[CSOptimizer] Add support for opened existential arguments

Check whether it would be possible to match a parameter type
by opening an existential type of the candidate argument.

Resolves: rdar://158159462

lib/Sema/CSOptimizer.cpp

@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "TypeChecker.h"
+#include "OpenedExistentials.h"
 #include "swift/AST/ConformanceLookup.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/GenericSignature.h"
@@ -1197,18 +1198,20 @@ static void determineBestChoicesInContext(
     // - Superclass conversion
     // - Array-to-pointer conversion
     // - Value to existential conversion
+    // - Existential opening
     // - Exact match on top-level types
     //
     // In situations when it's not possible to determine whether a candidate
     // type matches a parameter type (i.e. when partially resolved generic
     // types are matched) this function is going to produce \c std::nullopt
     // instead of `0` that indicates "not a match".
-    std::function<std::optional<double>(GenericSignature, ValueDecl *, Type,
-                                        Type, MatchOptions)>
+    std::function<std::optional<double>(GenericSignature, ValueDecl *,
+                                        std::optional<unsigned>, Type, Type,
+                                        MatchOptions)>
         scoreCandidateMatch =
             [&](GenericSignature genericSig, ValueDecl *choice,
-                Type candidateType, Type paramType,
-                MatchOptions options) -> std::optional<double> {
+                std::optional<unsigned> paramIdx, Type candidateType,
+                Type paramType, MatchOptions options) -> std::optional<double> {
       auto areEqual = [&](Type a, Type b) {
         return a->getDesugaredType()->isEqual(b->getDesugaredType());
       };
@@ -1260,7 +1263,7 @@ static void determineBestChoicesInContext(
           // This helps to determine whether there are any generic
           // overloads that are a possible match.
           auto score =
-              scoreCandidateMatch(genericSig, choice, candidateType,
+              scoreCandidateMatch(genericSig, choice, paramIdx, candidateType,
                                   paramType, options - MatchFlag::Literal);
           if (score == 0)
             return 0;
@@ -1347,8 +1350,8 @@ static void determineBestChoicesInContext(
           if ((paramOptionals.empty() &&
                paramType->is<GenericTypeParamType>()) ||
               paramOptionals.size() >= candidateOptionals.size()) {
-            auto score = scoreCandidateMatch(genericSig, choice, candidateType,
-                                             paramType, options);
+            auto score = scoreCandidateMatch(genericSig, choice, paramIdx,
+                                             candidateType, paramType, options);
 
             if (score > 0) {
               // Injection lowers the score slightly to comply with
@@ -1394,6 +1397,21 @@ static void determineBestChoicesInContext(
       if (paramType->isAny())
         return 1;
 
+      // Check if a candidate could be matched to a parameter by
+      // an existential opening.
+      if (options.contains(MatchFlag::OnParam) &&
+          candidateType->getMetatypeInstanceType()->isExistentialType()) {
+        if (auto *genericParam = paramType->getMetatypeInstanceType()
+                                     ->getAs<GenericTypeParamType>()) {
+          if (canOpenExistentialAt(choice, *paramIdx, genericParam,
+                                   candidateType->getMetatypeInstanceType())) {
+            // Lower the score slightly for operators to make sure that
+            // concrete overloads are always preferred over generic ones.
+            return choice->isOperator() ? 0.9 : 1;
+          }
+        }
+      }
+
       // Check protocol requirement(s) if this parameter is a
       // generic parameter type.
       if (genericSig && paramType->isTypeParameter()) {
@@ -1661,9 +1679,10 @@ static void determineBestChoicesInContext(
                 options |= MatchFlag::StringInterpolation;
 
               // The specifier for a candidate only matters for `inout` check.
-              auto candidateScore = scoreCandidateMatch(
-                  genericSig, decl, candidate.type->getWithoutSpecifierType(),
-                  paramType, options);
+              auto candidateScore =
+                  scoreCandidateMatch(genericSig, decl, paramIdx,
+                                      candidate.type->getWithoutSpecifierType(),
+                                      paramType, options);
 
               if (!candidateScore)
                 continue;
@@ -1706,6 +1725,7 @@ static void determineBestChoicesInContext(
               (score > 0 || !hasArgumentCandidates)) {
             if (llvm::any_of(resultTypes, [&](const Type candidateResultTy) {
                   return scoreCandidateMatch(genericSig, decl,
+                                             /*paramIdx=*/std::nullopt,
                                              overloadType->getResult(),
                                              candidateResultTy,
                                              /*options=*/{}) > 0;

test/Constraints/opened_existentials.swift

@@ -591,3 +591,18 @@ protocol PP3 {
     associatedtype A
 }
 
+protocol PP4 {
+}
+
+do {
+  func test<T>(env: T) where T: PP4 {}
+
+  func test(env: PP4? = nil) {
+    guard let env else {
+      return
+    }
+
+    // CHECK: open_existential_expr {{.*}} location={{.*}}:[[@LINE+1]]:{{[0-9]+}} range=
+    test(env: env)
+  }
+}

validation-test/compiler_crashers_2_fixed/cad1ff6235b5b3d.swift

@@ -1,4 +1,4 @@
 // {"kind":"typecheck","signature":"swift::constraints::ConstraintSystem::isArgumentGenericFunction(swift::Type, swift::Expr*)","signatureAssert":"Assertion failed: (!getFixedType(tyvar)), function getUnboundBindOverloadDisjunction"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 {
   print($0) $00 + 0. / 1