Merge pull request swiftlang#41475 from nkcsgexi/key-path-const

sema: keypath expression should be considered as compile-time constant

Author: nkcsgexi

lib/AST/Expr.cpp

@@ -209,6 +209,7 @@ bool Expr::isSemanticallyConstExpr() const {
   case ExprKind::BooleanLiteral:
   case ExprKind::FloatLiteral:
   case ExprKind::StringLiteral:
+  case ExprKind::KeyPath:
     return true;
   case ExprKind::Array:
   case ExprKind::Dictionary: {

test/Sema/const_keypath.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Article {
+  let id: String
+}
+
+let keypath = \Article.id
+func keypath_generator() -> KeyPath<Article, String> { return \.id }
+func const_map(_const _ map: KeyPath<Article, String>) {}
+
+const_map(\.id)
+const_map(\Article.id)
+
+const_map(keypath_generator()) // expected-error {{expect a compile-time constant literal}}
+const_map(keypath) // expected-error {{expect a compile-time constant literal}}