[ConstraintLocator] Add special locator for synthesized arguments

lib/Sema/CSSimplify.cpp

@@ -1264,8 +1264,8 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
     if (diff < 0) {
       for (unsigned i = func1Params.size(),
                     n = func2Params.size(); i != n; ++i) {
-        auto *argLoc =
-            getConstraintLocator(anchor, LocatorPathElt::getTupleElement(i));
+        auto *argLoc = getConstraintLocator(
+            anchor, LocatorPathElt::getSynthesizedArgument(i));
 
         auto arg = func2Params[i].withType(createTypeVariable(argLoc));
         func1Params.push_back(arg);

lib/Sema/ConstraintLocator.cpp

@@ -74,6 +74,7 @@ void ConstraintLocator::Profile(llvm::FoldingSetNodeID &id, Expr *anchor,
     case ImplicitlyUnwrappedDisjunctionChoice:
     case DynamicLookupResult:
     case ContextualType:
+    case SynthesizedArgument:
       if (unsigned numValues = numNumericValuesInPathElement(elt.getKind())) {
         id.AddInteger(elt.getValue());
         if (numValues > 1)
@@ -261,6 +262,10 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) {
     case ContextualType:
       out << "contextual type";
       break;
+
+    case SynthesizedArgument:
+      out << " synthesized argument #" << llvm::utostr(elt.getValue());
+      break;
     }
   }
 

lib/Sema/ConstraintLocator.h

@@ -125,6 +125,8 @@ public:
     DynamicLookupResult,
     /// The desired contextual type passed in to the constraint system.
     ContextualType,
+    /// The missing argument synthesized by the solver.
+    SynthesizedArgument,
   };
 
   /// Determine the number of numeric values used for the given path
@@ -162,6 +164,7 @@ public:
     case NamedTupleElement:
     case TupleElement:
     case KeyPathComponent:
+    case SynthesizedArgument:
       return 1;
 
     case TypeParameterRequirement:
@@ -217,6 +220,7 @@ public:
     case ImplicitlyUnwrappedDisjunctionChoice:
     case DynamicLookupResult:
     case ContextualType:
+    case SynthesizedArgument:
       return 0;
 
     case FunctionArgument:
@@ -361,6 +365,10 @@ public:
                          static_cast<unsigned>(kind));
     }
 
+    static PathElement getSynthesizedArgument(unsigned position) {
+      return PathElement(SynthesizedArgument, position);
+    }
+
     /// Retrieve the kind of path element.
     PathElementKind getKind() const {
       switch (static_cast<StoredKind>(storedKind)) {
@@ -447,6 +455,10 @@ public:
     bool isConditionalRequirement() const {
       return getKind() == PathElementKind::ConditionalRequirement;
     }
+
+    bool isSynthesizedArgument() const {
+      return getKind() == PathElementKind::SynthesizedArgument;
+    }
   };
 
   /// Return the summary flags for an entire path.