[C2y] Add diagnostics for alignof on an incomplete array

Clang implemented WG14 N3273 since Clang 3.5

Author: AaronBallman

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3547,6 +3547,12 @@ def warn_alignment_not_power_of_two : Warning<
   InGroup<DiagGroup<"non-power-of-two-alignment">>;
 def err_alignment_dependent_typedef_name : Error<
   "requested alignment is dependent but declaration is not dependent">;
+def ext_c2y_alignof_incomplete_array : Extension<
+  "'alignof' on an incomplete array type is a C2y extension">,
+  InGroup<C2y>;
+def warn_c2y_compat_alignof_incomplete_array : Warning<
+  "'alignof' on an incomplete array type is incompatible with C standards "
+  "before C2y">, InGroup<CPre2yCompat>, DefaultIgnore;
 
 def warn_alignment_builtin_useless : Warning<
   "%select{aligning a value|the result of checking whether a value is aligned}0"

clang/lib/Sema/SemaExpr.cpp

@@ -4486,8 +4486,16 @@ bool Sema::CheckUnaryExprOrTypeTraitOperand(QualType ExprType,
   //   When alignof or _Alignof is applied to an array type, the result
   //   is the alignment of the element type.
   if (ExprKind == UETT_AlignOf || ExprKind == UETT_PreferredAlignOf ||
-      ExprKind == UETT_OpenMPRequiredSimdAlign)
+      ExprKind == UETT_OpenMPRequiredSimdAlign) {
+    // If the trait is 'alignof' in C before C2y, the ability to apply the
+    // trait to an incomplete array is an extension.
+    if (ExprKind == UETT_AlignOf && !getLangOpts().CPlusPlus &&
+        ExprType->isIncompleteArrayType())
+      Diag(OpLoc, getLangOpts().C2y
+                      ? diag::warn_c2y_compat_alignof_incomplete_array
+                      : diag::ext_c2y_alignof_incomplete_array);
     ExprType = Context.getBaseElementType(ExprType);
+  }
 
   if (ExprKind == UETT_VecStep)
     return CheckVecStepTraitOperandType(*this, ExprType, OpLoc, ExprRange);

clang/test/C/C2y/n3273.c

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic -Wpre-c2y-compat %s
+// RUN: %clang_cc1 -verify=pre-c2y -std=c23 -Wall -pedantic %s
+
+/* WG14 N3273: Clang 3.5
+ * alignof of an incomplete array type
+ */
+
+static_assert(
+  alignof(int[]) == /* pre-c2y-warning {{'alignof' on an incomplete array type is a C2y extension}}
+                       expected-warning {{'alignof' on an incomplete array type is incompatible with C standards before C2y}}
+                     */
+  alignof(int)
+);
+