convert: Retain concrete types when converting from cty.DynamicPseudo…

…Type to concrete

cty/convert/conversion.go

@@ -43,14 +43,14 @@ func getConversion(in cty.Type, out cty.Type, unsafe bool) conversion {
 			out = out.WithoutOptionalAttributesDeep()
 
 			if !isKnown {
-				return cty.UnknownVal(out), nil
+				return cty.UnknownVal(dynamicReplace(in.Type(), out)), nil
 			}
 
 			if isNull {
 				// We'll pass through nulls, albeit type converted, and let
 				// the caller deal with whatever handling they want to do in
 				// case null values are considered valid in some applications.
-				return cty.NullVal(out), nil
+				return cty.NullVal(dynamicReplace(in.Type(), out)), nil
 			}
 		}
 

cty/convert/conversion_dynamic.go

@@ -31,3 +31,107 @@ func dynamicFixup(wantType cty.Type) conversion {
 func dynamicPassthrough(in cty.Value, path cty.Path) (cty.Value, error) {
 	return in, nil
 }
+
+// dynamicReplace aims to return the out type unchanged, but if it finds a
+// dynamic type either directly or in any descendent elements it replaces them
+// with the equivalent type from in.
+//
+// This function assumes that in and out are compatible from a Convert
+// perspective, and will panic if it finds that they are not. For example if
+// in is an object and out is a map, this function will still attempt to iterate
+// through both as if they were the same.
+func dynamicReplace(in, out cty.Type) cty.Type {
+	if in == cty.DynamicPseudoType || in == cty.NilType {
+		// Short circuit this case, there's no point worrying about this if in
+		// is a dynamic type or a nil type. Out is the best we can do.
+		return out
+	}
+
+	switch {
+	case out == cty.DynamicPseudoType:
+		// So replace out with in.
+		return in
+	case out.IsPrimitiveType(), out.IsCapsuleType():
+		// out is not dynamic and it doesn't contain descendent elements so just
+		// return it unchanged.
+		return out
+	case out.IsMapType():
+		var elemType cty.Type
+
+		// Maps are compatible with other maps or objects.
+		if in.IsMapType() {
+			elemType = dynamicReplace(in.ElementType(), out.ElementType())
+		}
+
+		if in.IsObjectType() {
+			var types []cty.Type
+			for _, t := range in.AttributeTypes() {
+				types = append(types, t)
+			}
+			unifiedType, _ := unify(types, true)
+			elemType = dynamicReplace(unifiedType, out.ElementType())
+		}
+
+		return cty.Map(elemType)
+	case out.IsObjectType():
+		// Objects are compatible with other objects and maps.
+		outTypes := map[string]cty.Type{}
+		if in.IsMapType() {
+			for attr, attrType := range out.AttributeTypes() {
+				outTypes[attr] = dynamicReplace(in.ElementType(), attrType)
+			}
+		}
+
+		if in.IsObjectType() {
+			for attr, attrType := range out.AttributeTypes() {
+				if !in.HasAttribute(attr) {
+					// If in does not have this attribute, then it is an
+					// optional attribute and there is nothing we can do except
+					// to return the type from out even if it is dynamic.
+					outTypes[attr] = attrType
+					continue
+				}
+				outTypes[attr] = dynamicReplace(in.AttributeType(attr), attrType)
+			}
+		}
+
+		return cty.Object(outTypes)
+	case out.IsSetType():
+		var elemType cty.Type
+
+		// Sets are compatible with other sets, lists, tuples.
+		if in.IsSetType() || in.IsListType() {
+			elemType = dynamicReplace(in.ElementType(), out.ElementType())
+		}
+
+		if in.IsTupleType() {
+			unifiedType, _ := unify(in.TupleElementTypes(), true)
+			elemType = dynamicReplace(unifiedType, out.ElementType())
+		}
+
+		return cty.Set(elemType)
+	case out.IsListType():
+		var elemType cty.Type
+
+		// Lists are compatible with other lists, sets, and tuples.
+		if in.IsSetType() || in.IsListType() {
+			elemType = dynamicReplace(in.ElementType(), out.ElementType())
+		}
+
+		if in.IsTupleType() {
+			unifiedType, _ := unify(in.TupleElementTypes(), true)
+			elemType = dynamicReplace(unifiedType, out.ElementType())
+		}
+
+		return cty.List(elemType)
+	case out.IsTupleType():
+		// Tuples are only compatible with other tuples
+		var types []cty.Type
+		for ix := 0; ix < len(out.TupleElementTypes()); ix++ {
+			types = append(types, dynamicReplace(in.TupleElementType(ix), out.TupleElementType(ix)))
+		}
+		return cty.Tuple(types)
+	default:
+		panic("unrecognized type " + out.FriendlyName())
+	}
+}

cty/convert/public_test.go

@@ -1514,6 +1514,91 @@ func TestConvert(t *testing.T) {
 				})),
 			}),
 		},
+		// Collections should prefer concrete types over dynamic types.
+		{
+			Value: cty.ListValEmpty(cty.Number),
+			Type:  cty.List(cty.DynamicPseudoType),
+			Want:  cty.ListValEmpty(cty.Number),
+		},
+		{
+			Value: cty.NullVal(cty.List(cty.Number)),
+			Type:  cty.List(cty.DynamicPseudoType),
+			Want:  cty.NullVal(cty.List(cty.Number)),
+		},
+		{
+			Value: cty.NullVal(cty.List(cty.Number)),
+			Type:  cty.Set(cty.DynamicPseudoType),
+			Want:  cty.NullVal(cty.Set(cty.Number)),
+		},
+		{
+			Value: cty.MapValEmpty(cty.Number),
+			Type:  cty.Map(cty.DynamicPseudoType),
+			Want:  cty.MapValEmpty(cty.Number),
+		},
+		{
+			Value: cty.NullVal(cty.Map(cty.Number)),
+			Type:  cty.Map(cty.DynamicPseudoType),
+			Want:  cty.NullVal(cty.Map(cty.Number)),
+		},
+		{
+			Value: cty.NullVal(cty.Map(cty.Number)),
+			Type: cty.Object(map[string]cty.Type{
+				"a": cty.DynamicPseudoType,
+			}),
+			Want: cty.NullVal(cty.Object(map[string]cty.Type{
+				"a": cty.Number,
+			})),
+		},
+		{
+			Value: cty.SetValEmpty(cty.Number),
+			Type:  cty.Set(cty.DynamicPseudoType),
+			Want:  cty.SetValEmpty(cty.Number),
+		},
+		{
+			Value: cty.NullVal(cty.Set(cty.Number)),
+			Type:  cty.Set(cty.DynamicPseudoType),
+			Want:  cty.NullVal(cty.Set(cty.Number)),
+		},
+		{
+			Value: cty.NullVal(cty.Set(cty.Number)),
+			Type:  cty.List(cty.DynamicPseudoType),
+			Want:  cty.NullVal(cty.List(cty.Number)),
+		},
+		{
+			Value: cty.NullVal(cty.Object(map[string]cty.Type{
+				"a": cty.String,
+			})),
+			Type: cty.Map(cty.DynamicPseudoType),
+			Want: cty.NullVal(cty.Map(cty.String)),
+		},
+		{
+			Value: cty.NullVal(cty.Object(map[string]cty.Type{
+				"a": cty.Object(map[string]cty.Type{
+					"b": cty.String,
+				}),
+			})),
+			Type: cty.Object(map[string]cty.Type{
+				"a": cty.Object(map[string]cty.Type{
+					"b": cty.DynamicPseudoType,
+				}),
+			}),
+			Want: cty.NullVal(cty.Object(map[string]cty.Type{
+				"a": cty.Object(map[string]cty.Type{
+					"b": cty.String,
+				}),
+			})),
+		},
+		{
+			Value: cty.NullVal(cty.Tuple([]cty.Type{
+				cty.String,
+			})),
+			Type: cty.Tuple([]cty.Type{
+				cty.DynamicPseudoType,
+			}),
+			Want: cty.NullVal(cty.Tuple([]cty.Type{
+				cty.String,
+			})),
+		},
 		// We should strip optional attributes out of types even if they match.
 		{
 			Value: cty.MapVal(map[string]cty.Value{