c++: alignas and alignof void [PR104944]

I started looking into this PR because in GCC 4.9 we were able to
detect the invalid

  struct alignas(void) S{};

but I broke it in r210262.

It's ill-formed code in C++:
[dcl.align]/3: "An alignment-specifier of the form alignas(type-id) has
the same effect as alignas(alignof(type-id))", and [expr.align]/1:
"The operand shall be a type-id representing a complete object type,
or an array thereof, or a reference to one of those types." and void
is not a complete type.

It's also invalid in C:
6.7.5: _Alignas(type-name) is equivalent to _Alignas(_Alignof(type-name))
6.5.3.4: "The _Alignof operator shall not be applied to a function type
or an incomplete type."

We have a GNU extension whereby we treat sizeof(void) as 1, but I assume
it doesn't apply to alignof, at least in C++.  However, __alignof__(void)
is still accepted with a -Wpedantic warning.

We still say "invalid application of 'alignof'" rather than 'alignas' in the
void diagnostic, but I felt that fixing that may not be suitable as part of
this patch.  The "incomplete type" diagnostic still always prints
'__alignof__'.

	PR c++/104944

gcc/cp/ChangeLog:

	* typeck.cc (cxx_sizeof_or_alignof_type): Diagnose alignof(void).
	(cxx_alignas_expr): Call cxx_sizeof_or_alignof_type with
	complain == true.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alignas20.C: New test.

2 files changed, 41 insertions, 6 deletions

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 516fa57..26a7cb4 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -1873,9 +1873,9 @@ compparms (const_tree parms1, const_tree parms2)
 }
 
 
-/* Process a sizeof or alignof expression where the operand is a
-   type. STD_ALIGNOF indicates whether an alignof has C++11 (minimum alignment)
-   or GNU (preferred alignment) semantics; it is ignored if op is
+/* Process a sizeof or alignof expression where the operand is a type.
+   STD_ALIGNOF indicates whether an alignof has C++11 (minimum alignment)
+   or GNU (preferred alignment) semantics; it is ignored if OP is
    SIZEOF_EXPR.  */
 
 tree
@@ -1899,6 +1899,13 @@ cxx_sizeof_or_alignof_type (location_t loc, tree type, enum tree_code op,
       else
 	return error_mark_node;
     }
+  else if (VOID_TYPE_P (type) && std_alignof)
+    {
+      if (complain)
+	error_at (loc, "invalid application of %qs to a void type",
+		  OVL_OP_INFO (false, op)->name);
+      return error_mark_node;
+    }
 
   bool dependent_p = dependent_type_p (type);
   if (!dependent_p)
@@ -2132,11 +2139,13 @@ cxx_alignas_expr (tree e)
     /* [dcl.align]/3:
        
 	   When the alignment-specifier is of the form
-	   alignas(type-id ), it shall have the same effect as
-	   alignas(alignof(type-id )).  */
+	   alignas(type-id), it shall have the same effect as
+	   alignas(alignof(type-id)).  */
 
     return cxx_sizeof_or_alignof_type (input_location,
-				       e, ALIGNOF_EXPR, true, false);
+				       e, ALIGNOF_EXPR,
+				       /*std_alignof=*/true,
+				       /*complain=*/true);
   
   /* If we reach this point, it means the alignas expression if of
      the form "alignas(assignment-expression)", so we should follow
diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas20.C b/gcc/testsuite/g++.dg/cpp0x/alignas20.C
new file mode 100644
index 0000000..01a55f3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas20.C
@@ -0,0 +1,26 @@
+// PR c++/104944
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wpedantic" }
+
+struct inc;
+
+struct alignas(inc) S1 { }; // { dg-error "invalid application" }
+struct alignas(void) S2 { }; // { dg-error "invalid application" }
+
+template <typename T>
+struct alignas(T) S4 {}; // { dg-error "invalid application" }
+
+template <typename T>
+struct alignas(T) S5 {}; // { dg-error "invalid application" }
+
+S4<void> s1;
+S5<inc> s2;
+
+void
+g ()
+{
+  auto s1 = alignof(void); // { dg-error "invalid application" }
+  auto s2 = alignof(const void); // { dg-error "invalid application" }
+  auto s3 = __alignof(void); // { dg-warning "invalid application" }
+  auto s4 = alignof(inc); // { dg-error "invalid application" }
+}