[ConstraintSolver] Use a flag to prevent solver from removing possible solutions

In some situations e.g. while trying to shrink domains of the type
variables before attempting search, use a flag to tell constraint
system to retain all of the viable solutions otherwise solver could
loose some of the information required to produce complete solution.

Resolves: rdar://problem/32726044

Author: xedin

lib/Sema/CSRanking.cpp

@@ -85,6 +85,9 @@ void ConstraintSystem::increaseScore(ScoreKind kind, unsigned value) {
 }
 
 bool ConstraintSystem::worseThanBestSolution() const {
+  if (retainAllSolutions())
+    return false;
+
   if (!solverState || !solverState->BestScore ||
       CurrentScore <= *solverState->BestScore)
     return false;

lib/Sema/CSSolver.cpp

@@ -110,7 +110,7 @@ Solution ConstraintSystem::finalize(
   Solution solution(*this, CurrentScore);
 
   // Update the best score we've seen so far.
-  if (solverState) {
+  if (solverState && !retainAllSolutions()) {
     assert(!solverState->BestScore || CurrentScore <= *solverState->BestScore);
     solverState->BestScore = CurrentScore;
   }
@@ -1499,7 +1499,8 @@ bool ConstraintSystem::Candidate::solve(
   };
 
   // Allocate new constraint system for sub-expression.
-  ConstraintSystem cs(TC, DC, None);
+  ConstraintSystem cs(TC, DC,
+                      ConstraintSystemFlags::ReturnAllDiscoveredSolutions);
 
   // Cleanup after constraint system generation/solving,
   // because it would assign types to expressions, which
@@ -2066,17 +2067,11 @@ bool ConstraintSystem::solve(SmallVectorImpl<Solution> &solutions,
   // Solve the system.
   solveRec(solutions, allowFreeTypeVariables);
 
-  // If there is more than one viable system, attempt to pick the best
-  // solution.
-  auto size = solutions.size();
-  if (size > 1 &&
-      !Options.contains(ConstraintSystemFlags::ReturnAllDiscoveredSolutions)) {
-    if (auto best = findBestSolution(solutions, /*minimize=*/false)) {
-      if (*best != 0)
-        solutions[0] = std::move(solutions[*best]);
-      solutions.erase(solutions.begin() + 1, solutions.end());
-    }
-  }
+  // Filter deduced solutions, try to figure out if there is
+  // a single best solution to use, if not explicitly disabled
+  // by constraint system options.
+  if (!retainAllSolutions())
+    filterSolutions(solutions);
 
   // We fail if there is no solution.
   return solutions.empty();
@@ -2289,11 +2284,8 @@ bool ConstraintSystem::solveRec(SmallVectorImpl<Solution> &solutions,
     auto &solutions = partialSolutions[component];
     // If there's a single best solution, keep only that one.
     // Otherwise, the set of solutions will at least have been minimized.
-    if (auto best = findBestSolution(solutions, /*minimize=*/true)) {
-      if (*best > 0)
-        solutions[0] = std::move(solutions[*best]);
-      solutions.erase(solutions.begin() + 1, solutions.end());
-    }
+    if (!retainAllSolutions())
+      filterSolutions(solutions, /*minimize=*/true);
   }
 
   // Produce all combinations of partial solutions.

lib/Sema/ConstraintSystem.h

@@ -1382,6 +1382,13 @@ class ConstraintSystem {
   bool hasFreeTypeVariables();
 
 private:
+  /// \brief Indicates if the constraint system should retain all of the
+  /// solutions it has deduced regardless of their score.
+  bool retainAllSolutions() const {
+    return Options.contains(
+        ConstraintSystemFlags::ReturnAllDiscoveredSolutions);
+  }
+
   /// \brief Finalize this constraint system; we're done attempting to solve
   /// it.
   ///
@@ -1403,8 +1410,28 @@ class ConstraintSystem {
   /// diagnostic for it and returning true.  If the fixit hint turned out to be
   /// bogus, this returns false and doesn't emit anything.
   bool applySolutionFix(Expr *expr, const Solution &solution, unsigned fixNo);
-  
-  
+
+  /// \brief If there is more than one viable solution,
+  /// attempt to pick the best solution and remove all of the rest.
+  ///
+  /// \param solutions The set of solutions to filter.
+  ///
+  /// \param minimize The flag which idicates if the
+  /// set of solutions should be filtered even if there is
+  /// no single best solution, see `findBestSolution` for
+  /// more details.
+  void filterSolutions(SmallVectorImpl<Solution> &solutions,
+                       bool minimize = false) {
+    if (solutions.size() < 2)
+      return;
+
+    if (auto best = findBestSolution(solutions, minimize)) {
+      if (*best != 0)
+        solutions[0] = std::move(solutions[*best]);
+      solutions.erase(solutions.begin() + 1, solutions.end());
+    }
+  }
+
   /// \brief Restore the type variable bindings to what they were before
   /// we attempted to solve this constraint system.
   ///

test/Constraints/diagnostics.swift

@@ -983,3 +983,21 @@ let _: KeyPath<R32101765, Float> = \R32101765.prop32101765.unknown
 for var i in 0..<10 { // expected-warning {{variable 'i' was never mutated; consider changing to 'let' constant}} {{5-9=}}
   _ = i + 1
 }
+
+// rdar://problem/32726044 - shrink reduced domains too far
+
+public protocol P_32726044 {}
+
+extension Int: P_32726044 {}
+extension Float: P_32726044 {}
+
+public func *(lhs: P_32726044, rhs: P_32726044) -> Double {
+  fatalError()
+}
+
+func rdar32726044() -> Float {
+  var f: Float = 0
+  f = Float(1) * 100 // Ok
+  let _: Float = Float(42) + 0 // Ok
+  return f
+}