[CSDiagnostics] Tailor diagnostic for Escapable conformance mismatch in key path context

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -667,6 +667,8 @@ ERROR(expr_keypath_generic_type,none,
       "key path cannot refer to generic type %0", (Identifier))
 ERROR(expr_keypath_noncopyable_type,none,
       "key path cannot refer to noncopyable type %0", (Type))
+ERROR(expr_keypath_nonescapable_type,none,
+      "key path cannot refer to nonescapable type %0", (Type))
 ERROR(expr_keypath_not_property,none,
       "%select{key path|dynamic key path member lookup}1 cannot refer to %kind0",
       (const ValueDecl *, bool))

lib/Sema/CSDiagnostics.cpp

@@ -689,6 +689,11 @@ bool MissingConformanceFailure::diagnoseAsError() {
         emitDiagnostic(diag::expr_keypath_noncopyable_type, nonConformingType);
         return true;
       }
+
+      if (P->isSpecificProtocol(KnownProtocolKind::Escapable)) {
+        emitDiagnostic(diag::expr_keypath_nonescapable_type, nonConformingType);
+        return true;
+      }
     }
   }
 

test/Sema/lifetime_attr.swift

@@ -44,3 +44,18 @@ func invalidTarget(_ result: inout NE, _ source: consuming NE) { // expected-err
 func immortalConflict(_ immortal: Int) -> NE { // expected-error{{conflict between the parameter name and 'immortal' contextual keyword}}
   NE()
 }
+
+do {
+  struct Test: ~Escapable {
+    var v1: Int
+    var v2: NE
+  }
+
+  _ = \Test.v1 // expected-error {{key path cannot refer to nonescapable type 'Test'}}
+  _ = \Test.v2 // expected-error {{key path cannot refer to nonescapable type 'Test'}} expected-error {{key path cannot refer to nonescapable type 'NE'}}
+
+  func use(t: Test) {
+    t[keyPath: \.v1] // expected-error {{key path cannot refer to nonescapable type 'Test'}}
+    t[keyPath: \.v2] // expected-error {{key path cannot refer to nonescapable type 'Test'}} expected-error {{key path cannot refer to nonescapable type 'NE'}}
+  }
+}