[clang] [clang] Deduce _BitInt(N) template parameter as size_t (PR #195534)
Jan Schultke via cfe-commits
cfe-commits at lists.llvm.org
Sun May 3 09:29:16 PDT 2026
https://github.com/eisenwave updated https://github.com/llvm/llvm-project/pull/195534
>From 9e700211401821a4238f6088729236aab2e52915 Mon Sep 17 00:00:00 2001
From: Eisenwave <me at eisenwave.net>
Date: Sun, 3 May 2026 17:17:24 +0200
Subject: [PATCH 1/3] [clang] Deduce _BitInt(N) template parameter as size_t
Update template argument deduction to deduce the N in _BitInt(N) as size_t rather than int.
This increases consistency with deduction of array sizes, and matches the behavior proposed in P3666.
---
clang/lib/Sema/SemaTemplateDeduction.cpp | 7 ++++--
clang/test/SemaTemplate/deduction.cpp | 27 ++++++++++++++++++++++++
2 files changed, 32 insertions(+), 2 deletions(-)
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index a287319cc4f88..3528fa001c201 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -2521,11 +2521,14 @@ static TemplateDeductionResult DeduceTemplateArgumentsByTypeMatch(
if (!NTTP)
return TemplateDeductionResult::Success;
- llvm::APSInt ArgSize(S.Context.getTypeSize(S.Context.IntTy), false);
+ // P3666 suggested wording for [temp.deduct.type]:
+ // The type of N in the type _BitInt(N) is std::size_t.
+ QualType T = S.Context.getSizeType();
+ llvm::APSInt ArgSize(S.Context.getTypeSize(T), false);
ArgSize = IA->getNumBits();
return DeduceNonTypeTemplateArgument(
- S, TemplateParams, NTTP, ArgSize, S.Context.IntTy, true, Info,
+ S, TemplateParams, NTTP, ArgSize, T, true, Info,
POK != PartialOrderingKind::None, Deduced, HasDeducedAnyParam);
}
diff --git a/clang/test/SemaTemplate/deduction.cpp b/clang/test/SemaTemplate/deduction.cpp
index a209615c36479..e1ac6a9e5d267 100644
--- a/clang/test/SemaTemplate/deduction.cpp
+++ b/clang/test/SemaTemplate/deduction.cpp
@@ -633,6 +633,33 @@ namespace dependent_list_deduction {
}
}
+namespace bitint_deduction {
+#if __cplusplus >= 201703L
+ template<auto X> void f(unsigned _BitInt(X)) {
+ static_assert(is_same<decltype(X), decltype(sizeof(0))>::value, "");
+ static_assert(X == 31, "");
+ }
+ template<auto X> void g(_BitInt(X)) {
+ static_assert(is_same<decltype(X), decltype(sizeof(0))>::value, "");
+ static_assert(X == 32, "");
+ }
+ template<typename T, T V> void i(unsigned _BitInt(V)) {
+ static_assert(is_same<T, decltype(sizeof(0))>::value, "");
+ static_assert(V == 33, "");
+ }
+ template<typename T, T V> void j(_BitInt(V)) {
+ static_assert(is_same<T, decltype(sizeof(0))>::value, "");
+ static_assert(V == 34, "");
+ }
+ void h() {
+ f(static_cast<unsigned _BitInt(31)>(0));
+ g(static_cast<_BitInt(32)>(0));
+ i(static_cast<unsigned _BitInt(33)>(0));
+ j(static_cast<_BitInt(34)>(0));
+ }
+#endif
+}
+
namespace designators {
template<typename T, int N> constexpr int f(T (&&)[N]) { return N; } // expected-note 2{{couldn't infer template argument 'T'}}
static_assert(f({1, 2, [20] = 3}) == 3, ""); // expected-error {{no matching function}} expected-warning 2{{C99}} expected-note {{}}
>From 6519a0e71345d54bb4853d02cbad325c052613c2 Mon Sep 17 00:00:00 2001
From: Eisenwave <me at eisenwave.net>
Date: Sun, 3 May 2026 18:27:47 +0200
Subject: [PATCH 2/3] Move test to ext-int.cpp, add size_t
---
clang/test/SemaCXX/ext-int.cpp | 50 ++++++++++++++++++++++-----
clang/test/SemaTemplate/deduction.cpp | 27 ---------------
2 files changed, 41 insertions(+), 36 deletions(-)
diff --git a/clang/test/SemaCXX/ext-int.cpp b/clang/test/SemaCXX/ext-int.cpp
index 5c566dafed931..cb43ada30d13b 100644
--- a/clang/test/SemaCXX/ext-int.cpp
+++ b/clang/test/SemaCXX/ext-int.cpp
@@ -61,6 +61,38 @@ template<typename T>
void deduced_whole_type(T){}
template<int I>
void deduced_bound(_BitInt(I)){}
+template<int I>
+void deduced_bound_unsigned(unsigned _BitInt(I)){}
+
+template <typename T, typename U>
+struct is_same {
+ static constexpr bool value = false;
+};
+template <typename T>
+struct is_same<T,T> {
+ static constexpr bool value = true;
+};
+
+using size_t = decltype(sizeof(0));
+
+#if __cplusplus >= 201703L
+template<auto X> void deduced_bound_auto(_BitInt(X)) {
+ static_assert(is_same<decltype(X), size_t>::value, "");
+ static_assert(X == 9, "");
+}
+template<auto X> void deduced_bound_auto_unsigned(unsigned _BitInt(X)) {
+ static_assert(is_same<decltype(X), size_t>::value, "");
+ static_assert(X == 11, "");
+}
+template<typename T, T V> void deduced_bound_dependent(_BitInt(V)) {
+ static_assert(is_same<T, size_t>::value, "");
+ static_assert(V == 9, "");
+}
+template<typename T, T V> void deduced_bound_dependent_unsigned(unsigned _BitInt(V)) {
+ static_assert(is_same<T, size_t>::value, "");
+ static_assert(V == 11, "");
+}
+#endif
// Ensure ext-int can be used in template places.
void Templates() {
@@ -69,20 +101,20 @@ void Templates() {
ExtIntTemplParam<b> c;
constexpr _BitInt(9) d = 1;
ExtIntTemplParam<b> e;
+ constexpr unsigned _BitInt(11) f = 1;
deduced_whole_type(b);
+ deduced_whole_type(f);
deduced_bound(b);
+ deduced_bound_unsigned(f);
+#if __cplusplus >= 201703L
+ deduced_bound_auto(d);
+ deduced_bound_auto_unsigned(f);
+ deduced_bound_dependent(d);
+ deduced_bound_dependent_unsigned(f);
+#endif
}
-template <typename T, typename U>
-struct is_same {
- static constexpr bool value = false;
-};
-template <typename T>
-struct is_same<T,T> {
- static constexpr bool value = true;
-};
-
// Reject vector types:
// expected-error at +1{{'_BitInt' vector element width must be a power of 2}}
typedef _BitInt(5) __attribute__((vector_size(16))) VecTy3;
diff --git a/clang/test/SemaTemplate/deduction.cpp b/clang/test/SemaTemplate/deduction.cpp
index e1ac6a9e5d267..a209615c36479 100644
--- a/clang/test/SemaTemplate/deduction.cpp
+++ b/clang/test/SemaTemplate/deduction.cpp
@@ -633,33 +633,6 @@ namespace dependent_list_deduction {
}
}
-namespace bitint_deduction {
-#if __cplusplus >= 201703L
- template<auto X> void f(unsigned _BitInt(X)) {
- static_assert(is_same<decltype(X), decltype(sizeof(0))>::value, "");
- static_assert(X == 31, "");
- }
- template<auto X> void g(_BitInt(X)) {
- static_assert(is_same<decltype(X), decltype(sizeof(0))>::value, "");
- static_assert(X == 32, "");
- }
- template<typename T, T V> void i(unsigned _BitInt(V)) {
- static_assert(is_same<T, decltype(sizeof(0))>::value, "");
- static_assert(V == 33, "");
- }
- template<typename T, T V> void j(_BitInt(V)) {
- static_assert(is_same<T, decltype(sizeof(0))>::value, "");
- static_assert(V == 34, "");
- }
- void h() {
- f(static_cast<unsigned _BitInt(31)>(0));
- g(static_cast<_BitInt(32)>(0));
- i(static_cast<unsigned _BitInt(33)>(0));
- j(static_cast<_BitInt(34)>(0));
- }
-#endif
-}
-
namespace designators {
template<typename T, int N> constexpr int f(T (&&)[N]) { return N; } // expected-note 2{{couldn't infer template argument 'T'}}
static_assert(f({1, 2, [20] = 3}) == 3, ""); // expected-error {{no matching function}} expected-warning 2{{C99}} expected-note {{}}
>From 084dc0cbb1326dde31c3f1c75ba25cdb117047c8 Mon Sep 17 00:00:00 2001
From: Jan Schultke <me at eisenwave.net>
Date: Sun, 3 May 2026 18:29:06 +0200
Subject: [PATCH 3/3] Update clang/lib/Sema/SemaTemplateDeduction.cpp
Co-authored-by: Corentin Jabot <corentinjabot at gmail.com>
---
clang/lib/Sema/SemaTemplateDeduction.cpp | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 3528fa001c201..947f742600a64 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -2521,8 +2521,7 @@ static TemplateDeductionResult DeduceTemplateArgumentsByTypeMatch(
if (!NTTP)
return TemplateDeductionResult::Success;
- // P3666 suggested wording for [temp.deduct.type]:
- // The type of N in the type _BitInt(N) is std::size_t.
+ // Deduce the size parameter of _BitInt as std::size_t
QualType T = S.Context.getSizeType();
llvm::APSInt ArgSize(S.Context.getTypeSize(T), false);
ArgSize = IA->getNumBits();
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