[TypeChecker] Always emit a fallback error if type-check failed witho…

include/swift/AST/ASTContext.h

@@ -758,6 +758,10 @@ class ASTContext final {
     bool IsError;
   };
 
+  /// Check whether current context has any errors associated with
+  /// ill-formed protocol conformances which haven't been produced yet.
+  bool hasDelayedConformanceErrors() const;
+
   /// Add a delayed diagnostic produced while type-checking a
   /// particular protocol conformance.
   void addDelayedConformanceDiag(NormalProtocolConformance *conformance,

include/swift/AST/Builtins.def

@@ -572,6 +572,7 @@ BUILTIN_SANITIZER_OPERATION(TSanInoutAccess, "tsanInoutAccess", "")
 BUILTIN_TYPE_CHECKER_OPERATION(TypeJoin, type_join)
 BUILTIN_TYPE_CHECKER_OPERATION(TypeJoinInout, type_join_inout)
 BUILTIN_TYPE_CHECKER_OPERATION(TypeJoinMeta, type_join_meta)
+BUILTIN_TYPE_CHECKER_OPERATION(TriggerFallbackDiagnostic, trigger_fallback_diagnostic)
 
 #undef BUILTIN_TYPE_CHECKER_OPERATION
 

include/swift/AST/DiagnosticsSema.def

@@ -2767,6 +2767,10 @@ ERROR(type_of_expression_is_ambiguous,none,
 ERROR(specific_type_of_expression_is_ambiguous,none,
       "expression type %0 is ambiguous without more context", (Type))
 
+ERROR(failed_to_produce_diagnostic,Fatal,
+      "failed to produce diagnostic for expression; "
+      "please file a bug report with your project", ())
+
 
 ERROR(missing_protocol,none,
       "missing protocol %0", (Identifier))

lib/AST/ASTContext.cpp

@@ -1999,6 +1999,19 @@ LazyGenericContextData *ASTContext::getOrCreateLazyGenericContextData(
                                                               lazyLoader);
 }
 
+bool ASTContext::hasDelayedConformanceErrors() const {
+  for (const auto &entry : getImpl().DelayedConformanceDiags) {
+    auto &diagnostics = entry.getSecond();
+    if (std::any_of(diagnostics.begin(), diagnostics.end(),
+                    [](const ASTContext::DelayedConformanceDiag &diag) {
+                      return diag.IsError;
+                    }))
+      return true;
+  }
+
+  return false;
+}
+
 void ASTContext::addDelayedConformanceDiag(
        NormalProtocolConformance *conformance,
        DelayedConformanceDiag fn) {

lib/AST/Builtins.cpp

@@ -1012,6 +1012,12 @@ static ValueDecl *getTypeJoinMetaOperation(ASTContext &Context, Identifier Id) {
   return builder.build(Id);
 }
 
+static ValueDecl *getTriggerFallbackDiagnosticOperation(ASTContext &Context,
+                                                        Identifier Id) {
+  // () -> Void
+  return getBuiltinFunction(Id, {}, Context.TheEmptyTupleType);
+}
+
 static ValueDecl *getCanBeObjCClassOperation(ASTContext &Context,
                                              Identifier Id) {
   // <T> T.Type -> Builtin.Int8
@@ -1876,7 +1882,10 @@ ValueDecl *swift::getBuiltinValueDecl(ASTContext &Context, Identifier Id) {
     return getTypeJoinInoutOperation(Context, Id);
 
   case BuiltinValueKind::TypeJoinMeta:
-    return getTypeJoinMetaOperation(Context, Id);      
+    return getTypeJoinMetaOperation(Context, Id);
+
+  case BuiltinValueKind::TriggerFallbackDiagnostic:
+    return getTriggerFallbackDiagnosticOperation(Context, Id);
   }
 
   llvm_unreachable("bad builtin value!");

lib/Sema/CSDiag.cpp

@@ -1892,6 +1892,10 @@ Expr *FailureDiagnosis::typeCheckChildIndependently(
   // the context is missing).
   TypeCheckExprOptions TCEOptions = TypeCheckExprFlags::DisableStructuralChecks;
 
+  // Make sure that typechecker knows that this is an attempt
+  // to diagnose a problem.
+  TCEOptions |= TypeCheckExprFlags::SubExpressionDiagnostics;
+
   // Don't walk into non-single expression closure bodies, because
   // ExprTypeSaver and TypeNullifier skip them too.
   TCEOptions |= TypeCheckExprFlags::SkipMultiStmtClosures;

lib/Sema/CSGen.cpp

@@ -1579,6 +1579,11 @@ namespace {
       if (typeOperation != TypeOperation::None)
         return CS.getASTContext().TheAnyType;
 
+      // If this is `Builtin.trigger_fallback_diagnostic()`, fail
+      // without producing any diagnostics, in order to test fallback error.
+      if (isTriggerFallbackDiagnosticBuiltin(expr, CS.getASTContext()))
+        return Type();
+
       // Open a member constraint for constructor delegations on the
       // subexpr type.
       if (CS.TC.getSelfForInitDelegationInConstructor(CS.DC, expr)) {
@@ -3129,6 +3134,19 @@ namespace {
       return tv;
     }
 
+    static bool isTriggerFallbackDiagnosticBuiltin(UnresolvedDotExpr *UDE,
+                                                   ASTContext &Context) {
+      auto *DRE = dyn_cast<DeclRefExpr>(UDE->getBase());
+      if (!DRE)
+        return false;
+
+      if (DRE->getDecl() != Context.TheBuiltinModule)
+        return false;
+
+      auto member = UDE->getName().getBaseName().userFacingName();
+      return member.equals("trigger_fallback_diagnostic");
+    }
+
     enum class TypeOperation { None,
                                Join,
                                JoinInout,

lib/Sema/CSSolver.cpp

@@ -1089,6 +1089,8 @@ ConstraintSystem::solveImpl(Expr *&expr,
   if (auto generatedExpr = generateConstraints(expr))
     expr = generatedExpr;
   else {
+    if (listener)
+      listener->constraintGenerationFailed(expr);
     return SolutionKind::Error;
   }
 

lib/Sema/TypeCheckConstraints.cpp

@@ -29,6 +29,7 @@
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/ParameterList.h"
 #include "swift/AST/PrettyStackTrace.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/SubstitutionMap.h"
 #include "swift/AST/TypeCheckerDebugConsumer.h"
 #include "swift/Basic/Statistic.h"
@@ -1914,6 +1915,11 @@ Expr *ExprTypeCheckListener::appliedSolution(Solution &solution, Expr *expr) {
   return expr;
 }
 
+void ExprTypeCheckListener::preCheckFailed(Expr *expr) {}
+void ExprTypeCheckListener::constraintGenerationFailed(Expr *expr) {}
+void ExprTypeCheckListener::applySolutionFailed(Solution &solution,
+                                                Expr *expr) {}
+
 void ParentConditionalConformance::diagnoseConformanceStack(
     DiagnosticEngine &diags, SourceLoc loc,
     ArrayRef<ParentConditionalConformance> conformances) {
@@ -1941,20 +1947,115 @@ bool GenericRequirementsCheckListener::diagnoseUnsatisfiedRequirement(
   return false;
 }
 
+/// Sometimes constraint solver fails without producing any diagnostics,
+/// that leads to crashes down the line in AST Verifier or SILGen
+/// which, as a result, are much harder to figure out.
+///
+/// This class is intended to guard against situations like that by
+/// keeping track of failures of different type-check phases, and
+/// emitting fallback fatal error if any of them fail without producing
+/// error diagnostic, and there were no errors emitted or scheduled to be
+/// emitted previously.
+class FallbackDiagnosticListener : public ExprTypeCheckListener {
+  TypeChecker &TC;
+  TypeCheckExprOptions Options;
+  ExprTypeCheckListener *BaseListener;
+
+public:
+  FallbackDiagnosticListener(TypeChecker &TC, TypeCheckExprOptions options,
+                             ExprTypeCheckListener *base)
+      : TC(TC), Options(options), BaseListener(base) {}
+
+  bool builtConstraints(ConstraintSystem &cs, Expr *expr) override {
+    return BaseListener ? BaseListener->builtConstraints(cs, expr) : false;
+  }
+
+  Expr *foundSolution(Solution &solution, Expr *expr) override {
+    return BaseListener ? BaseListener->foundSolution(solution, expr) : expr;
+  }
+
+  Expr *appliedSolution(Solution &solution, Expr *expr) override {
+    return BaseListener ? BaseListener->appliedSolution(solution, expr) : expr;
+  }
+
+  void preCheckFailed(Expr *expr) override {
+    if (BaseListener)
+      BaseListener->preCheckFailed(expr);
+    maybeProduceFallbackDiagnostic(expr);
+  }
+
+  void constraintGenerationFailed(Expr *expr) override {
+    if (BaseListener)
+      BaseListener->constraintGenerationFailed(expr);
+    maybeProduceFallbackDiagnostic(expr);
+  }
+
+  void applySolutionFailed(Solution &solution, Expr *expr) override {
+    if (BaseListener)
+      BaseListener->applySolutionFailed(solution, expr);
+
+    if (hadAnyErrors())
+      return;
+
+    // If solution involves invalid or incomplete conformances that's
+    // a probable cause of failure to apply it without producing an error,
+    // which is going to be diagnosed later, so let's not produce
+    // fallback diagnostic in this case.
+    if (llvm::any_of(
+            solution.Conformances,
+            [](const std::pair<ConstraintLocator *, ProtocolConformanceRef>
+                   &conformance) -> bool {
+              auto &ref = conformance.second;
+              return ref.isConcrete() && ref.getConcrete()->isInvalid();
+            }))
+      return;
+
+    maybeProduceFallbackDiagnostic(expr);
+  }
+
+private:
+  bool hadAnyErrors() const { return TC.Context.Diags.hadAnyError(); }
+
+  void maybeProduceFallbackDiagnostic(Expr *expr) const {
+    if (Options.contains(TypeCheckExprFlags::SubExpressionDiagnostics) ||
+        Options.contains(TypeCheckExprFlags::SuppressDiagnostics))
+      return;
+
+    // Before producing fatal error here, let's check if there are any "error"
+    // diagnostics already emitted or waiting to be emitted. Because they are
+    // a better indication of the problem.
+    if (!(hadAnyErrors() || TC.Context.hasDelayedConformanceErrors()))
+      TC.diagnose(expr->getLoc(), diag::failed_to_produce_diagnostic);
+  }
+};
+
 #pragma mark High-level entry points
 Type TypeChecker::typeCheckExpression(Expr *&expr, DeclContext *dc,
                                       TypeLoc convertType,
                                       ContextualTypePurpose convertTypePurpose,
                                       TypeCheckExprOptions options,
                                       ExprTypeCheckListener *listener,
                                       ConstraintSystem *baseCS) {
+  FallbackDiagnosticListener diagListener(*this, options, listener);
+  return typeCheckExpressionImpl(expr, dc, convertType, convertTypePurpose,
+                                 options, diagListener, baseCS);
+}
+
+Type TypeChecker::typeCheckExpressionImpl(Expr *&expr, DeclContext *dc,
+                                          TypeLoc convertType,
+                                          ContextualTypePurpose convertTypePurpose,
+                                          TypeCheckExprOptions options,
+                                          ExprTypeCheckListener &listener,
+                                          ConstraintSystem *baseCS) {
   FrontendStatsTracer StatsTracer(Context.Stats, "typecheck-expr", expr);
   PrettyStackTraceExpr stackTrace(Context, "type-checking", expr);
 
   // First, pre-check the expression, validating any types that occur in the
   // expression and folding sequence expressions.
-  if (preCheckExpression(expr, dc))
+  if (preCheckExpression(expr, dc)) {
+    listener.preCheckFailed(expr);
     return Type();
+  }
 
   // Construct a constraint system from this expression.
   ConstraintSystemOptions csOptions = ConstraintSystemFlags::AllowFixes;
@@ -2009,10 +2110,9 @@ Type TypeChecker::typeCheckExpression(Expr *&expr, DeclContext *dc,
     convertTo = getOptionalType(expr->getLoc(), var);
   }
 
-  // Attempt to solve the constraint system.
   SmallVector<Solution, 4> viable;
-  if (cs.solve(expr, convertTo, listener, viable,
-               allowFreeTypeVariables))
+  // Attempt to solve the constraint system.
+  if (cs.solve(expr, convertTo, &listener, viable, allowFreeTypeVariables))
     return Type();
 
   // If the client allows the solution to have unresolved type expressions,
@@ -2026,29 +2126,26 @@ Type TypeChecker::typeCheckExpression(Expr *&expr, DeclContext *dc,
 
   auto result = expr;
   auto &solution = viable[0];
-  if (listener) {
-    result = listener->foundSolution(solution, result);
-    if (!result)
-      return Type();
-  }
+  result = listener.foundSolution(solution, result);
+  if (!result)
+    return Type();
 
   // Apply the solution to the expression.
   result = cs.applySolution(
       solution, result, convertType.getType(),
       options.contains(TypeCheckExprFlags::IsDiscarded),
       options.contains(TypeCheckExprFlags::SkipMultiStmtClosures));
+
   if (!result) {
+    listener.applySolutionFailed(solution, expr);
     // Failure already diagnosed, above, as part of applying the solution.
     return Type();
   }
 
-  // If there's a listener, notify it that we've applied the solution.
-  if (listener) {
-    result = listener->appliedSolution(solution, result);
-    if (!result) {
-      return Type();
-    }
-  }
+  // Notify listener that we've applied the solution.
+  result = listener.appliedSolution(solution, result);
+  if (!result)
+    return Type();
 
   if (getLangOpts().DebugConstraintSolver) {
     auto &log = Context.TypeCheckerDebug->getStream();

lib/Sema/TypeChecker.h

@@ -263,6 +263,14 @@ enum class TypeCheckExprFlags {
   /// If set, a conversion constraint should be specified so that the result of
   /// the expression is an optional type.
   ExpressionTypeMustBeOptional = 0x200,
+
+  /// FIXME(diagnostics): Once diagnostics are completely switched to new
+  /// framework, this flag could be removed as obsolete.
+  ///
+  /// If set, this is a sub-expression, and it is being re-typechecked
+  /// as part of the expression diagnostics, which is attempting to narrow
+  /// down failure location.
+  SubExpressionDiagnostics = 0x400,
 };
 
 using TypeCheckExprOptions = OptionSet<TypeCheckExprFlags>;
@@ -381,6 +389,18 @@ class ExprTypeCheckListener {
   /// failure.
   virtual Expr *appliedSolution(constraints::Solution &solution,
                                 Expr *expr);
+
+  /// Callback invoked if expression is structurally unsound and can't
+  /// be correctly processed by the constraint solver.
+  virtual void preCheckFailed(Expr *expr);
+
+  /// Callback invoked if constraint system failed to generate
+  /// constraints for a given expression.
+  virtual void constraintGenerationFailed(Expr *expr);
+
+  /// Callback invoked if application of chosen solution to
+  /// expression has failed.
+  virtual void applySolutionFailed(constraints::Solution &solution, Expr *expr);
 };
 
 /// A conditional conformance that implied some other requirements. That is, \c
@@ -1330,7 +1350,15 @@ class TypeChecker final : public LazyResolver {
                                TypeCheckExprOptions(), listener);
   }
 
+private:
+  Type typeCheckExpressionImpl(Expr *&expr, DeclContext *dc,
+                               TypeLoc convertType,
+                               ContextualTypePurpose convertTypePurpose,
+                               TypeCheckExprOptions options,
+                               ExprTypeCheckListener &listener,
+                               constraints::ConstraintSystem *baseCS);
 
+public:
   /// Type check the given expression and return its type without
   /// applying the solution.
   ///

test/Sema/rdar38885760.swift

@@ -0,0 +1,10 @@
+// RUN: %empty-directory(%t)
+//
+// RUN: not %target-swift-frontend -typecheck %s -parse-stdlib 2>%t/fallback_diagnostic.txt
+// RUN: %FileCheck %s --check-prefix FALLBACK-DIAGNOSTIC <%t/fallback_diagnostic.txt
+//
+// FALLBACK-DIAGNOSTIC: error: failed to produce diagnostic for expression; please file a bug report with your project
+
+import Swift
+
+Builtin.trigger_fallback_diagnostic()