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Merged
nv-cmeerw merged 10 commits into from
Oct 9, 2026
+238 −62
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diagnose auto deduction from void more specifically, fix for [GH #27] #206

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748bb42
diagnose auto deduction from void more specifically. [GH #27]
mohitmishra786 Oct 3, 2026
61356ec
report an incomplete type when plain auto is deduced from void. [GH #27]
mohitmishra786 Oct 5, 2026
ed026f4
Keep the GH #27 Changes entry above the GH #213 note. [GH #27]
mohitmishra786 Oct 5, 2026
bcef7db
Keep the GH #27 Changes entry at the top of version 7.1. [GH #27]
mohitmishra786 Oct 6, 2026
90a5e26
Address review comments on the incomplete-auto deduction change. [GH …
mohitmishra786 Oct 6, 2026
aef05b9
Update recordings for auto deduction from an incomplete type. [GH #27]
mohitmishra786 Oct 7, 2026
3056726
Show why incomplete auto is not diagnosed in a template definition. […
mohitmishra786 Oct 8, 2026
a629bee
Place the GH #27 Changes entry above the GH #203 note. [GH #27]
mohitmishra786 Oct 8, 2026
d6f5618
Keep incomplete-type errors in template definitions outside GNU and M…
mohitmishra786 Oct 8, 2026
4589f92
Add only the GH #27 entry to Changes and record GH27_a for edg_x86_32…
mohitmishra786 Oct 8, 2026
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15 changes: 15 additions & 0 deletions src/Changes
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Original file line number Diff line number Diff line change
@@ -1,5 +1,20 @@
Version 7.1

10/9/26 [GH #27]
Clearer diagnostic when auto is deduced from an incomplete type

Deducing a plain "auto" (including cv-qualified "auto") from void or another
incomplete type used to be reported as a deduction failure. The type is now
deduced, and the usual incomplete-type diagnostic is issued, as it already
is for decltype(auto). For example:

void g();
auto x = g(); // Error: incomplete type "void" is not allowed
auto *p = g(); // Error: cannot deduce "auto" type

Forms that are not a plain "auto", such as "auto *", still fail deduction.


10/7/26 [GH #176]
C-generating back end: always_inline attribute and inlined setjmp calls

Expand Down
34 changes: 26 additions & 8 deletions src/overload.c
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Expand Up @@ -65,7 +65,8 @@ static a_boolean adjust_deduction_pair(
a_template_arg_ptr template_arg_list,
a_type_ptr *qc_param_type,
a_type_ptr *qc_arg_type,
a_boolean *consider_nondeduced);
a_boolean *consider_nondeduced,
a_boolean allow_incomplete_arg);
static a_boolean check_narrowing_conversion(an_operand *source_operand,
a_type_ptr dest_type,
a_boolean check_enum_target,
Expand Down Expand Up @@ -1485,7 +1486,8 @@ from previous arguments; in the standard case, it is always NULL.
(an_operand *)NULL, templ_params,
(a_template_arg *)NULL,
(a_type_ptr*)NULL, (a_type_ptr*)NULL,
(a_boolean *)NULL) &&
(a_boolean *)NULL,
/*allow_incomplete_arg=*/FALSE) &&
tentatively_matches_template_type(local_arg_type,
local_param_type,
templ_params,
Expand Down Expand Up @@ -4272,7 +4274,8 @@ static a_boolean adjust_deduction_pair(
a_template_arg_ptr template_arg_list,
a_type_ptr *qc_param_type,
a_type_ptr *qc_arg_type,
a_boolean *consider_nondeduced)
a_boolean *consider_nondeduced,
a_boolean allow_incomplete_arg)
/*
Adjust the types *p_param_type (a parameter type of a function template or a
type involving the "auto" type specifier) and *p_arg_type (the type of the
Expand All @@ -4295,7 +4298,10 @@ are not removed. If consider_nondeduced is non-NULL and the reason for
failure is that an indefinite function matches several ways, return
*consider_nondeduced TRUE. template_arg_list is used in some nonstandard
modes to introduce knowledge from previous arguments; in the standard case,
it is always NULL.
it is always NULL. allow_incomplete_arg allows an incomplete argument
type (including void) to be used for deduction. It is used for placeholder
type deduction, where an incomplete type error is diagnosed by the caller
instead of a deduction failure.
*/
{
a_boolean adjustment_okay = FALSE, indefinite_function_designator = FALSE;
Expand Down Expand Up @@ -4399,7 +4405,8 @@ it is always NULL.
void m() { f(*p); }
*/
complete_type_is_needed(arg_type);
if (is_incomplete_type(arg_type) && !is_managed_nullptr_type(arg_type)) {
if (is_incomplete_type(arg_type) && !is_managed_nullptr_type(arg_type) &&
!allow_incomplete_arg) {
/* Although the managed (C++/CLI) nullptr type is incomplete and
cannot be used as the type of an object, for example, the Microsoft
compiler allows it as a template argument. */
Expand Down Expand Up @@ -4621,7 +4628,8 @@ deduction was successful: some cases are treated as "nondeduced contexts").
elem_operand,
templ_params, *template_arg_list,
&qc_param_type, &qc_arg_type,
&consider_nondeduced)) {
&consider_nondeduced,
/*allow_incomplete_arg=*/FALSE)) {
if (consider_nondeduced) continue;
deduction_okay = FALSE;
break;
Expand Down Expand Up @@ -4775,7 +4783,8 @@ deduction succeeds, FALSE if it fails.
if (!adjust_deduction_pair(&param_type, &arg_type, operand,
templ_params, *template_arg_list,
&qc_param_type, &qc_arg_type,
&consider_nondeduced)) {
&consider_nondeduced,
/*allow_incomplete_arg=*/FALSE)) {
if (consider_nondeduced) {
/* The argument is an indefinite function that can match in more than
one way. Keep going without adding anything to the template
Expand Down Expand Up @@ -31235,10 +31244,19 @@ TRUE and FALSE is returned.
/* Adjust the argument and parameter types for deduction. Some types can
never succeed: Issue an error and don't attempt deduction any
further. */
/* GCC and Microsoft don't diagnose incomplete-type errors in template
definitions, but for now the only way for us to emulate that is by
failing deduction in these cases (which we then allow, see
prescan_initializer_for_auto_type_deduction). */
a_boolean allow_incomplete_arg =
!((gpp_version_is(any_version) || ms_version_is(any_version)) &&

@nv-cmeerw nv-cmeerw Oct 7, 2026

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The need for this change isn't entirely obvious. Was there any test case that would show undesirable results without this change? If so, it might be worth adding one example to your GH27.sft.cpp test to demonstrate the need for this change.

Thanks for your continued work on this issue.

@mohitmishra786 mohitmishra786 Oct 8, 2026

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yes. Without that check, a template definition is diagnosed even when the type is completed before instantiation:

struct Later;
extern Later later_obj;
template<int> void use_later() { auto x = later_obj; }
struct Later { int n; };
void call_later() { use_later<0>(); }

A deduction failure in the template definition is already suppressed in GNU and Microsoft modes. Diagnosing an incomplete type there instead turned decomp44 and decomp45 into errors. The check keeps that suppression. GH27 now has a --gnu_version run with this template, and it produces no diagnostic for use_later. The errors outside the template are unchanged.

@nv-cmeerw nv-cmeerw Oct 8, 2026

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Thanks for the explanation.

Unfortunately, it's a bit more complicated. So GCC and MSVC don't diagnose substitution failures and incomplete-type errors in template definition. We are only emulating the substitution failure part, but because we treated an incomplete type deduction as a substitution failure we kind of emulated that part as well.

Your change would mean that for auto var = void() inside a template definition we wouldn't get the "incomplete type" error in standard or Clang modes any more (but instead get the old deduction failure error).

I think for now we'll have to duplicate the condition here:

      /* GCC and Microsoft don't diagnose incomplete-type errors in template
         definitions, but for now the only way for us to emulate that is by
         failing deduction in these cases (which we then allow, see
         prescan_initializer_for_auto_type_deduction). */
      a_boolean  allow_incomplete_arg =
               !((gpp_version_is(any_version) || ms_version_is(any_version)) &&
                 scope_stack_top().in_prototype_instantiation &&
                 innermost_function_scope != NULL);

and run the template definition test not just in GCC mode, but all possible modes, i.e. --c++11 -A:--c++11 --gnu_version 160200:--c++11 --clang_version 230100:--ms_c++20 --microsoft_version 1951

@mohitmishra786 mohitmishra786 Oct 8, 2026

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Thanks, that makes sense. I had folded the incomplete-type case into the GNU/Microsoft suppression, so standard and Clang modes went back to the deduction failure error for auto var = void() in a template definition.

I've used your condition and comment as is, so incomplete-type deduction is only failed in a template definition in GNU and Microsoft modes, where prescan_initializer_for_auto_type_deduction then allows it.

The template definition test is now a separate test, changes/GH27_a, because GH27 needs --c++23 for the explicit object parameter. GH27_a runs with --c++11 -A:--c++11 --gnu_version 160200:--c++11 --clang_version 230100:--ms_c++20 --microsoft_version 1951. It also has a non-template auto v = void() so that every mode has an error, as the test type requires. Results:

  • strict mode: "incomplete type" errors in the template definitions
  • GNU and Microsoft modes: no diagnostic in the template definitions
  • Clang mode: "incomplete type" errors in the template definitions with the C++-generating back end config (recorded as edg_x86_64_cp.3.1.txt). In the default config, Clang mode reports nothing inside an uninstantiated template body. That isn't specific to this change: a template body with no auto and int x = "s"; isn't diagnosed there either.

scope_stack_top().in_prototype_instantiation &&
innermost_function_scope != NULL);
if (!adjust_deduction_pair(&type, &arg_type, initializer_operand,
templ_param, (a_template_arg *)NULL,
&qc_param_type, &qc_arg_type,
(a_boolean *)NULL)) {
(a_boolean *)NULL,
allow_incomplete_arg)) {
okay = FALSE;
} else if (!deduce_from_one_pair(type, arg_type,
qc_param_type, qc_arg_type,
Expand Down
30 changes: 30 additions & 0 deletions tests/tests/changes/.GH27.rto/default.1.1.txt
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@@ -0,0 +1,30 @@
fe_only -DTEST_NUMBER=1 --c++23 Test_name.c
"Test_name.c", line 27: error: incomplete type "void" is not allowed
auto v1 = void();
^

"Test_name.c", line 28: error: incomplete type "void" is not allowed
decltype(auto) v2 = void();
^

"Test_name.c", line 29: error: incomplete type "Incomplete" is not allowed
auto v3 = getIncomplete();
^

"Test_name.c", line 30: error: incomplete type "void" is not allowed
auto opts = Args{}.parse();
^

"Test_name.c", line 32: error: cannot deduce "auto" type
auto *v4 = void();
^

"Test_name.c", line 33: error: cannot deduce "auto" type
auto *v5 = 1;
^

"Test_name.c", line 34: error: cannot deduce "auto" type
auto *p = g();
^

7 errors detected in the compilation of "Test_name.c".
14 changes: 14 additions & 0 deletions tests/tests/changes/.GH27_a.rto/default.1.1.txt
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@@ -0,0 +1,14 @@
fe_only -DTEST_NUMBER=1 --c++11 -A Test_name.c
"Test_name.c", line 14: error: incomplete type "void" is not allowed
auto v = void();
^

"Test_name.c", line 20: error: incomplete type "void" is not allowed
auto v = void();
^

"Test_name.c", line 29: error: incomplete type "Later" is not allowed
auto x = later_obj;
^

3 errors detected in the compilation of "Test_name.c".
6 changes: 6 additions & 0 deletions tests/tests/changes/.GH27_a.rto/default.2.1.txt
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@@ -0,0 +1,6 @@
fe_only -DTEST_NUMBER=2 --c++11 --gnu_version 160200 Test_name.c
"Test_name.c", line 14: error: incomplete type "void" is not allowed
auto v = void();
^

1 error detected in the compilation of "Test_name.c".
6 changes: 6 additions & 0 deletions tests/tests/changes/.GH27_a.rto/default.3.1.txt
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@@ -0,0 +1,6 @@
fe_only -DTEST_NUMBER=3 --c++11 --clang_version 230100 Test_name.c
"Test_name.c", line 14: error: incomplete type "void" is not allowed
auto v = void();
^

1 error detected in the compilation of "Test_name.c".
6 changes: 6 additions & 0 deletions tests/tests/changes/.GH27_a.rto/default.4.1.txt
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fe_only -DTEST_NUMBER=4 --ms_c++20 --microsoft_version 1951 Test_name.c
"Test_name.c", line 14: error: incomplete type "void" is not allowed
auto v = void();
^

1 error detected in the compilation of "Test_name.c".
14 changes: 14 additions & 0 deletions tests/tests/changes/.GH27_a.rto/edg_x86_32_cp.3.1.txt
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@@ -0,0 +1,14 @@
fe_only -DTEST_NUMBER=3 --c++11 --clang_version 230100 Test_name.c
"Test_name.c", line 14: error: incomplete type "void" is not allowed
auto v = void();
^

"Test_name.c", line 20: error: incomplete type "void" is not allowed
auto v = void();
^

"Test_name.c", line 29: error: incomplete type "Later" is not allowed
auto x = later_obj;
^

3 errors detected in the compilation of "Test_name.c".
14 changes: 14 additions & 0 deletions tests/tests/changes/.GH27_a.rto/edg_x86_64_cp.3.1.txt
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@@ -0,0 +1,14 @@
fe_only -DTEST_NUMBER=3 --c++11 --clang_version 230100 Test_name.c
"Test_name.c", line 14: error: incomplete type "void" is not allowed
auto v = void();
^

"Test_name.c", line 20: error: incomplete type "void" is not allowed
auto v = void();
^

"Test_name.c", line 29: error: incomplete type "Later" is not allowed
auto x = later_obj;
^

3 errors detected in the compilation of "Test_name.c".
36 changes: 36 additions & 0 deletions tests/tests/changes/GH27.sft.cpp
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@@ -0,0 +1,36 @@
//type:fn
//options:--c++23
//remark:[GH #27] Clearer diagnostic when auto deduces to void
// 10/9/26 [GH #27]
//
// A plain auto deduced from void or another incomplete type gets the
// incomplete-type diagnostic, matching decltype(auto). auto * still
// fails deduction.

struct Incomplete;

struct Incomplete &getIncomplete();

struct Clap {
template <typename Spec>
auto parse(this Spec const& spec) {
}
};

struct Args : Clap {
};

void g();

void f()
{
auto v1 = void();
decltype(auto) v2 = void();
auto v3 = getIncomplete();
auto opts = Args{}.parse();

auto *v4 = void();
auto *v5 = 1;
auto *p = g();
}

37 changes: 37 additions & 0 deletions tests/tests/changes/GH27_a.sft.cpp
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//type:fn
//options:--c++11 -A:--c++11 --gnu_version 160200:--c++11 --clang_version 230100:--ms_c++20 --microsoft_version 1951
//remark:[GH #27] Incomplete auto deduction in a template definition
// 10/9/26 [GH #27]
//
// In standard and Clang modes, a plain auto deduced from an incomplete
// type in a template definition gets the incomplete-type diagnostic. GCC
// and Microsoft don't diagnose that in a template definition, so in GNU and
// Microsoft modes the definition is still accepted. Outside a template,
// the incomplete-type diagnostic is issued in every mode.

void not_a_template()
{
auto v = void();
}

template<int>
void use_void()
{
auto v = void();
}

struct Later;
extern Later later_obj;

template<int>
void use_later()
{
auto x = later_obj;
}

struct Later { int n; };

void call_later()
{
use_later<0>();
}
4 changes: 2 additions & 2 deletions ...imported/clang/cpp/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.spec.auto/.p2-1z.rto/default.1.1.txt
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Expand Up @@ -12,8 +12,8 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
^
detected during instantiation of "auto d<k>() [with k=0]" at line 41

"Test_name.c", line 46: error: cannot deduce "auto" type
"Test_name.c", line 46: error: incomplete type "void" is not allowed
auto e = []{ if constexpr (false) return 0; }(); // expected-error {{variable has incomplete type 'void'}}
^
^

2 errors detected in the compilation of "Test_name.c".
2 changes: 1 addition & 1 deletion .../tests/imported/clang/cpp/CXX/expr/expr.prim/expr.prim.lambda/.p11-1y.rto/default.1.1.txt
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Expand Up @@ -31,7 +31,7 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
auto bad_init_3 = [&a(void_fn())] {}; // expected-error {{cannot form a reference to 'void'}}
^

"Test_name.c", line 51: error: cannot deduce "auto" type
"Test_name.c", line 51: error: incomplete type "void" is not allowed
auto bad_init_4 = [a(void_fn())] {}; // expected-error {{has incomplete type 'void'}}
^

Expand Down
6 changes: 1 addition & 5 deletions tests/tests/imported/clang/cpp/CXX/stmt.stmt/stmt.iter/stmt.ranged/.p1.rto/default.1.1.txt
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Expand Up @@ -127,10 +127,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
for (auto u : NoEnd()) { // expected-error {{no viable 'end' function available}}
^

"Test_name.c", line 180: error: cannot deduce "auto" type
for (auto u : NoIncr()) { // expected-error {{arithmetic on a pointer to void}}\
^

"Test_name.c", line 180: error: incomplete type "void" is not allowed
for (auto u : NoIncr()) { // expected-error {{arithmetic on a pointer to void}}\
^
Expand Down Expand Up @@ -305,4 +301,4 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
^
detected during instantiation of "void rdar13712739::foo(const T &) [with T=int]" at line 290

55 errors detected in the compilation of "Test_name.c".
54 errors detected in the compilation of "Test_name.c".
6 changes: 1 addition & 5 deletions tests/tests/imported/clang/cpp/CXX/stmt.stmt/stmt.iter/stmt.ranged/.p1.rto/default.2.1.txt
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Expand Up @@ -127,10 +127,6 @@ fe_only -DTEST_NUMBER=2 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
for (auto u : NoEnd()) { // expected-error {{no viable 'end' function available}}
^

"Test_name.c", line 180: error: cannot deduce "auto" type
for (auto u : NoIncr()) { // expected-error {{arithmetic on a pointer to void}}\
^

"Test_name.c", line 180: error: incomplete type "void" is not allowed
for (auto u : NoIncr()) { // expected-error {{arithmetic on a pointer to void}}\
^
Expand Down Expand Up @@ -305,4 +301,4 @@ fe_only -DTEST_NUMBER=2 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
^
detected during instantiation of "void rdar13712739::foo(const T &) [with T=int]" at line 290

55 errors detected in the compilation of "Test_name.c".
54 errors detected in the compilation of "Test_name.c".
6 changes: 1 addition & 5 deletions tests/tests/imported/clang/cpp/CXX/stmt.stmt/stmt.iter/stmt.ranged/.p1.rto/default.3.1.txt
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Expand Up @@ -131,10 +131,6 @@ fe_only -DTEST_NUMBER=3 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
for (auto u : NoEnd()) { // expected-error {{no viable 'end' function available}}
^

"Test_name.c", line 180: error: cannot deduce "auto" type
for (auto u : NoIncr()) { // expected-error {{arithmetic on a pointer to void}}\
^

"Test_name.c", line 180: error: incomplete type "void" is not allowed
for (auto u : NoIncr()) { // expected-error {{arithmetic on a pointer to void}}\
^
Expand Down Expand Up @@ -309,4 +305,4 @@ fe_only -DTEST_NUMBER=3 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
^
detected during instantiation of "void rdar13712739::foo(const T &) [with T=int]" at line 290

56 errors detected in the compilation of "Test_name.c".
55 errors detected in the compilation of "Test_name.c".
4 changes: 2 additions & 2 deletions tests/tests/imported/clang/cpp/SemaCXX/.PR172001.rto/default.3.1.txt
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@@ -1,6 +1,6 @@
fe_only -DTEST_NUMBER=3 --clang -tused -e 200 --no_wrap_diagnostics --clang_version 999999 --c++20 -DTEST3 Test_name.c
"Test_name.c", line 46: error: cannot deduce "auto" type
"Test_name.c", line 46: error: incomplete type "std::partial_ordering" is not allowed
auto res = a <=> b;
^
^

1 error detected in the compilation of "Test_name.c".
10 changes: 5 additions & 5 deletions tests/tests/imported/clang/cpp/SemaCXX/.auto-subst-failure.rto/default.1.1.txt
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_version 999999 --c++11 Test_name.c
"Test_name.c", line 6: error: cannot deduce "auto" type
"Test_name.c", line 6: error: incomplete type "void" is not allowed
auto a = f(); // expected-error {{variable has incomplete type 'void'}}
^
^

"Test_name.c", line 7: error: cannot deduce "auto" type
auto &b = f(); // expected-error {{cannot form a reference to 'void'}}
Expand All @@ -11,9 +11,9 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
auto *c = f(); // expected-error {{incompatible initializer of type 'void'}}
^

"Test_name.c", line 10: error: cannot deduce "auto" type
"Test_name.c", line 10: error: incomplete type "void" is not allowed
auto d(f()); // expected-error {{variable has incomplete type 'void'}}
^
^

"Test_name.c", line 11: error: cannot deduce "auto" type
auto &&e(f()); // expected-error {{cannot form a reference to 'void'}}
Expand All @@ -23,7 +23,7 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
auto *g(f()); // expected-error {{incompatible initializer of type 'void'}}
^

"Test_name.c", line 14: error: cannot deduce "auto" type
"Test_name.c", line 14: error: incomplete type "void" is not allowed
(void)new auto(f()); // expected-error {{allocation of incomplete type 'void'}}
^

Expand Down
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