[ASTGen] Fix '@differentiable' attribute

* Typo: '_liner' -> '_linear'
* Accept '@differentiable(_linear)' type attribute

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -603,7 +603,7 @@ extension ASTGenVisitor {
     switch text {
     case "reverse": return .reverse
     case "wrt", "withRespectTo": return .normal
-    case "_liner": return .linear
+    case "_linear": return .linear
     case "_forward": return .forward
     default: return .nonDifferentiable
     }

lib/ASTGen/Sources/ASTGen/TypeAttrs.swift

@@ -206,10 +206,17 @@ extension ASTGenVisitor {
       differentiabilityLoc = nil
     }
 
-    // Only 'reverse' is supported today.
-    guard differentiability == .reverse else {
+    // Only 'reverse' is formally supported today. '_linear' works for testing
+    // purposes. '_forward' is rejected.
+    switch differentiability {
+    case .normal, .nonDifferentiable:
       // TODO: Diagnose
       fatalError("Only @differentiable(reverse) is supported")
+    case .forward:
+      // TODO: Diagnose
+      fatalError("Only @differentiable(reverse) is supported")
+    case .reverse, .linear:
+      break
     }
 
     return .createParsed(

test/ASTGen/autodiff.swift

@@ -17,6 +17,10 @@ func testDifferentiableTypeAttr(_ fn: @escaping @differentiable(reverse) (Float)
     -> @differentiable(reverse) (Float) -> Float {
   return fn
 }
+func testDifferentiableTypeAttrLinear(_ fn: @escaping @differentiable(_linear) (Float) -> Float)
+    -> @differentiable(_linear) (Float) -> Float {
+  return fn
+}
 
 @differentiable(reverse)
 func testDifferentiableSimple(_ x: Float) -> Float { return x * x }