[CS][Experiment] Re-activate all inactive constraints

include/swift/AST/DiagnosticsSema.def

@@ -1057,6 +1057,10 @@ ERROR(missing_unwrap_optional_try,none,
 ERROR(missing_forced_downcast,none,
       "%0 is not convertible to %1; "
       "did you mean to use 'as!' to force downcast?", (Type, Type))
+WARNING(coercion_may_fail_warning,none,
+        "coercion from %0 to %1 may fail; use 'as?' or 'as!' instead",
+        (Type, Type))
+
 ERROR(missing_explicit_conversion,none,
       "%0 is not implicitly convertible to %1; "
       "did you mean to use 'as' to explicitly convert?", (Type, Type))

lib/Sema/CSDiagnostics.cpp

@@ -6184,3 +6184,11 @@ bool MissingQuialifierInMemberRefFailure::diagnoseAsError() {
   emitDiagnostic(choice, diag::decl_declared_here, choice->getFullName());
   return true;
 }
+
+bool CoercionAsForceCastFailure::diagnoseAsError() {
+  auto *coercion = cast<CoerceExpr>(getRawAnchor());
+  emitDiagnostic(coercion->getLoc(), diag::coercion_may_fail_warning,
+                 getFromType(), getToType())
+      .highlight(coercion->getSourceRange());
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -1977,6 +1977,16 @@ class MissingQuialifierInMemberRefFailure final : public FailureDiagnostic {
   bool diagnoseAsError();
 };
 
+/// Emits a warning about an attempt to use the 'as' operator as the 'as!'
+/// operator.
+class CoercionAsForceCastFailure final : public ContextualFailure {
+public:
+  CoercionAsForceCastFailure(const Solution &solution, Type fromType,
+                             Type toType, ConstraintLocator *locator)
+      : ContextualFailure(solution, fromType, toType, locator) {}
+
+  bool diagnoseAsError() override;
+};
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -1276,3 +1276,17 @@ AddQualifierToAccessTopLevelName::create(ConstraintSystem &cs,
                                          ConstraintLocator *locator) {
   return new (cs.getAllocator()) AddQualifierToAccessTopLevelName(cs, locator);
 }
+
+bool AllowCoercionToForceCast::diagnose(const Solution &solution,
+                                        bool asNote) const {
+  CoercionAsForceCastFailure failure(solution, getFromType(), getToType(),
+                                     getLocator());
+  return failure.diagnose(asNote);
+}
+
+AllowCoercionToForceCast *
+AllowCoercionToForceCast::create(ConstraintSystem &cs, Type fromType,
+                                 Type toType, ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      AllowCoercionToForceCast(cs, fromType, toType, locator);
+}

lib/Sema/CSFix.h

@@ -248,6 +248,9 @@ enum class FixKind : uint8_t {
   /// Member shadows a top-level name, such a name could only be accessed by
   /// prefixing it with a module name.
   AddQualifierToAccessTopLevelName,
+
+  /// A warning fix that allows a coercion to perform a force-cast.
+  AllowCoercionToForceCast,
 };
 
 class ConstraintFix {
@@ -1728,6 +1731,34 @@ class AllowNonClassTypeToConvertToAnyObject final : public ContextualMismatch {
   create(ConstraintSystem &cs, Type type, ConstraintLocator *locator);
 };
 
+/// A warning fix to maintain compatibility with the following:
+///
+/// \code
+/// func foo(_ arr: [Any]?) {
+///  _ = (arr ?? []) as [NSObject]
+/// }
+/// \endcode
+///
+/// which performs a force-cast of the array's elements, despite being spelled
+/// as a coercion.
+class AllowCoercionToForceCast final : public ContextualMismatch {
+  AllowCoercionToForceCast(ConstraintSystem &cs, Type fromType, Type toType,
+                           ConstraintLocator *locator)
+      : ContextualMismatch(cs, FixKind::AllowCoercionToForceCast, fromType,
+                           toType, locator, /*warning*/ true) {}
+
+public:
+  std::string getName() const {
+    return "allow coercion to be treated as a force-cast";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const;
+
+  static AllowCoercionToForceCast *create(ConstraintSystem &cs, Type fromType,
+                                          Type toType,
+                                          ConstraintLocator *locator);
+};
+
 } // end namespace constraints
 } // end namespace swift