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);
+    }
+}