[CS] Fix invalid key path crasher (#30832)

[CS] Fix invalid key path crasher

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -7543,6 +7543,7 @@ ConstraintSystem::simplifyKeyPathConstraint(
   } capability = Writable;
 
   bool anyComponentsUnresolved = false;
+  bool didOptionalChain = false;
 
   for (unsigned i : indices(keyPath->getComponents())) {
     auto &component = keyPath->getComponents()[i];
@@ -7642,26 +7643,27 @@ ConstraintSystem::simplifyKeyPathConstraint(
     }
 
     case KeyPathExpr::Component::Kind::OptionalChain:
-      // Optional chains force the entire key path to be read-only.
-      capability = ReadOnly;
-      goto done;
+      didOptionalChain = true;
+      break;
 
     case KeyPathExpr::Component::Kind::OptionalForce:
       // Forcing an optional preserves its lvalue-ness.
       break;
 
     case KeyPathExpr::Component::Kind::OptionalWrap:
-      // An optional chain should already have forced the entire key path to
-      // be read-only.
-      assert(capability == ReadOnly);
+      // An optional chain should already have been recorded.
+      assert(didOptionalChain);
       break;
 
     case KeyPathExpr::Component::Kind::TupleElement:
       llvm_unreachable("not implemented");
       break;
     }
   }
-done:
+
+  // Optional chains force the entire key path to be read-only.
+  if (didOptionalChain)
+    capability = ReadOnly;
 
   // Resolve the type.
   NominalTypeDecl *kpDecl;

test/expr/unary/keypath/keypath.swift

@@ -811,6 +811,16 @@ func test_keypath_with_method_refs() {
   let _ = \A.Type.faz.bar // expected-error {{key path cannot refer to static method 'faz()'}}
 }
 
+// SR-12519: Compiler crash on invalid method reference in key path.
+protocol Zonk {
+  func wargle()
+}
+typealias Blatz = (gloop: String, zoop: Zonk?)
+
+func sr12519(fleep: [Blatz]) {
+  fleep.compactMap(\.zoop?.wargle) // expected-error {{key path cannot refer to instance method 'wargle()'}}
+}
+
 // SR-10467 - Argument type 'KeyPath<String, Int>' does not conform to expected type 'Any'
 func test_keypath_in_any_context() {
   func foo(_: Any) {}