript operator.
+      if (result == SolutionKind::Solved)
+        return recordFix(UseSubscriptOperator::create(*this, locator))
+                   ? SolutionKind::Error
+                   : SolutionKind::Solved;
+    }
   }
   return SolutionKind::Error;
 }
@@ -5442,6 +5453,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
     return recordFix(fix) ? SolutionKind::Error : SolutionKind::Solved;
   }
 
+  case FixKind::UseSubscriptOperator:
   case FixKind::InsertCall:
   case FixKind::ExplicitlyEscaping:
   case FixKind::CoerceToCheckedCast: