stdlib: ContainsFunc allows testing for presence of null value

The defaults for what to allow in a function argument are intentionally
conservative so that function implementations don't all need to reimplement
similar validation code for commonly-rejected cases, but this function
doesn't do anythign with the "value" argument except compare it with
another value using Value.Equals and so null values can be permitted here
for the same reason that EqualFunc allows its operands to be null.

Author: apparentlymart

CHANGELOG.md

@@ -1,5 +1,6 @@
 # 1.18.1 (Unreleased)
 
+- stdlib: `ContainsFunc` now allows its second argument to be null, to test whether the given collection contains any null elements.
 
 # 1.18.0 (February 23, 2026)
 

cty/function/stdlib/collection.go

@@ -321,8 +321,9 @@ var ContainsFunc = function.New(&function.Spec{
 			Type: cty.DynamicPseudoType,
 		},
 		{
-			Name: "value",
-			Type: cty.DynamicPseudoType,
+			Name:      "value",
+			Type:      cty.DynamicPseudoType,
+			AllowNull: true,
 		},
 	},
 	Type:         function.StaticReturnType(cty.Bool),