[CS] Re-order subscript constraint generation

Add the application constraint before the member constraint, which
allows us to get rid of the special-cased delaying logic for dynamic
member subscripts. The diagnostic change here is due to the fact that
we no longer have a simplified type for the result in CSGen, which
would also be the case if we had a disjunction for the member.

lib/Sema/CSGen.cpp

@@ -1158,6 +1158,18 @@ namespace {
       if (addedTypeVars)
         addedTypeVars->push_back(memberTy);
 
+      SmallVector<AnyFunctionType::Param, 8> params;
+      getMatchingParams(argList, params);
+
+      // Add the constraint that the index expression's type be convertible
+      // to the input type of the subscript operator.
+      // We add this as an unsolved constraint before adding the member
+      // constraint since some member constraints such as key-path dynamic
+      // member require the applicable function constraint to be available.
+      CS.addUnsolvedConstraint(Constraint::createApplicableFunction(
+          CS, FunctionType::get(params, outputTy), memberTy,
+          /*trailingClosureMatching=*/std::nullopt, CurDC, fnLocator));
+
       // FIXME: synthesizeMaterializeForSet() wants to statically dispatch to
       // a known subscript here. This might be cleaner if we split off a new
       // UnresolvedSubscriptExpr from SubscriptExpr.
@@ -1173,15 +1185,6 @@ namespace {
                                     /*outerAlternatives=*/{}, memberLocator);
       }
 
-      SmallVector<AnyFunctionType::Param, 8> params;
-      getMatchingParams(argList, params);
-
-      // Add the constraint that the index expression's type be convertible
-      // to the input type of the subscript operator.
-      CS.addApplicationConstraint(FunctionType::get(params, outputTy), memberTy,
-                                  /*trailingClosureMatching=*/std::nullopt,
-                                  CurDC, fnLocator);
-
       if (CS.performanceHacksEnabled()) {
         Type fixedOutputType =
             CS.getFixedTypeRecursive(outputTy, /*wantRValue=*/false);

lib/Sema/CSSimplify.cpp

@@ -11465,22 +11465,6 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
     if (result.actualBaseType)
       baseTy = result.actualBaseType;
 
-    // If only possible choice to refer to member is via keypath
-    // dynamic member dispatch, let's delay solving this constraint
-    // until constraint generation phase is complete, because
-    // subscript dispatch relies on presence of function application.
-    if (result.ViableCandidates.size() == 1) {
-      auto &choice = result.ViableCandidates.front();
-      if (Phase == ConstraintSystemPhase::ConstraintGeneration &&
-          choice.isKeyPathDynamicMemberLookup() &&
-          member.getBaseName().isSubscript()) {
-        // Let's move this constraint to the active
-        // list so it could be picked up right after
-        // constraint generation is done.
-        return formUnsolved(/*activate=*/true);
-      }
-    }
-
     generateOverloadConstraints(
         candidates, memberTy, result.ViableCandidates, useDC, locator,
         result.getFavoredIndex(), /*requiresFix=*/false,

lib/Sema/CSSolver.cpp

@@ -1798,12 +1798,6 @@ ConstraintSystem::filterDisjunction(
     // constraint, so instead let's keep the disjunction, but disable all
     // unviable choices.
     if (choice->getOverloadChoice().isKeyPathDynamicMemberLookup()) {
-      // Early simplification of the "keypath dynamic member lookup" choice
-      // is impossible because it requires constraints associated with
-      // subscript index expression to be present.
-      if (Phase == ConstraintSystemPhase::ConstraintGeneration)
-        return SolutionKind::Unsolved;
-
       for (auto *currentChoice : disjunction->getNestedConstraints()) {
         if (currentChoice->isDisabled())
           continue;

test/Constraints/function.swift

@@ -115,11 +115,11 @@ func f_45262(block: () -> (), other: () -> Int) {
 
 struct S {
   init<T>(_ x: T, _ y: T) {} // expected-note {{generic parameters are always considered '@escaping'}}
-  subscript<T>() -> (T, T) -> Void { { _, _ in } } // expected-note {{generic parameters are always considered '@escaping'}}
+  subscript<T>() -> (T, T) -> Void { { _, _ in } }
 
-  init(fn: () -> Int) {
+  init(fn: () -> Int) { // expected-note {{parameter 'fn' is implicitly non-escaping}}
     self.init({ 0 }, fn) // expected-error {{converting non-escaping parameter 'fn' to generic parameter 'T' may allow it to escape}}
-    _ = self[]({ 0 }, fn) // expected-error {{converting non-escaping parameter 'fn' to generic parameter 'T' may allow it to escape}}
+    _ = self[]({ 0 }, fn) // expected-error {{passing non-escaping parameter 'fn' to function expecting an '@escaping' closure}}
   }
 }