[CS] Only use unsolved applicable fn for @dynamicMemberLookup cases

This works around the fact that existential opening does not currently
work correctly in cases where the argument isn't resolved before the
applicable fn is solved.

lib/Sema/CSGen.cpp

@@ -1162,12 +1162,27 @@ namespace {
 
       // 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));
+      auto addApplicableFn = [&]() {
+        CS.addApplicationConstraint(
+            FunctionType::get(params, outputTy), memberTy,
+            /*trailingClosureMatching=*/std::nullopt, CurDC, fnLocator);
+      };
+
+      // If we have a dynamic member base we need to add the applicable function
+      // first since solving the member constraint requires the constraint to be
+      // available since it may retire it. We can't yet do this in the general
+      // case since the simplifying of the applicable fn in CSGen is currently
+      // load-bearing for existential opening.
+      // FIXME: Once existential opening is no longer sensitive to solving
+      // order, we ought to be able to just always record the applicable fn as
+      // an unsolved constraint before the member.
+      auto hasDynamicMemberLookup = CS.simplifyType(baseTy)
+                                        ->getRValueType()
+                                        ->getMetatypeInstanceType()
+                                        ->eraseDynamicSelfType()
+                                        ->hasDynamicMemberLookupAttribute();
+      if (hasDynamicMemberLookup)
+        addApplicableFn();
 
       // FIXME: synthesizeMaterializeForSet() wants to statically dispatch to
       // a known subscript here. This might be cleaner if we split off a new
@@ -1184,6 +1199,11 @@ namespace {
                                     /*outerAlternatives=*/{}, memberLocator);
       }
 
+      // If we don't have a dynamic member, we add the application after the
+      // member, see the above comment.
+      if (!hasDynamicMemberLookup)
+        addApplicableFn();
+
       if (CS.performanceHacksEnabled()) {
         Type fixedOutputType =
             CS.getFixedTypeRecursive(outputTy, /*wantRValue=*/false);

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

test/Constraints/keypath_dynamic_member_lookup.swift

@@ -601,3 +601,18 @@ struct TestIssue56837 {
         _ = value[type: Int8.max]
     }
 }
+
+@dynamicMemberLookup
+class TestDynamicSelf {
+  struct S {
+    subscript() -> Int { 0 }
+  }
+  func foo() -> Self {
+    // Make sure we can do dynamic member lookup on a dynamic self.
+    _ = self[]
+    return self
+  }
+  subscript<T>(dynamicMember dynamicMember: KeyPath<S, T>) -> T {
+    fatalError()
+  }
+}

test/Constraints/opened_existentials_overload.swift

@@ -3,7 +3,7 @@
 protocol P {}
 
 func g(_: some P) {}
-// expected-note@-1 {{required by global function 'g' where 'some P' = 'any P'}}
+// expected-note@-1 2{{required by global function 'g' where 'some P' = 'any P'}}
 
 // rdar://problem/160389221
 func good(_ x: Array<any P>) {
@@ -28,3 +28,25 @@ func bad(_ x: Array<any P>) {
   // expected-error@-1 {{type 'any P' cannot conform to 'P'}}
   // expected-note@-2 {{only concrete types such as structs, enums and classes can conform to protocols}}
 }
+
+func testSubscript() {
+  struct S1<T> {
+    subscript() -> T { fatalError() }
+  }
+  func foo(_ xs: S1<any P>) {
+    g(xs[])
+  }
+
+  struct S2<T> {
+    subscript() -> Int { fatalError() }
+    subscript() -> T { fatalError() }
+  }
+  func foo(_ xs: S2<any P>) {
+    // FIXME: This should work, if you fix this you can also remove the
+    // dynamic member lookup hack in `addSubscriptConstraints`, we should always
+    // just add the applicable fn as an unsolved constraint before the member.
+    g(xs[])
+    // expected-error@-1 {{type 'any P' cannot conform to 'P'}}
+    // expected-note@-2 {{only concrete types such as structs, enums and classes can conform to protocols}}
+  }
+}