Reject dictionaries passed as lists

src/main/java/com/google/devtools/build/lib/packages/Type.java

@@ -732,6 +732,10 @@ public List<ElemT> convert(Object x, Object what, LabelConverter labelConverter)
         throws ConversionException {
       Iterable<?> iterable;
 
+      if (x instanceof Map) {
+        throw new ConversionException(this, x, what);
+      }
+
       if (x instanceof Iterable) {
         iterable = (Iterable<?>) x;
       } else if (x instanceof Depset) {

src/test/java/com/google/devtools/build/lib/packages/TypeTest.java

@@ -289,6 +289,19 @@ public void testNonStringList() throws Exception {
         .isEqualTo("expected value of type 'list(string)' for blah, but got 3 (int)");
   }
 
+  @Test
+  public void testStringListRejectsDict() throws Exception {
+    Type.ConversionException e =
+        assertThrows(
+            Type.ConversionException.class,
+            () -> Types.STRING_LIST.convert(ImmutableMap.of("foo", "bar"), "myattr"));
+    assertThat(e)
+        .hasMessageThat()
+        .isEqualTo(
+            "expected value of type 'list(string)' for myattr,"
+                + " but got {\"foo\": \"bar\"} (Map)");
+  }
+
   @Test
   public void testStringListBadElements() throws Exception {
     Object input = Arrays.<Object>asList("foo", "bar", StarlarkInt.of(1));