Make the error for missing existential erasure coercions configurable.

Author: DougGregor

include/swift/Sema/CSFix.h

@@ -3025,8 +3025,10 @@ class AddExplicitExistentialCoercion final : public ConstraintFix {
   Type ErasedResultType;
 
   AddExplicitExistentialCoercion(ConstraintSystem &cs, Type erasedResultTy,
-                                 ConstraintLocator *locator)
-      : ConstraintFix(cs, FixKind::AddExplicitExistentialCoercion, locator),
+                                 ConstraintLocator *locator,
+                                 FixBehavior fixBehavior)
+      : ConstraintFix(cs, FixKind::AddExplicitExistentialCoercion, locator,
+                      fixBehavior),
         ErasedResultType(erasedResultTy) {}
 
 public:

lib/Sema/CSDiagnostics.h

@@ -2749,8 +2749,9 @@ class MissingExplicitExistentialCoercion final : public FailureDiagnostic {
 public:
   MissingExplicitExistentialCoercion(const Solution &solution,
                                      Type erasedResultTy,
-                                     ConstraintLocator *locator)
-      : FailureDiagnostic(solution, locator),
+                                     ConstraintLocator *locator,
+                                     FixBehavior fixBehavior)
+      : FailureDiagnostic(solution, locator, fixBehavior),
         ErasedResultType(resolveType(erasedResultTy)) {}
 
   SourceRange getSourceRange() const override {

lib/Sema/CSFix.cpp

@@ -2233,7 +2233,7 @@ IgnoreDefaultExprTypeMismatch::create(ConstraintSystem &cs, Type argType,
 bool AddExplicitExistentialCoercion::diagnose(const Solution &solution,
                                               bool asNote) const {
   MissingExplicitExistentialCoercion failure(solution, ErasedResultType,
-                                             getLocator());
+                                             getLocator(), diagfixBehavior());
   return failure.diagnose(asNote);
 }
 
@@ -2465,8 +2465,9 @@ bool AddExplicitExistentialCoercion::isRequired(
 AddExplicitExistentialCoercion *
 AddExplicitExistentialCoercion::create(ConstraintSystem &cs, Type resultTy,
                                        ConstraintLocator *locator) {
+  FixBehavior fixBehavior = FixBehavior::Error;
   return new (cs.getAllocator())
-      AddExplicitExistentialCoercion(cs, resultTy, locator);
+      AddExplicitExistentialCoercion(cs, resultTy, locator, fixBehavior);
 }
 
 bool RenameConflictingPatternVariables::diagnose(const Solution &solution,