Diagnose extensions in 'offsetof'

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm made very
clear that it is an UB having type definitions with in offsetof.
Clang supports defining a type as the first argument as a conforming
extension due to how many projects use the construct in C99 and earlier
to calculate the alignment of a type. GCC also supports defining a type
as the first argument.

This adds extension warnings and documentation for the functionality
Clang explicitly supports.

Fixes #57065
Reverts the revert of 39da55e8f548a11f7dadefa73ea73d809a5f1729

Co-authored-by: Yingchi Long <i@lyc.dev>
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

Differential Revision: https://reviews.llvm.org/D133574

1 files changed, 11 insertions, 8 deletions

diff --git a/clang/test/Sema/offsetof.c b/clang/test/Sema/offsetof.c
index 3a5ddc4..8fd9ad6 100644
--- a/clang/test/Sema/offsetof.c
+++ b/clang/test/Sema/offsetof.c
@@ -5,35 +5,38 @@
 typedef struct P { int i; float f; } PT;
 struct external_sun3_core
 {
- unsigned c_regs; 
+ unsigned c_regs;
 
   PT  X[100];
-  
+
 };
 
+// Ensure the builtin works as a constant expression
+int i = offsetof(PT, f);
+
 void swap(void)
 {
   int x;
   x = offsetof(struct external_sun3_core, c_regs);
   x = __builtin_offsetof(struct external_sun3_core, X[42].f);
-  
+
   x = __builtin_offsetof(struct external_sun3_core, X[42].f2);  // expected-error {{no member named 'f2'}}
   x = __builtin_offsetof(int, X[42].f2);  // expected-error {{offsetof requires struct}}
-  
+
   int a[__builtin_offsetof(struct external_sun3_core, X) == 4 ? 1 : -1];
   int b[__builtin_offsetof(struct external_sun3_core, X[42]) == 340 ? 1 : -1];
   int c[__builtin_offsetof(struct external_sun3_core, X[42].f2) == 344 ? 1 : -1];  // expected-error {{no member named 'f2'}}
-}    
+}
 
 extern int f(void);
 
-struct s1 { int a; }; 
+struct s1 { int a; };
 int v1 = offsetof (struct s1, a) == 0 ? 0 : f();
 
-struct s2 { int a; }; 
+struct s2 { int a; };
 int v2 = (int)(&((struct s2 *) 0)->a) == 0 ? 0 : f();
 
-struct s3 { int a; }; 
+struct s3 { int a; };
 int v3 = __builtin_offsetof(struct s3, a) == 0 ? 0 : f();
 
 // PR3396