flatten: null dynamic values are known

When flattening a list of arguments, the special case of a null
dynamic value should not force the result to be unknown. We do not check
directly for "InKnown" here, because it is specifically the dynamic type
which will cause flatten to become unknown, while unknown values alone
are preserved.

cty/function/stdlib/collection.go

@@ -538,7 +538,7 @@ func flattener(flattenList cty.Value) ([]cty.Value, []cty.ValueMarks, bool) {
 
 		// Any dynamic types could result in more collections that need to be
 		// flattened, so the type cannot be known.
-		if val.Type().Equals(cty.DynamicPseudoType) {
+		if val.Type().Equals(cty.DynamicPseudoType) && !val.IsNull() {
 			isKnown = false
 		}
 

cty/function/stdlib/collection_test.go

@@ -2031,6 +2031,39 @@ func TestFlatten(t *testing.T) {
 			}),
 			"",
 		},
+		{
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.TupleVal([]cty.Value{
+					cty.StringVal("c"),
+				}),
+			}),
+			cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("b"),
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.StringVal("c"),
+			}),
+			"",
+		},
+		{
+			cty.TupleVal([]cty.Value{
+				cty.TupleVal([]cty.Value{
+					cty.StringVal("a"),
+					cty.StringVal("b"),
+				}),
+				cty.UnknownVal(cty.DynamicPseudoType),
+				cty.TupleVal([]cty.Value{
+					cty.StringVal("c"),
+				}),
+			}),
+			cty.UnknownVal(cty.DynamicPseudoType),
+			"",
+		},
 	}
 
 	for _, test := range tests {