stdlib: no MergeFunc crash with null values of object type

This was failing due to the special case of returning the type of the first
argument when all arguments have the same type. This function treats a
null value of any object type as if it were a non-null value of the empty
object type, but the Type function wasn't respecting that special case and
so it was proposing a different return type than the Impl function actually
produces.

Author: apparentlymart

CHANGELOG.md

@@ -1,6 +1,7 @@
 # 1.18.1 (Unreleased)
 
 - stdlib: `ContainsFunc` now allows its second argument to be null, to test whether the given collection contains any null elements.
+- stdlib: `MergeFunc` no longer panics if all of its arguments are null values of the same object type with at least one attribute.
 
 # 1.18.0 (February 23, 2026)
 

cty/function/stdlib/collection.go

@@ -3,6 +3,7 @@ package stdlib
 import (
 	"errors"
 	"fmt"
+	"maps"
 	"sort"
 
 	"github.com/zclconf/go-cty/cty"
@@ -794,10 +795,13 @@ var MergeFunc = function.New(&function.Spec{
 			arg, _ = arg.Unmark()
 
 			switch {
-			case ty.IsObjectType() && !arg.IsNull():
-				for attr, aty := range ty.AttributeTypes() {
-					attrs[attr] = aty
+			case ty.IsObjectType():
+				if arg.IsNull() {
+					// Null objects are treated by this function as if they have
+					// no attributes at all.
+					ty = cty.EmptyObject
 				}
+				maps.Copy(attrs, ty.AttributeTypes())
 			case ty.IsMapType():
 				switch {
 				case arg.IsNull():
@@ -817,11 +821,11 @@ var MergeFunc = function.New(&function.Spec{
 
 			// record the first argument type for comparison
 			if i == 0 {
-				first = arg.Type()
+				first = ty
 				continue
 			}
 
-			if !ty.Equals(first) && matching {
+			if matching && !ty.Equals(first) {
 				matching = false
 			}
 		}

cty/function/stdlib/collection_test.go

@@ -529,7 +529,54 @@ func TestMerge(t *testing.T) {
 			cty.EmptyObjectVal,
 			false,
 		},
-		{ // single null input
+		{ // single null object with attributes
+			[]cty.Value{
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"a": cty.String,
+				})),
+			},
+			cty.EmptyObjectVal,
+			false,
+		},
+		{ // multible null objects with the same attributes
+			[]cty.Value{
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"a": cty.String,
+				})),
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"a": cty.String,
+				})),
+			},
+			cty.EmptyObjectVal,
+			false,
+		},
+		{ // multible null objects with the differing attributes
+			[]cty.Value{
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"a": cty.String,
+				})),
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"b": cty.String,
+				})),
+			},
+			cty.EmptyObjectVal,
+			false,
+		},
+		{ // mixture of null and non-null objects of the same type
+			[]cty.Value{
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"a": cty.String,
+				})),
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.StringVal("a value"),
+				}),
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("a value"),
+			}),
+			false,
+		},
+		{ // single empty map
 			[]cty.Value{
 				cty.MapValEmpty(cty.String),
 			},