[Type checker] Improve diagnostics when an optional value is not unwrapped

include/swift/AST/DiagnosticsSema.def

@@ -902,10 +902,21 @@ NOTE(note_init_parameter,none,
 ERROR(missing_nullary_call,none,
      "function produces expected type %0; did you mean to call it with '()'?",
      (Type))
-ERROR(missing_unwrap_optional,none,
-      "value of optional type %0 not unwrapped; did you mean to use '!' "
-      "or '?'?",
-     (Type))
+ERROR(optional_not_unwrapped,none,
+      "value of optional type %0 must be unwrapped to a value of type %1",
+     (Type, Type))
+NOTE(unwrap_with_default_value,none,
+     "coalesce using '?" "?' to provide a default when the optional value "
+     "contains 'nil'", ())
+NOTE(unwrap_with_force_value,none,
+     "force-unwrap using '!' to abort execution if the optional value contains "
+     "'nil'", ())
+ERROR(optional_base_not_unwrapped,none,
+      "value of optional type %0 must be unwrapped to refer to member %1 of "
+      "wrapped base type %2", (Type, DeclName, Type))
+NOTE(optional_base_chain,none,
+     "chain the optional using '?' to access member %0 only for non-'nil' "
+     "base values", (DeclName))
 ERROR(missing_unwrap_optional_try,none,
       "value of optional type %0 not unwrapped; did you mean to use 'try!' "
       "or chain with '?'?",

include/swift/AST/KnownIdentifiers.def

@@ -135,6 +135,7 @@ IDENTIFIER(AssignmentPrecedence)
 IDENTIFIER(CastingPrecedence)
 IDENTIFIER(DefaultPrecedence)
 IDENTIFIER(FunctionArrowPrecedence)
+IDENTIFIER(NilCoalescingPrecedence)
 IDENTIFIER(TernaryPrecedence)
 
 // Builtins and literals

lib/Sema/CSApply.cpp

@@ -7794,6 +7794,27 @@ static bool exprNeedsParensAfterAddingAs(TypeChecker &TC, DeclContext *DC,
   return exprNeedsParensOutsideFollowingOperator(TC, DC, expr, rootExpr, asPG);
 }
 
+bool swift::exprNeedsParensBeforeAddingNilCoalescing(TypeChecker &TC,
+                                                     DeclContext *DC,
+                                                     Expr *expr) {
+  auto asPG =
+    TC.lookupPrecedenceGroup(DC, DC->getASTContext().Id_NilCoalescingPrecedence,
+                             SourceLoc());
+  if (!asPG) return true;
+  return exprNeedsParensInsideFollowingOperator(TC, DC, expr, asPG);
+}
+
+bool swift::exprNeedsParensAfterAddingNilCoalescing(TypeChecker &TC,
+                                                    DeclContext *DC,
+                                                    Expr *expr,
+                                                    Expr *rootExpr) {
+  auto asPG =
+    TC.lookupPrecedenceGroup(DC, DC->getASTContext().Id_NilCoalescingPrecedence,
+                             SourceLoc());
+  if (!asPG) return true;
+  return exprNeedsParensOutsideFollowingOperator(TC, DC, expr, rootExpr, asPG);
+}
+
 namespace {
   class ExprWalker : public ASTWalker {
     ExprRewriter &Rewriter;
@@ -7955,7 +7976,7 @@ bool ConstraintSystem::applySolutionFix(Expr *expr,
 
   switch (fix.first.getKind()) {
   case FixKind::ForceOptional: {
-    const Expr *unwrapped = affected->getValueProvidingExpr();
+    Expr *unwrapped = affected->getValueProvidingExpr();
     auto type = solution.simplifyType(getType(affected))
                   ->getRValueObjectType();
 
@@ -7966,25 +7987,17 @@ bool ConstraintSystem::applySolutionFix(Expr *expr,
                       "try!");
 
     } else {
-      auto diag = TC.diagnose(affected->getLoc(),
-                              diag::missing_unwrap_optional, type);
-      if (affected->canAppendPostfixExpression(true)) {
-        diag.fixItInsertAfter(affected->getEndLoc(), "!");
-      } else {
-        diag.fixItInsert(affected->getStartLoc(), "(")
-            .fixItInsertAfter(affected->getEndLoc(), ")!");
-      }
+      return diagnoseUnwrap(TC, DC, unwrapped, type);
     }
     return true;
   }
 
-  case FixKind::OptionalChaining: {
+  case FixKind::UnwrapOptionalBase: {
     auto type = solution.simplifyType(getType(affected))
                 ->getRValueObjectType();
-    auto diag = TC.diagnose(affected->getLoc(),
-                            diag::missing_unwrap_optional, type);
-    diag.fixItInsertAfter(affected->getEndLoc(), "?");
-    return true;
+    DeclName memberName = fix.first.getDeclNameArgument(*this);
+    return diagnoseBaseUnwrapForMemberAccess(affected, type, memberName,
+                                             SourceRange());
   }
 
   case FixKind::ForceDowncast: {

lib/Sema/CSDiag.cpp

@@ -2419,15 +2419,6 @@ Expr *FailureDiagnosis::typeCheckChildIndependently(
       allowFreeTypeVariables)
     TCEOptions |= TypeCheckExprFlags::AllowUnresolvedTypeVariables;
 
-  // If we're not passing down contextual type information this time, but the
-  // original failure had type info that wasn't an optional type,
-  // then set the flag to prefer fixits with force unwrapping.
-  if (!convertType) {
-    auto previousType = CS.getContextualType();
-    if (previousType && previousType->getOptionalObjectType().isNull())
-      TCEOptions |= TypeCheckExprFlags::PreferForceUnwrapToOptional;
-  }
-
   auto resultTy = CS.TC.typeCheckExpression(
       subExpr, CS.DC, TypeLoc::withoutLoc(convertType), convertTypePurpose,
       TCEOptions, listener, &CS);
@@ -8067,15 +8058,9 @@ bool FailureDiagnosis::diagnoseMemberFailures(
       }
 
       if (!optionalResult.ViableCandidates.empty()) {
-        // By default we assume that the LHS type is not optional.
-        StringRef fixIt = "!";
-        auto contextualType = CS.getContextualType();
-        if (contextualType && isa<OptionalType>(contextualType.getPointer()))
-          fixIt = "?";
-
-        diagnose(BaseLoc, diag::missing_unwrap_optional, baseObjTy)
-            .fixItInsertAfter(baseExpr->getEndLoc(), fixIt);
-        return true;
+        if (diagnoseBaseUnwrapForMemberAccess(baseExpr, baseObjTy, memberName,
+                                              memberRange))
+          return true;
       }
     }
 
@@ -8982,3 +8967,76 @@ bool ConstraintSystem::salvage(SmallVectorImpl<Solution> &viable, Expr *expr) {
   diagnoseFailureForExpr(expr);
   return true;
 }
+
+bool swift::diagnoseUnwrap(TypeChecker &TC, DeclContext *DC,
+                           Expr *expr, Type type) {
+  Type unwrappedType = type->getOptionalObjectType();
+  if (!unwrappedType)
+    return false;
+
+  TC.diagnose(expr->getLoc(), diag::optional_not_unwrapped, type,
+              unwrappedType);
+
+  // Suggest a default value via ?? <default value>
+  {
+    auto diag =
+      TC.diagnose(expr->getLoc(), diag::unwrap_with_default_value);
+
+    // Figure out what we need to parenthesize.
+    bool needsParensInside =
+      exprNeedsParensBeforeAddingNilCoalescing(TC, DC, expr);
+    bool needsParensOutside =
+      exprNeedsParensAfterAddingNilCoalescing(TC, DC, expr, expr);
+
+    llvm::SmallString<2> insertBefore;
+    llvm::SmallString<32> insertAfter;
+    if (needsParensOutside) {
+      insertBefore += "(";
+    }
+    if (needsParensInside) {
+      insertBefore += "(";
+      insertAfter += ")";
+    }
+    insertAfter += " ?? <" "#default value#" ">";
+    if (needsParensOutside)
+      insertAfter += ")";
+
+    if (!insertBefore.empty()) {
+      diag.fixItInsert(expr->getStartLoc(), insertBefore);
+    }
+    diag.fixItInsertAfter(expr->getEndLoc(), insertAfter);
+  }
+
+  // Suggest a force-unwrap.
+  {
+    auto diag =
+      TC.diagnose(expr->getLoc(), diag::unwrap_with_force_value);
+    if (expr->canAppendPostfixExpression(true)) {
+      diag.fixItInsertAfter(expr->getEndLoc(), "!");
+    } else {
+      diag.fixItInsert(expr->getStartLoc(), "(")
+          .fixItInsertAfter(expr->getEndLoc(), ")!");
+    }
+  }
+
+  return true;
+}
+
+bool swift::diagnoseBaseUnwrapForMemberAccess(Expr *baseExpr, Type baseType,
+                                              DeclName memberName,
+                                              SourceRange memberRange) {
+  auto unwrappedBaseType = baseType->getOptionalObjectType();
+  if (!unwrappedBaseType)
+    return false;
+
+  ASTContext &ctx = baseType->getASTContext();
+  DiagnosticEngine &diags = ctx.Diags;
+  diags.diagnose(baseExpr->getLoc(), diag::optional_base_not_unwrapped,
+                 baseType, memberName, unwrappedBaseType);
+
+  diags.diagnose(baseExpr->getLoc(), diag::optional_base_chain, memberName)
+    .fixItInsertAfter(baseExpr->getEndLoc(), "?");
+  diags.diagnose(baseExpr->getLoc(), diag::unwrap_with_force_value)
+    .fixItInsertAfter(baseExpr->getEndLoc(), "!");
+  return true;
+}

lib/Sema/CSSimplify.cpp

@@ -3554,18 +3554,9 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
   // Value member lookup has some hacks too.
   if (shouldAttemptFixes() && baseObjTy->getOptionalObjectType()) {
     // If the base type was an optional, look through it.
-
-    // Determine whether or not we want to provide an optional chaining fixit or
-    // a force unwrap fixit.
-    bool optionalChain;
-    if (!getContextualType())
-      optionalChain = !(Options & ConstraintSystemFlags::PreferForceUnwrapToOptional);
-    else
-      optionalChain = !getContextualType()->getOptionalObjectType().isNull();
-    auto fixKind = optionalChain ? FixKind::OptionalChaining : FixKind::ForceOptional;
-
-    // Note the fix.
-    if (recordFix(fixKind, locator))
+
+    // We're unwrapping the base to perform a member access.
+    if (recordFix(Fix::getUnwrapOptionalBase(*this, member), locator))
       return SolutionKind::Error;
 
     // Look through one level of optional.
@@ -4877,8 +4868,8 @@ ConstraintSystem::simplifyFixConstraint(Fix fix, Type type1, Type type2,
     getDefaultDecompositionOptions(flags) | TMF_ApplyingFix;
   switch (fix.getKind()) {
   case FixKind::ForceOptional:
-  case FixKind::OptionalChaining: {
-    // Assume that '!' was applied to the first type.
+  case FixKind::UnwrapOptionalBase: {
+    // Assume that we've unwrapped the first type.
     auto result =
         matchTypes(type1->getRValueObjectType()->getOptionalObjectType(), type2,
                    matchKind, subflags, locator);

lib/Sema/CalleeCandidateInfo.cpp

@@ -1012,10 +1012,10 @@ bool CalleeCandidateInfo::diagnoseGenericParameterErrors(Expr *badArgExpr) {
     // Check for optional near miss.
     if (auto argOptType = substitution->getOptionalObjectType()) {
       if (isSubstitutableFor(argOptType, paramArchetype, CS.DC)) {
-        CS.TC.diagnose(badArgExpr->getLoc(), diag::missing_unwrap_optional,
-                       argType);
-        foundFailure = true;
-        break;
+        if (diagnoseUnwrap(CS.TC, CS.DC, badArgExpr, substitution)) {
+          foundFailure = true;
+          break;
+        }
       }
     }
 

lib/Sema/Constraint.cpp

@@ -487,17 +487,29 @@ Fix Fix::getForcedDowncast(ConstraintSystem &cs, Type toType) {
   return Fix(FixKind::ForceDowncast, index);
 }
 
+Fix Fix::getUnwrapOptionalBase(ConstraintSystem &cs, DeclName memberName) {
+  unsigned index = cs.FixedDeclNames.size();
+  cs.FixedDeclNames.push_back(memberName);
+  return Fix(FixKind::UnwrapOptionalBase, index);
+}
+
 Type Fix::getTypeArgument(ConstraintSystem &cs) const {
   assert(getKind() == FixKind::ForceDowncast);
   return cs.FixedTypes[Data];
 }
 
+/// If this fix has a name argument, retrieve it.
+DeclName Fix::getDeclNameArgument(ConstraintSystem &cs) const {
+  assert(getKind() == FixKind::UnwrapOptionalBase);
+  return cs.FixedDeclNames[Data];
+}
+
 StringRef Fix::getName(FixKind kind) {
   switch (kind) {
   case FixKind::ForceOptional:
     return "fix: force optional";
-  case FixKind::OptionalChaining:
-    return "fix: optional chaining";
+  case FixKind::UnwrapOptionalBase:
+    return "fix: unwrap optional base of member lookup";
   case FixKind::ForceDowncast:
     return "fix: force downcast";
   case FixKind::AddressOf:

lib/Sema/Constraint.h

@@ -233,8 +233,8 @@ enum class FixKind : uint8_t {
   /// Introduce a '!' to force an optional unwrap.
   ForceOptional,
 
-  /// Introduce a '?.' to begin optional chaining.
-  OptionalChaining,
+  /// Unwrap an optional base when we have a member access.
+  UnwrapOptionalBase,
 
   /// Append 'as! T' to force a downcast to the specified type.
   ForceDowncast,
@@ -265,17 +265,26 @@ class Fix {
 public:
   Fix(FixKind kind) : Kind(kind), Data(0) {
     assert(kind != FixKind::ForceDowncast && "Use getForceDowncast()");
+    assert(kind != FixKind::UnwrapOptionalBase &&
+           "Use getUnwrapOptionalBase()");
   }
 
   /// Produce a new fix that performs a forced downcast to the given type.
   static Fix getForcedDowncast(ConstraintSystem &cs, Type toType);
 
+  /// Produce a new fix that unwraps an optional base for an access to a member
+  /// with the given name.
+  static Fix getUnwrapOptionalBase(ConstraintSystem &cs, DeclName memberName);
+
   /// Retrieve the kind of fix.
   FixKind getKind() const { return Kind; }
 
   /// If this fix has a type argument, retrieve it.
   Type getTypeArgument(ConstraintSystem &cs) const;
 
+  /// If this fix has a name argument, retrieve it.
+  DeclName getDeclNameArgument(ConstraintSystem &cs) const;
+
   /// Return a string representation of a fix.
   static llvm::StringRef getName(FixKind kind);
 

lib/Sema/ConstraintSystem.h

@@ -787,10 +787,6 @@ enum class ConstraintSystemFlags {
   /// Whether we allow the solver to attempt fixes to the system.
   AllowFixes = 0x01,
 
-  /// Set if the client prefers fixits to be in the form of force unwrapping
-  /// or optional chaining to return an optional.
-  PreferForceUnwrapToOptional = 0x02,
-
   /// If set, this is going to prevent constraint system from erasing all
   /// discovered solutions except the best one.
   ReturnAllDiscoveredSolutions = 0x04,
@@ -992,6 +988,9 @@ class ConstraintSystem {
   /// Types used in fixes.
   std::vector<Type> FixedTypes;
 
+  /// Declaration names used in fixes.
+  std::vector<DeclName> FixedDeclNames;
+
   /// \brief The set of remembered disjunction choices used to reach
   /// the current constraint system.
   SmallVector<std::pair<ConstraintLocator*, unsigned>, 32>
@@ -3528,6 +3527,35 @@ class InputMatcher {
 
   size_t getNumSkippedParameters() const { return NumSkippedParameters; }
 };
+
+/// Diagnose an attempt to recover when we have a value of optional type
+/// that needs to be unwrapped.
+///
+/// \returns true if a diagnostic was produced.
+bool diagnoseUnwrap(TypeChecker &TC, DeclContext *DC, Expr *expr, Type type);
+
+/// Diagnose an attempt to recover from a member access into a value of
+/// optional type which needed to be unwrapped for the member to be found.
+///
+/// \returns true if a diagnostic was produced.
+bool diagnoseBaseUnwrapForMemberAccess(Expr *baseExpr, Type baseType,
+                                       DeclName memberName,
+                                       SourceRange memberRange);
+
+// Return true if, when replacing "<expr>" with "<expr> ?? T", parentheses need
+// to be added around <expr> first in order to maintain the correct precedence.
+bool exprNeedsParensBeforeAddingNilCoalescing(TypeChecker &TC,
+                                              DeclContext *DC,
+                                              Expr *expr);
+
+// Return true if, when replacing "<expr>" with "<expr> as T", parentheses need
+// to be added around the new expression in order to maintain the correct
+// precedence.
+bool exprNeedsParensAfterAddingNilCoalescing(TypeChecker &TC,
+                                             DeclContext *DC,
+                                             Expr *expr,
+                                             Expr *rootExpr);
+
 } // end namespace swift
 
 #endif // LLVM_SWIFT_SEMA_CONSTRAINT_SYSTEM_H

lib/Sema/TypeCheckConstraints.cpp

@@ -1953,8 +1953,6 @@ Type TypeChecker::typeCheckExpression(Expr *&expr, DeclContext *dc,
 
   // Construct a constraint system from this expression.
   ConstraintSystemOptions csOptions = ConstraintSystemFlags::AllowFixes;
-  if (options.contains(TypeCheckExprFlags::PreferForceUnwrapToOptional))
-    csOptions |= ConstraintSystemFlags::PreferForceUnwrapToOptional;
   ConstraintSystem cs(*this, dc, csOptions);
   cs.baseCS = baseCS;
   CleanupIllFormedExpressionRAII cleanup(expr);