[CS] Fix invalid key path crasher

Previously we were bailing early on encountering
an optional chain in the key path. However this
could cause us to miss invalid components further
down the line. Instead, set a flag and force the
key path to be read-only if we encountered an
optional chain.

Resolves SR-12519.

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) {}