[Clang] allow restrict qualifier for array types with pointer types as element types (#120896)

Fixes #92847

---

> Types other than pointer types whose referenced type is an object type
and (possibly multi-dimensional) array types with such pointer types as
element type shall not be restrict-qualified.

clang/docs/ReleaseNotes.rst

@@ -93,6 +93,7 @@ C Language Changes
 
 - Clang now allows an ``inline`` specifier on a typedef declaration of a
   function type in Microsoft compatibility mode. #GH124869
+- Clang now allows ``restrict`` qualifier for array types with pointer elements (#GH92847).
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7413,6 +7413,11 @@ def warn_c23_compat_utf8_string : Warning<
 def note_cxx20_c23_compat_utf8_string_remove_u8 : Note<
   "remove 'u8' prefix to avoid a change of behavior; "
   "Clang encodes unprefixed narrow string literals as UTF-8">;
+def warn_c23_compat_restrict_on_array_of_pointers : Warning<
+  "'restrict' qualifier on an array of pointers is incompatible with C standards before C23">,
+  InGroup<CPre23Compat>, DefaultIgnore;
+def ext_restrict_on_array_of_pointers_c23 : Extension<
+  "'restrict' qualifier on an array of pointers is a C23 extension">, InGroup<C23>;
 def err_array_init_different_type : Error<
   "cannot initialize array %diff{of type $ with array of type $|"
   "with different type of array}0,1">;

clang/lib/Sema/SemaType.cpp

@@ -1593,35 +1593,38 @@ QualType Sema::BuildQualifiedType(QualType T, SourceLocation Loc,
   // object or incomplete types shall not be restrict-qualified."
   if (Qs.hasRestrict()) {
     unsigned DiagID = 0;
-    QualType ProblemTy;
+    QualType EltTy = Context.getBaseElementType(T);
 
-    if (T->isAnyPointerType() || T->isReferenceType() ||
-        T->isMemberPointerType()) {
-      QualType EltTy;
-      if (T->isObjCObjectPointerType())
-        EltTy = T;
-      else if (const MemberPointerType *PTy = T->getAs<MemberPointerType>())
+    if (EltTy->isAnyPointerType() || EltTy->isReferenceType() ||
+        EltTy->isMemberPointerType()) {
+
+      if (const auto *PTy = EltTy->getAs<MemberPointerType>())
         EltTy = PTy->getPointeeType();
       else
-        EltTy = T->getPointeeType();
+        EltTy = EltTy->getPointeeType();
 
       // If we have a pointer or reference, the pointee must have an object
       // incomplete type.
-      if (!EltTy->isIncompleteOrObjectType()) {
+      if (!EltTy->isIncompleteOrObjectType())
         DiagID = diag::err_typecheck_invalid_restrict_invalid_pointee;
-        ProblemTy = EltTy;
-      }
+
     } else if (!isDependentOrGNUAutoType(T)) {
       // For an __auto_type variable, we may not have seen the initializer yet
       // and so have no idea whether the underlying type is a pointer type or
       // not.
       DiagID = diag::err_typecheck_invalid_restrict_not_pointer;
-      ProblemTy = T;
+      EltTy = T;
     }
 
+    Loc = DS ? DS->getRestrictSpecLoc() : Loc;
     if (DiagID) {
-      Diag(DS ? DS->getRestrictSpecLoc() : Loc, DiagID) << ProblemTy;
+      Diag(Loc, DiagID) << EltTy;
       Qs.removeRestrict();
+    } else {
+      if (T->isArrayType())
+        Diag(Loc, getLangOpts().C23
+                      ? diag::warn_c23_compat_restrict_on_array_of_pointers
+                      : diag::ext_restrict_on_array_of_pointers_c23);
     }
   }
 

clang/test/Sema/pre-c2x-restrict-qualifier.c

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -std=c23 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c17 -fsyntax-only -pedantic -verify=pedantic,expected %s
+// RUN: %clang_cc1 -std=c2x -fsyntax-only -Wpre-c2x-compat -verify=c2x-compat,expected %s
+
+typedef int (*T1)[2];
+restrict T1 t1;
+
+typedef int *T2[2];
+restrict T2 t2;         // pedantic-warning {{'restrict' qualifier on an array of pointers is a C23 extension}} \
+                        // c2x-compat-warning {{'restrict' qualifier on an array of pointers is incompatible with C standards before C23}}
+
+typedef int *T3[2][2];
+restrict T3 t3;         // pedantic-warning {{'restrict' qualifier on an array of pointers is a C23 extension}} \
+                        // c2x-compat-warning {{'restrict' qualifier on an array of pointers is incompatible with C standards before C23}}
+
+typedef int (*t4)();    // pedantic-warning {{a function declaration without a prototype is deprecated in all versions of C}}
+typedef t4 t5[2];
+typedef t5 restrict t6; // // expected-error-re {{pointer to function type 'int {{\((void)?\)}}' may not be 'restrict' qualified}}
+
+typedef int t7[2];
+typedef t7 restrict t8; // expected-error {{restrict requires a pointer or reference ('t7' (aka 'int[2]') is invalid)}}

clang/test/Sema/restrict-qualifier.c

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -std=c2y -fsyntax-only -verify -pedantic %s
+
+typedef int (*T1)[2];
+restrict T1 t1;
+static_assert(_Generic(typeof (t1), int (*restrict)[2] : 1, default : 0));
+
+typedef int *T2[2];
+restrict T2 t2;
+static_assert(_Generic(typeof (t2), int *restrict[2] : 1, default : 0));
+
+typedef int *T3[2][2];
+restrict T3 t3;
+static_assert(_Generic(typeof (t3), int *restrict[2][2] : 1, default : 0));
+static_assert(_Generic(void(T3 restrict), void(int *restrict (*)[2]): 1, default: 0));
+
+typedef int (*t4)();
+typedef t4 t5[2];
+typedef t5 restrict t6; // expected-error {{pointer to function type 'int (void)' may not be 'restrict' qualified}}
+
+typedef int t7[2];
+typedef t7 restrict t8; // expected-error {{restrict requires a pointer or reference ('t7' (aka 'int[2]')}}

clang/test/Sema/types.c

@@ -9,20 +9,20 @@ typedef int (*T)[2];
 restrict T x;
 
 typedef int *S[2];
-restrict S y; // expected-error {{restrict requires a pointer or reference ('S' (aka 'int *[2]') is invalid)}}
-
-
+restrict S y; // expected-warning {{'restrict' qualifier on an array of pointers is a C23 extension}}
 
 // int128_t is available.
-int a(void) {
+void a(void) {
   __int128_t s;
   __uint128_t t;
 }
+
 // but not a keyword
-int b(void) {
+void b(void) {
   int __int128_t;
   int __uint128_t;
 }
+
 // __int128 is a keyword
 int c(void) {
   __int128 i;