Add the flag --incompatible_keyword_only_syntax to forbid keyword-only syntax.

Keyword-only syntax is when a parameter happens after the `*` paramter, e.g.
  def foo(a, *, b): pass

or:
  def foo(a, *b, c): pass

The syntax is not compatible with Python 2 (it's supported only in Python 3),
and breaking tooling based on Python2.

RELNOTES:
  Keyword-only syntax in a function definition is deprecated
  (e.g. `def foo(a, *, b)` or `def foo(a, *b, c)`) and will be removed in the future.
PiperOrigin-RevId: 155071704

2 files changed, 19 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/FunctionDefStatement.java b/src/main/java/com/google/devtools/build/lib/syntax/FunctionDefStatement.java
index 70c4c95943..cde876e58a 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/FunctionDefStatement.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/FunctionDefStatement.java
@@ -49,12 +49,21 @@ public class FunctionDefStatement extends Statement {
         defaultValues.add(expr.eval(env));
       }
     }
+
+    FunctionSignature sig = signature.getSignature();
+    if (env.getSemantics().incompatibleKeywordOnlySyntax
+        && sig.getShape().getMandatoryNamedOnly() > 0) {
+      throw new EvalException(
+          getLocation(),
+          "Keyword-only argument is forbidden. You can temporarily disable this "
+              + "error using the flag --incompatible_keyword_only_syntax=false");
+    }
+
     env.update(
         ident.getName(),
         new UserDefinedFunction(
             ident,
-            FunctionSignature.WithValues.<Object, SkylarkType>create(
-                signature.getSignature(), defaultValues, types),
+            FunctionSignature.WithValues.<Object, SkylarkType>create(sig, defaultValues, types),
             statements,
             env.getGlobals()));
   }
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
index 5ed2643e90..0c83c25760 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
@@ -50,4 +50,12 @@ public class SkylarkSemanticsOptions extends OptionsBase implements Serializable
     help = "If set to true, disables the deprecated `set` constructor for depsets."
   )
   public boolean incompatibleDepsetConstructor;
+
+  @Option(
+    name = "incompatible_keyword_only_syntax",
+    defaultValue = "false",
+    category = "incompatible changes",
+    help = "If set to true, disables the keyword-only argument syntax in function definition."
+  )
+  public boolean incompatibleKeywordOnlySyntax;
 }