[Sema] Diagnostic: improve diagnostics for ApplyExpr.

lib/Sema/CSDiag.cpp

@@ -5665,6 +5665,34 @@ static bool shouldTypeCheckFunctionExpr(TypeChecker &TC, DeclContext *DC,
   return true;
 }
 
+// Check if any candidate of the overload set can accept a specified
+// number of arguments, regardless of parameter type or label information.
+static bool isViableOverloadSet(const CalleeCandidateInfo &CCI,
+                                size_t numArgs) {
+  for (unsigned i = 0; i < CCI.size(); ++i) {
+    auto &&cand = CCI[i];
+    auto funcDecl = dyn_cast_or_null<AbstractFunctionDecl>(cand.getDecl());
+    if (!funcDecl)
+      continue;
+
+    auto params = cand.getParameters();
+    bool hasVariadicParameter = false;
+    auto pairMatcher = [&](unsigned argIdx, unsigned paramIdx) {
+      hasVariadicParameter |= params[paramIdx].isVariadic();
+      return true;
+    };
+
+    auto defaultMap = computeDefaultMap(params, funcDecl, cand.level);
+    InputMatcher IM(params, defaultMap);
+    auto result = IM.match(numArgs, pairMatcher);
+    if (result == InputMatcher::IM_Succeeded)
+      return true;
+    if (result == InputMatcher::IM_HasUnclaimedInput && hasVariadicParameter)
+      return true;
+  }
+  return false;
+}
+
 bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
   // If this call involves trailing closure as an argument,
   // let's treat it specially, because re-typecheck of the
@@ -5826,6 +5854,41 @@ bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
   // checked function.
   CalleeCandidateInfo calleeInfo(fnExpr, hasTrailingClosure, CS);
 
+  // In the case that function subexpression was resolved independently in
+  // the first place, the resolved type may not provide the best diagnostic.
+  // We consider the number of arguments to decide whether we'd go with it or
+  // stay with the original one.
+  if (fnExpr != callExpr->getFn()) {
+    bool isInstanceMethodAsCurriedMemberOnType = false;
+    if (!calleeInfo.empty()) {
+      auto &&cand = calleeInfo[0];
+      auto decl = cand.getDecl();
+      if (decl && decl->isInstanceMember() && cand.level == 0 &&
+          cand.getParameters().size() == 1)
+        isInstanceMethodAsCurriedMemberOnType = true;
+    }
+
+    // In terms of instance method as curried member on type, we should not
+    // take the number of arguments into account.
+    if (!isInstanceMethodAsCurriedMemberOnType) {
+      size_t numArgs = 1;
+      auto arg = callExpr->getArg();
+      if (auto tuple = dyn_cast<TupleExpr>(arg)) {
+        numArgs = tuple->getNumElements();
+      }
+
+      if (!isViableOverloadSet(calleeInfo, numArgs)) {
+        CalleeCandidateInfo calleeInfoOrig(callExpr->getFn(),
+                                           hasTrailingClosure, CS);
+        if (isViableOverloadSet(calleeInfoOrig, numArgs)) {
+          fnExpr = callExpr->getFn();
+          fnType = getFuncType(CS.getType(fnExpr));
+          calleeInfo = calleeInfoOrig;
+        }
+      }
+    }
+  }
+
   // Filter list of the candidates based on the known function type.
   if (auto fn = fnType->getAs<AnyFunctionType>()) {
     using Closeness = CalleeCandidateInfo::ClosenessResultTy;

lib/Sema/CSRanking.cpp

@@ -606,8 +606,6 @@ static bool isDeclAsSpecializedAs(TypeChecker &tc, DeclContext *dc,
         auto funcTy2 = openedType2->castTo<FunctionType>();
         auto params1 = funcTy1->getParams();
         auto params2 = funcTy2->getParams();
-        llvm::SmallBitVector defaultMapType2 =
-          computeDefaultMap(params2, decl2, outerDC2->isTypeContext());
 
         unsigned numParams1 = params1.size();
         unsigned numParams2 = params2.size();
@@ -620,8 +618,6 @@ static bool isDeclAsSpecializedAs(TypeChecker &tc, DeclContext *dc,
               params1.back().getType()->is<AnyFunctionType>() &&
               params2.back().getType()->is<AnyFunctionType>()) {
             compareTrailingClosureParamsSeparately = true;
-            --numParams1;
-            --numParams2;
           }
         }
 
@@ -646,56 +642,38 @@ static bool isDeclAsSpecializedAs(TypeChecker &tc, DeclContext *dc,
           return true;
         };
 
-        for (unsigned param1 = 0, param2 = 0; param2 != numParams2; ++param2) {
-          // If there is a default for parameter in the second function
-          // while there are still some parameters left unclaimed in first,
-          // it could only mean that default parameters are intermixed e.g.
-          //
-          // ```swift
-          // func foo(a: Int) {}
-          // func foo(q: String = "", a: Int) {}
-          // ```
-          // or
-          // ```swift
-          // func foo(a: Int, c: Int) {}
-          // func foo(a: Int, b: Int = 0, c: Int) {}
-          // ```
-          // and we shouldn't claim parameter from the first function.
-          if (param1 < numParams1 && numParams1 != numParams2 &&
-              defaultMapType2[param2]) {
-            fewerEffectiveParameters = true;
-            continue;
-          }
-
-          // If we've claimed all of the parameters from first
-          // function, the rest of the parameters in second should
-          // be either default or variadic.
-          if (param1 >= numParams1) {
-            if (!defaultMapType2[param2] && !params2[param2].isVariadic())
-              return false;
-
-            fewerEffectiveParameters = true;
-            continue;
-          }
-
+        auto pairMatcher = [&](unsigned idx1, unsigned idx2) -> bool {
           // Emulate behavior from when IUO was a type, where IUOs
           // were considered subtypes of plain optionals, but not
           // vice-versa.  This wouldn't normally happen, but there are
           // cases where we can rename imported APIs so that we have a
           // name collision, and where the parameter type(s) are the
           // same except for details of the kind of optional declared.
-          auto param1IsIUO = paramIsIUO(decl1, param1);
-          auto param2IsIUO = paramIsIUO(decl2, param2);
+          auto param1IsIUO = paramIsIUO(decl1, idx1);
+          auto param2IsIUO = paramIsIUO(decl2, idx2);
           if (param2IsIUO && !param1IsIUO)
             return false;
 
-          if (!maybeAddSubtypeConstraint(params1[param1], params2[param2]))
+          if (!maybeAddSubtypeConstraint(params1[idx1], params2[idx2]))
             return false;
 
-          // claim the parameter as used.
-          ++param1;
+          return true;
+        };
+
+        auto defaultMap = computeDefaultMap(
+            params2, decl2, decl2->getDeclContext()->isTypeContext());
+        auto params2ForMatching = params2;
+        if (compareTrailingClosureParamsSeparately) {
+          --numParams1;
+          params2ForMatching = params2.drop_back();
         }
 
+        InputMatcher IM(params2ForMatching, defaultMap);
+        if (IM.match(numParams1, pairMatcher) != InputMatcher::IM_Succeeded)
+          return false;
+
+        fewerEffectiveParameters |= (IM.getNumSkippedParameters() != 0);
+
         if (compareTrailingClosureParamsSeparately)
           if (!maybeAddSubtypeConstraint(params1.back(), params2.back()))
             knownNonSubtype = true;
@@ -1385,3 +1363,55 @@ SolutionDiff::SolutionDiff(ArrayRef<Solution> solutions) {
     }
   }
 }
+
+InputMatcher::InputMatcher(const ArrayRef<AnyFunctionType::Param> params,
+                           const llvm::SmallBitVector &defaultValueMap)
+    : NumSkippedParameters(0), DefaultValueMap(defaultValueMap),
+      Params(params) {}
+
+InputMatcher::Result
+InputMatcher::match(int numInputs,
+                    std::function<bool(unsigned, unsigned)> pairMatcher) {
+
+  int inputIdx = 0;
+  int numParams = Params.size();
+  for (int i = 0; i < numParams; ++i) {
+    // If we've claimed all of the inputs, the rest of the parameters should
+    // be either default or variadic.
+    if (inputIdx == numInputs) {
+      if (!DefaultValueMap[i] && !Params[i].isVariadic())
+        return IM_HasUnmatchedParam;
+      ++NumSkippedParameters;
+      continue;
+    }
+
+    // If there is a default for parameter, while there are still some
+    // input left unclaimed, it could only mean that default parameters
+    // are intermixed e.g.
+    //
+    // inputs: (a: Int)
+    // params: (q: String = "", a: Int)
+    //
+    // or
+    // inputs: (a: Int, c: Int)
+    // params: (a: Int, b: Int = 0, c: Int)
+    //
+    // and we shouldn't claim any input and just skip such parameter.
+    if ((numInputs - inputIdx) < (numParams - i) && DefaultValueMap[i]) {
+      ++NumSkippedParameters;
+      continue;
+    }
+
+    // Call custom function to match the input-parameter pair.
+    if (!pairMatcher(inputIdx, i))
+      return IM_CustomPairMatcherFailed;
+
+    // claim the input as used.
+    ++inputIdx;
+  }
+
+  if (inputIdx < numInputs)
+    return IM_HasUnclaimedInput;
+
+  return IM_Succeeded;
+}

lib/Sema/CalleeCandidateInfo.cpp

@@ -880,6 +880,25 @@ CalleeCandidateInfo::CalleeCandidateInfo(Type baseType,
     declName = candidates[0].getDecl()->getBaseName().userFacingName();
 }
 
+CalleeCandidateInfo &CalleeCandidateInfo::
+operator=(const CalleeCandidateInfo &CCI) {
+  if (this != &CCI) {
+    // If the reference member (i.e., CS) is identical, just copy remaining
+    // members; otherwise, reconstruct the object.
+    if (&CS == &CCI.CS) {
+      declName = CCI.declName;
+      hasTrailingClosure = CCI.hasTrailingClosure;
+      candidates = CCI.candidates;
+      closeness = CCI.closeness;
+      failedArgument = CCI.failedArgument;
+    } else {
+      this->~CalleeCandidateInfo();
+      new (this) CalleeCandidateInfo(CCI);
+    }
+  }
+  return *this;
+}
+
 /// Given a set of parameter lists from an overload group, and a list of
 /// arguments, emit a diagnostic indicating any partially matching overloads.
 void CalleeCandidateInfo::

lib/Sema/CalleeCandidateInfo.h

@@ -188,6 +188,12 @@ namespace swift {
                         bool hasTrailingClosure, ConstraintSystem &CS,
                         bool selfAlreadyApplied = true);
 
+    ~CalleeCandidateInfo() = default;
+    CalleeCandidateInfo(const CalleeCandidateInfo &CCI) = default;
+    CalleeCandidateInfo &operator=(const CalleeCandidateInfo &CCI);
+    CalleeCandidateInfo(CalleeCandidateInfo &&CCI) = delete;
+    CalleeCandidateInfo &operator=(CalleeCandidateInfo &&CCI) = delete;
+
     using ClosenessResultTy = std::pair<CandidateCloseness, FailedArgumentInfo>;
     using ClosenessPredicate =
         const std::function<ClosenessResultTy(UncurriedCandidate)> &;

lib/Sema/ConstraintSystem.h

@@ -3486,6 +3486,48 @@ class OverloadSetCounter : public ASTWalker {
     return { true, expr };
   }
 };
+
+/// \brief Matches array of function parameters to candidate inputs,
+/// which can be anything suitable (e.g., parameters, arguments).
+///
+/// It claims inputs sequentially and tries to pair between an input
+/// and the next appropriate parameter. The detailed matching behavior
+/// of each pair is specified by a custom function (i.e., pairMatcher).
+/// It considers variadic and defaulted arguments when forming proper
+/// input-parameter pairs; however, other information like label and
+/// type information is not directly used here. It can be implemented
+/// in the custom function when necessary.
+class InputMatcher {
+  size_t NumSkippedParameters;
+  const llvm::SmallBitVector DefaultValueMap;
+  const ArrayRef<AnyFunctionType::Param> Params;
+
+public:
+  enum Result {
+    /// The specified inputs are successfully matched.
+    IM_Succeeded,
+    /// There are input(s) left unclaimed while all parameters are matched.
+    IM_HasUnclaimedInput,
+    /// There are parateter(s) left unmatched while all inputs are claimed.
+    IM_HasUnmatchedParam,
+    /// Custom pair matcher function failure.
+    IM_CustomPairMatcherFailed,
+  };
+
+  InputMatcher(const ArrayRef<AnyFunctionType::Param> params,
+               const llvm::SmallBitVector &defaultValueMap);
+
+  /// Matching a given array of inputs.
+  ///
+  /// \param numInputs The number of inputs.
+  /// \param pairMatcher Custom matching behavior of an input-parameter pair.
+  /// \return the matching result.
+  Result
+  match(int numInputs,
+        std::function<bool(unsigned inputIdx, unsigned paramIdx)> pairMatcher);
+
+  size_t getNumSkippedParameters() const { return NumSkippedParameters; }
+};
 } // end namespace swift
 
 #endif // LLVM_SWIFT_SEMA_CONSTRAINT_SYSTEM_H