[CSRanking] Detect cases where overload choices are incomparable

lib/Sema/CSRanking.cpp

@@ -799,6 +799,21 @@ SolutionCompareResult ConstraintSystem::compareSolutions(
     if (sameOverloadChoice(choice1, choice2))
       continue;
 
+    // If constraint system is underconstrained e.g. because there are
+    // editor placeholders, it's possible to end up with multiple solutions
+    // where each ambiguous declaration is going to have its own overload kind:
+    //
+    // func foo(_: Int) -> [Int] { ... }
+    // func foo(_: Double) -> (result: String, count: Int) { ... }
+    //
+    // _ = foo(<#arg#>).count
+    //
+    // In this case solver would produce 2 solutions: one where `count`
+    // is a property reference on `[Int]` and another one is tuple access
+    // for a `count:` element.
+    if (choice1.isDecl() != choice2.isDecl())
+      return SolutionCompareResult::Incomparable;
+
     auto decl1 = choice1.getDecl();
     auto dc1 = decl1->getDeclContext();
     auto decl2 = choice2.getDecl();

test/Sema/editor_placeholders.swift

@@ -29,3 +29,10 @@ f(<#T##String#>) // expected-error{{editor placeholder in source file}} expected
 for x in <#T#> { // expected-error{{editor placeholder in source file}} expected-error{{for-in loop requires '()' to conform to 'Sequence'}}
 
 }
+
+// rdar://problem/49712598 - crash while trying to rank solutions with different kinds of overloads
+func test_ambiguity_with_placeholders(pairs: [(rank: Int, count: Int)]) -> Bool {
+  return pairs[<#^ARG^#>].count == 2
+  // expected-error@-1 {{editor placeholder in source file}}
+  // expected-error@-2 {{ambiguous use of 'subscript(_:)'}}
+}