lang/funcs: Don't panic if coalescelist gets an unknown list

lang/funcs/collection.go

@@ -147,6 +147,12 @@ var CoalesceListFunc = function.New(&function.Spec{
 
 		vals := make([]cty.Value, 0, len(args))
 		for _, arg := range args {
+			if !arg.IsKnown() {
+				// If we run into an unknown list at some point, we can't
+				// predict the final result yet. (If there's a known, non-empty
+				// arg before this then we won't get here.)
+				return cty.UnknownVal(retType), nil
+			}
 
 			// We already know this will succeed because of the checks in our Type func above
 			arg, _ = convert.Convert(arg, retType)

lang/funcs/collection_test.go

@@ -355,6 +355,36 @@ func TestCoalesceList(t *testing.T) {
 			cty.NilVal,
 			true,
 		},
+		{ // unknown list
+			[]cty.Value{
+				cty.ListVal([]cty.Value{
+					cty.StringVal("third"), cty.StringVal("fourth"),
+				}),
+				cty.UnknownVal(cty.List(cty.String)),
+			},
+			cty.ListVal([]cty.Value{
+				cty.StringVal("third"), cty.StringVal("fourth"),
+			}),
+			false,
+		},
+		{ // unknown list
+			[]cty.Value{
+				cty.ListValEmpty(cty.String),
+				cty.UnknownVal(cty.List(cty.String)),
+			},
+			cty.UnknownVal(cty.List(cty.String)),
+			false,
+		},
+		{ // unknown list
+			[]cty.Value{
+				cty.UnknownVal(cty.List(cty.String)),
+				cty.ListVal([]cty.Value{
+					cty.StringVal("third"), cty.StringVal("fourth"),
+				}),
+			},
+			cty.UnknownVal(cty.List(cty.String)),
+			false,
+		},
 	}
 
 	for _, test := range tests {