compiler assertion tripped with multiple comptime fn calls and default initialized array

[andrewrk]

const std = @import("std");

pub const CrossTarget = struct {
 dynamic_linker: DynamicLinker = DynamicLinker{},

 pub fn parse() void {
 var result: CrossTarget = .{ };
 result.getCpuArch();
 }

 pub fn getCpuArch(self: CrossTarget) void { }
};

pub const DynamicLinker = struct {
 buffer: [255]u8 = undefined,
};

comptime {
 CrossTarget.parse();
 CrossTarget.parse();
}

pub fn panic(msg: []const u8, stack_trace: ?*const std.builtin.StackTrace) noreturn {
 @breakpoint();
 unreachable;
}

[nix-shell:~/Downloads/zig/build]$ ./zig build-obj test.zig 
Assertion failed at /home/andy/Downloads/zig/src/analyze.cpp:5403 in can_mutate_comptime_var_state. This is a bug in the Zig compiler.
/home/andy/Downloads/zig/src-self-hosted/stage2.zig:36:5: 0xff003c in stage2_panic (zigstage2)
 @panic(ptr[0..len]);
 ^
/home/andy/Downloads/zig/src/util.cpp:20:17: 0xfb2914 in zig_panic (/home/andy/Downloads/zig/src/util.cpp)
 stage2_panic("", 0);
 ^
/home/andy/Downloads/zig/src/util_base.hpp:53:18: 0xfb302f in zig_assert (/home/andy/Downloads/zig/src/analyze.cpp)
 zig_panic("Assertion failed at %s:%d in %s. This is a bug in the Zig compiler.", file, line, func);
 ^
/home/andy/Downloads/zig/src/analyze.cpp:5403:5: 0xfc39a1 in can_mutate_comptime_var_state (/home/andy/Downloads/zig/src/analyze.cpp)
 assert(value != nullptr);
 ^
/home/andy/Downloads/zig/src/analyze.cpp:5440:58: 0xfc3a78 in can_mutate_comptime_var_state (/home/andy/Downloads/zig/src/analyze.cpp)
 if (can_mutate_comptime_var_state(&value->data.x_array.data.s_none.elements[i]))
 ^
/home/andy/Downloads/zig/src/analyze.cpp:5448:50: 0xfc3ae7 in can_mutate_comptime_var_state (/home/andy/Downloads/zig/src/analyze.cpp)
 if (can_mutate_comptime_var_state(value->data.x_struct.fields[i]))
 ^
/home/andy/Downloads/zig/src/analyze.cpp:5448:50: 0xfc3ae7 in can_mutate_comptime_var_state (/home/andy/Downloads/zig/src/analyze.cpp)
 if (can_mutate_comptime_var_state(value->data.x_struct.fields[i]))
 ^
/home/andy/Downloads/zig/src/analyze.cpp:5522:46: 0xfc3d0f in fn_eval_cacheable (/home/andy/Downloads/zig/src/analyze.cpp)
 if (can_mutate_comptime_var_state(var_scope->var->const_value))
 ^
/home/andy/Downloads/zig/src/ir.cpp:19155:43: 0xf56b5d in ir_analyze_fn_call (/home/andy/Downloads/zig/src/ir.cpp)
 bool cacheable = fn_eval_cacheable(exec_scope, return_type);
 ^
/home/andy/Downloads/zig/src/ir.cpp:19571:43: 0xf587a2 in ir_analyze_fn_call_src (/home/andy/Downloads/zig/src/ir.cpp)
 IrInstGen *result = ir_analyze_fn_call(ira, &call_instruction->base.base, fn_entry, fn_type, fn_ref,
 ^
/home/andy/Downloads/zig/src/ir.cpp:19757:42: 0xf59367 in ir_analyze_instruction_call (/home/andy/Downloads/zig/src/ir.cpp)
 return ir_analyze_fn_call_src(ira, call_instruction, fn_table_entry, fn_table_entry->type_entry,
 ^
/home/andy/Downloads/zig/src/ir.cpp:29159:47: 0xf7c5ba in ir_analyze_instruction_base (/home/andy/Downloads/zig/src/ir.cpp)
 return ir_analyze_instruction_call(ira, (IrInstSrcCall *)instruction);
 ^
/home/andy/Downloads/zig/src/ir.cpp:29473:65: 0xf7d54c in ir_analyze (/home/andy/Downloads/zig/src/ir.cpp)
 IrInstGen *new_instruction = ir_analyze_instruction_base(ira, old_instruction);
 ^
/home/andy/Downloads/zig/src/ir.cpp:13003:38: 0xf40fe2 in ir_eval_const_value (/home/andy/Downloads/zig/src/ir.cpp)
 ZigType *result_type = ir_analyze(codegen, ir_executable, analyzed_executable,
 ^
/home/andy/Downloads/zig/src/ir.cpp:19168:43: 0xf56c7a in ir_analyze_fn_call (/home/andy/Downloads/zig/src/ir.cpp)
 if ((err = ir_eval_const_value(ira->codegen, exec_scope, body_node, result_ptr,
 ^
/home/andy/Downloads/zig/src/ir.cpp:19571:43: 0xf587a2 in ir_analyze_fn_call_src (/home/andy/Downloads/zig/src/ir.cpp)
 IrInstGen *result = ir_analyze_fn_call(ira, &call_instruction->base.base, fn_entry, fn_type, fn_ref,
 ^
/home/andy/Downloads/zig/src/ir.cpp:19749:42: 0xf592ac in ir_analyze_instruction_call (/home/andy/Downloads/zig/src/ir.cpp)
 return ir_analyze_fn_call_src(ira, call_instruction, fn_table_entry, fn_type,
 ^
/home/andy/Downloads/zig/src/ir.cpp:29159:47: 0xf7c5ba in ir_analyze_instruction_base (/home/andy/Downloads/zig/src/ir.cpp)
 return ir_analyze_instruction_call(ira, (IrInstSrcCall *)instruction);
 ^
/home/andy/Downloads/zig/src/ir.cpp:29473:65: 0xf7d54c in ir_analyze (/home/andy/Downloads/zig/src/ir.cpp)
 IrInstGen *new_instruction = ir_analyze_instruction_base(ira, old_instruction);
 ^
/home/andy/Downloads/zig/src/ir.cpp:13003:38: 0xf40fe2 in ir_eval_const_value (/home/andy/Downloads/zig/src/ir.cpp)
 ZigType *result_type = ir_analyze(codegen, ir_executable, analyzed_executable,
 ^
/home/andy/Downloads/zig/src/analyze.cpp:1119:35: 0xfb6bfd in analyze_const_value (/home/andy/Downloads/zig/src/analyze.cpp)
 if ((err = ir_eval_const_value(g, scope, node, result_ptr,
 ^
/home/andy/Downloads/zig/src/analyze.cpp:3588:24: 0xfbe370 in resolve_decl_comptime (/home/andy/Downloads/zig/src/analyze.cpp)
 analyze_const_value(g, tld_comptime->base.parent_scope, expr_node, g->builtin_types.entry_void,
 ^
/home/andy/Downloads/zig/src/analyze.cpp:4182:34: 0xfbff13 in resolve_top_level_decl (/home/andy/Downloads/zig/src/analyze.cpp)
 resolve_decl_comptime(g, tld_comptime);
 ^
/home/andy/Downloads/zig/src/analyze.cpp:4981:35: 0xfc26bc in semantic_analyze (/home/andy/Downloads/zig/src/analyze.cpp)
 resolve_top_level_decl(g, tld, source_node, false);
 ^
/home/andy/Downloads/zig/src/codegen.cpp:9497:25: 0xeff751 in gen_root_source (/home/andy/Downloads/zig/src/codegen.cpp)
 semantic_analyze(g);
 ^
/home/andy/Downloads/zig/src/codegen.cpp:10506:28: 0xf031ee in codegen_build_and_link (/home/andy/Downloads/zig/src/codegen.cpp)
 gen_root_source(g);
 ^
/home/andy/Downloads/zig/src/main.cpp:1301:39: 0xed78e4 in main0 (/home/andy/Downloads/zig/src/main.cpp)
 codegen_build_and_link(g);
 ^
/home/andy/Downloads/zig/src/main.cpp:1448:24: 0xed8158 in main (/home/andy/Downloads/zig/src/main.cpp)
 auto result = main0(argc, argv);
 ^

[LemonBoy]

You beat me to the punch, I had the following patch ready:

diff --git a/src/analyze.cpp b/src/analyze.cpp
index 9b776da93..192d08fc1 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -5399,7 +5399,7 @@ bool generic_fn_type_id_eql(GenericFnTypeId *a, GenericFnTypeId *b) {
 return true;
 }
 
-static bool can_mutate_comptime_var_state(ZigValue *value) {
+static bool can_mutate_comptime_var_state(CodeGen *g, ZigValue *value) {
 assert(value != nullptr);
 switch (value->type->id) {
 case ZigTypeIdInvalid:
@@ -5431,21 +5431,26 @@ static bool can_mutate_comptime_var_state(ZigValue *value) {
 case ZigTypeIdArray:
 if (value->type->data.array.len == 0)
 return false;
+
+ expand_undef_array(g, value);
+
 switch (value->data.x_array.special) {
 case ConstArraySpecialUndef:
 case ConstArraySpecialBuf:
 return false;
 case ConstArraySpecialNone:
 for (uint32_t i = 0; i < value->type->data.array.len; i += 1) {
- if (can_mutate_comptime_var_state(&value->data.x_array.data.s_none.elements[i]))
+ if (can_mutate_comptime_var_state(g, &value->data.x_array.data.s_none.elements[i]))
 return true;
 }
 return false;
 }
 zig_unreachable();
 case ZigTypeIdStruct:
+ expand_undef_struct(g, value);
+
 for (uint32_t i = 0; i < value->type->data.structure.src_field_count; i += 1) {
- if (can_mutate_comptime_var_state(value->data.x_struct.fields[i]))
+ if (can_mutate_comptime_var_state(g, value->data.x_struct.fields[i]))
 return true;
 }
 return false;
@@ -5455,16 +5460,16 @@ static bool can_mutate_comptime_var_state(ZigValue *value) {
 return value->data.x_ptr.mut == ConstPtrMutComptimeVar;
 if (value->data.x_optional == nullptr)
 return false;
- return can_mutate_comptime_var_state(value->data.x_optional);
+ return can_mutate_comptime_var_state(g, value->data.x_optional);
 
 case ZigTypeIdErrorUnion:
 if (value->data.x_err_union.error_set->data.x_err_set != nullptr)
 return false;
 assert(value->data.x_err_union.payload != nullptr);
- return can_mutate_comptime_var_state(value->data.x_err_union.payload);
+ return can_mutate_comptime_var_state(g, value->data.x_err_union.payload);
 
 case ZigTypeIdUnion:
- return can_mutate_comptime_var_state(value->data.x_union.payload);
+ return can_mutate_comptime_var_state(g, value->data.x_union.payload);
 }
 zig_unreachable();
 }
@@ -5519,7 +5524,7 @@ bool fn_eval_cacheable(Scope *scope, ZigType *return_type) {
 return false;
 if (var_scope->var->const_value->special == ConstValSpecialUndef)
 return false;
- if (can_mutate_comptime_var_state(var_scope->var->const_value))
+ if (can_mutate_comptime_var_state(scope->codegen, var_scope->var->const_value))
 return false;
 } else if (scope->id == ScopeIdFnDef) {
 return true;
@@ -6723,9 +6728,15 @@ static bool const_values_equal_array(CodeGen *g, ZigValue *a, ZigValue *b, size_
 }
 
 bool const_values_equal(CodeGen *g, ZigValue *a, ZigValue *b) {
- if (a->type->id != b->type->id) return false;
+ if (a->type->id != b->type->id)
+ return false;
+
+ if (a->special == ConstValSpecialUndef || b->special == ConstValSpecialUndef)
+ return a->special == b->special;
+
 assert(a->special == ConstValSpecialStatic);
 assert(b->special == ConstValSpecialStatic);
+
 if (a->type == b->type) {
 switch (type_has_one_possible_value(g, a->type)) {
 case OnePossibleValueInvalid:
@@ -6736,6 +6747,7 @@ bool const_values_equal(CodeGen *g, ZigValue *a, ZigValue *b) {
 return true;
 }
 }
+
 switch (a->type->id) {
 case ZigTypeIdOpaque:
 zig_unreachable();
@@ -6795,6 +6807,9 @@ bool const_values_equal(CodeGen *g, ZigValue *a, ZigValue *b) {
 return const_values_equal_array(g, a, b, a->type->data.array.len);
 }
 case ZigTypeIdStruct:
+ expand_undef_struct(g, a);
+ expand_undef_struct(g, b);
+
 for (size_t i = 0; i < a->type->data.structure.src_field_count; i += 1) {
 ZigValue *field_a = a->data.x_struct.fields[i];
 ZigValue *field_b = b->data.x_struct.fields[i];
diff --git a/test/stage1/behavior/undefined.zig b/test/stage1/behavior/undefined.zig
index 114b0262b..bfe0c6ec5 100644
--- a/test/stage1/behavior/undefined.zig
+++ b/test/stage1/behavior/undefined.zig
@@ -67,3 +67,16 @@ test "type name of undefined" {
 const x = undefined;
 expect(mem.eql(u8, @typeName(@TypeOf(x)), "(undefined)"));
 }
+
+test "comptime caching of struct with array initialized to undefined" {
+ const S = struct {
+ buffer: [255]u8 = undefined,
+ pub fn frob(obj: @This()) void {}
+ };
+
+ comptime {
+ var result: S = .{};
+ S.frob(result);
+ S.frob(result);
+ }
+}