convert: fix panic when converting nested objects

When converting a list of objects, it is necessary to unify the types of
the child elements.

cty/convert/conversion_collection.go

@@ -265,6 +265,7 @@ func conversionTupleToList(tupleType cty.Type, listEty cty.Type, unsafe bool) co
 	// element conversions in elemConvs
 	return func(val cty.Value, path cty.Path) (cty.Value, error) {
 		elems := make([]cty.Value, 0, len(elemConvs))
+		elemTys := make([]cty.Type, 0, len(elems))
 		elemPath := append(path.Copy(), nil)
 		i := int64(0)
 		it := val.ElementIterator()
@@ -284,10 +285,15 @@ func conversionTupleToList(tupleType cty.Type, listEty cty.Type, unsafe bool) co
 				}
 			}
 			elems = append(elems, val)
+			elemTys = append(elemTys, val.Type())
 
 			i++
 		}
 
+		elems, err := conversionUnifyListElements(elems, elemPath, unsafe)
+		if err != nil {
+			return cty.NilVal, err
+		}
 		return cty.ListVal(elems), nil
 	}
 }
@@ -441,6 +447,7 @@ func conversionUnifyCollectionElements(elems map[string]cty.Value, path cty.Path
 	}
 	unifiedType, _ := unify(elemTypes, unsafe)
 	if unifiedType == cty.NilType {
+		return nil, path.NewErrorf("collection elements cannot be unified")
 	}
 
 	unifiedElems := make(map[string]cty.Value)
@@ -486,3 +493,37 @@ func conversionCheckMapElementTypes(elems map[string]cty.Value, path cty.Path) e
 
 	return nil
 }
+
+func conversionUnifyListElements(elems []cty.Value, path cty.Path, unsafe bool) ([]cty.Value, error) {
+	elemTypes := make([]cty.Type, len(elems))
+	for i, elem := range elems {
+		elemTypes[i] = elem.Type()
+	}
+	unifiedType, _ := unify(elemTypes, unsafe)
+	if unifiedType == cty.NilType {
+		return nil, path.NewErrorf("collection elements cannot be unified")
+	}
+
+	ret := make([]cty.Value, len(elems))
+	elemPath := append(path.Copy(), nil)
+
+	for i, elem := range elems {
+		if elem.Type().Equals(unifiedType) {
+			ret[i] = elem
+			continue
+		}
+		conv := getConversion(elem.Type(), unifiedType, unsafe)
+		if conv == nil {
+		}
+		elemPath[len(elemPath)-1] = cty.IndexStep{
+			Key: cty.NumberIntVal(int64(i)),
+		}
+		val, err := conv(elem, elemPath)
+		if err != nil {
+			return nil, err
+		}
+		ret[i] = val
+	}
+
+	return ret, nil
+}

cty/convert/public_test.go

@@ -649,6 +649,34 @@ func TestConvert(t *testing.T) {
 				"b": cty.MapValEmpty(cty.String),
 			}),
 		},
+		// https://github.com/hashicorp/terraform/issues/21588:
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.EmptyObjectVal,
+					"b": cty.NumberIntVal(2),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.ObjectVal(map[string]cty.Value{"var1": cty.StringVal("val1")}),
+					"b": cty.StringVal("2"),
+				}),
+			}),
+			Type: cty.List(cty.Object(map[string]cty.Type{
+				"a": cty.DynamicPseudoType,
+				"b": cty.String,
+			})),
+			Want: cty.ListVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.MapValEmpty(cty.String),
+					"b": cty.StringVal("2"),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.MapVal(map[string]cty.Value{"var1": cty.StringVal("val1")}),
+					"b": cty.StringVal("2"),
+				}),
+			}),
+			WantError: false,
+		},
 	}
 
 	for _, test := range tests {