Merge pull request swiftlang#35744 from hborla/solver-disjunction-selection

[ConstraintSystem] Tweak disjunction selection

Author: hborla

include/swift/Sema/Constraint.h

@@ -686,6 +686,11 @@ class Constraint final : public llvm::ilist_node<Constraint>,
     });
   }
 
+  /// Returns the number of resolved argument types for an applied disjunction
+  /// constriant. This is always zero for disjunctions that do not represent
+  /// an applied overload.
+  unsigned countResolvedArgumentTypes(ConstraintSystem &cs) const;
+
   /// Determine if this constraint represents explicit conversion,
   /// e.g. coercion constraint "as X" which forms a disjunction.
   bool isExplicitConversion() const;

include/swift/Sema/ConstraintSystem.h

@@ -4983,7 +4983,7 @@ class ConstraintSystem {
 
   // If the given constraint is an applied disjunction, get the argument function
   // that the disjunction is applied to.
-  const FunctionType *getAppliedDisjunctionArgumentFunction(Constraint *disjunction) {
+  const FunctionType *getAppliedDisjunctionArgumentFunction(const Constraint *disjunction) {
     assert(disjunction->getKind() == ConstraintKind::Disjunction);
     return AppliedDisjunctions[disjunction->getLocator()];
   }

lib/Sema/CSSolver.cpp

@@ -2295,20 +2295,27 @@ Constraint *ConstraintSystem::selectDisjunction() {
           return first->countActiveNestedConstraints() < second->countActiveNestedConstraints();
 
         if (firstFavored == secondFavored) {
-          // Look for additional choices to favor
+          // Look for additional choices that are "favored"
           SmallVector<unsigned, 4> firstExisting;
           SmallVector<unsigned, 4> secondExisting;
 
           existingOperatorBindingsForDisjunction(*cs, first->getNestedConstraints(), firstExisting);
-          firstFavored = firstExisting.size() ? firstExisting.size() : first->countActiveNestedConstraints();
+          firstFavored += firstExisting.size();
           existingOperatorBindingsForDisjunction(*cs, second->getNestedConstraints(), secondExisting);
-          secondFavored = secondExisting.size() ? secondExisting.size() : second->countActiveNestedConstraints();
-
-          return firstFavored < secondFavored;
+          secondFavored += secondExisting.size();
         }
 
         firstFavored = firstFavored ? firstFavored : first->countActiveNestedConstraints();
         secondFavored = secondFavored ? secondFavored : second->countActiveNestedConstraints();
+
+        // Everything else equal, choose the disjunction with the greatest
+        // number of resoved argument types. The number of resolved argument
+        // types is always zero for disjunctions that don't represent applied
+        // overloads.
+        if (firstFavored == secondFavored) {
+          return first->countResolvedArgumentTypes(*this) > second->countResolvedArgumentTypes(*this);
+        }
+
         return firstFavored < secondFavored;
       });
 

lib/Sema/Constraint.cpp

@@ -606,6 +606,17 @@ gatherReferencedTypeVars(Constraint *constraint,
   }
 }
 
+unsigned Constraint::countResolvedArgumentTypes(ConstraintSystem &cs) const {
+  auto *argumentFuncType = cs.getAppliedDisjunctionArgumentFunction(this);
+  if (!argumentFuncType)
+    return 0;
+
+  return llvm::count_if(argumentFuncType->getParams(), [&](const AnyFunctionType::Param arg) {
+    auto argType = cs.getFixedTypeRecursive(arg.getPlainType(), /*wantRValue=*/true);
+    return !argType->isTypeVariableOrMember();
+  });
+}
+
 bool Constraint::isExplicitConversion() const {
   assert(Kind == ConstraintKind::Disjunction);
 

validation-test/Sema/type_checker_perf/fast/rdar73892556.gyb

@@ -0,0 +1,9 @@
+// RUN: %scale-test --begin 10 --end 20 --step 1 --select NumLeafScopes %s
+// REQUIRES: asserts,no_asan
+
+func test(_ arr: [Int]) -> [Int] {
+%for i in range(0, N):
+  arr +
+%end
+  (arr.isEmpty ? arr : [Int]())
+}