stage2: Panic when initializing Tuple with explicit type

[topolarity]

Zig Version

0.10.0-dev.1335+c64279b15

Steps to Reproduce

test {
    const T = @TypeOf(.{ @as(i32, 0), @as(u32, 0) });
    var a = T{ 0, 0 };
    _ = a;
}

Expected Behavior

Test should pass.

Test is a minor variant of #11159, but the root cause is quite different. In this case, AstGen and the AIR expect to be able to refer to the "elem_type" of the initialized array.

Tuples are initialized like arrays but their fields are heterogeneous like structs, so this fails in Sema.

Actual Behavior

Test crashes:

thread 16327 panic: reached unreachable code
Analyzing testme.zig: testme.zig:test_0
      %2 = dbg_stmt(2, 5)
      %3 = typeof_builtin({
        %4 = as_node(@Ref.i32_type, @Ref.zero) node_offset:2:35
        %5 = as_node(@Ref.u32_type, @Ref.zero) node_offset:2:48
        %6 = array_init_anon(.{%4, %5}) node_offset:2:24
        %7 = break_inline(%3, %6)
      }) node_offset:2:15
      %8 = dbg_var_val(%3, "T")
      %9 = dbg_stmt(3, 5)
      %10 = alloc_inferred_mut() node_offset:3:5
      %11 = validate_array_init_ty(%3) node_offset:3:14
    > %12 = elem_type(%3) node_offset:3:13
      %13 = coerce_result_ptr(%3, %10)
      %14 = elem_ptr_imm(%13, 0) node_offset:3:16
      %15 = store_node(%14, @Ref.zero) node_offset:3:16
      %16 = elem_ptr_imm(%13, 1) node_offset:3:19
      %17 = store_node(%16, @Ref.zero) node_offset:3:19
      %18 = validate_array_init({
        %14 = elem_ptr_imm(%13, 0) node_offset:3:16
        %16 = elem_ptr_imm(%13, 1) node_offset:3:19
      }) node_offset:3:14
      %19 = resolve_inferred_alloc(%10) node_offset:3:5
      %20 = dbg_var_ptr(%10, "a")
      %21 = dbg_stmt(4, 5)
      %22 = load(%10) node_offset:4:9
      %23 = ensure_result_non_error(%22) node_offset:4:9
      %24 = ret_tok(@Ref.void_value) token_offset:5:1
    For full context, use the command
      zig ast-check -t testme.zig


/home/topolarity/repos/zig/src/type.zig:3374:21: 0x2c85805 in type.Type.childType (zig)
            else => unreachable,
                    ^
/home/topolarity/repos/zig/src/Sema.zig:5204:42: 0x331096b in Sema.zirElemType (zig)
    const elem_type = array_type.elemType();
                                         ^
/home/topolarity/repos/zig/src/Sema.zig:671:66: 0x31cc82c in Sema.analyzeBodyInner (zig)
            .elem_type                    => try sema.zirElemType(block, inst),
                                                                 ^
/home/topolarity/repos/zig/src/Sema.zig:541:30: 0x31c035a in Sema.analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
    ```