[clang][ExprConstant] Reject integral casts of addr-label-diffs...

... if the result is narrower than 32 bits.

See the discussion in #136135

Author: tbaederr

clang/lib/AST/ExprConstant.cpp

@@ -18606,12 +18606,15 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
 
     if (!Result.isInt()) {
       // Allow casts of address-of-label differences if they are no-ops
-      // or narrowing.  (The narrowing case isn't actually guaranteed to
+      // or narrowing, if the result is at least 32 bits wide.
+      // (The narrowing case isn't actually guaranteed to
       // be constant-evaluatable except in some narrow cases which are hard
       // to detect here.  We let it through on the assumption the user knows
       // what they are doing.)
-      if (Result.isAddrLabelDiff())
-        return Info.Ctx.getTypeSize(DestType) <= Info.Ctx.getTypeSize(SrcType);
+      if (Result.isAddrLabelDiff()) {
+        unsigned DestBits = Info.Ctx.getTypeSize(DestType);
+        return DestBits >= 32 && DestBits <= Info.Ctx.getTypeSize(SrcType);
+      }
       // Only allow casts of lvalues if they are lossless.
       return Info.Ctx.getTypeSize(DestType) == Info.Ctx.getTypeSize(SrcType);
     }

clang/test/CodeGenCXX/const-init.cpp

@@ -73,6 +73,10 @@ __int128_t PR11705 = (__int128_t)&PR11705;
 // CHECK: @_ZZ23UnfoldableAddrLabelDiffvE1x = internal global i128 0
 void UnfoldableAddrLabelDiff() { static __int128_t x = (long)&&a-(long)&&b; a:b:return;}
 
+// CHECK: @_ZZ24UnfoldableAddrLabelDiff2vE1x = internal global i16 0
+void UnfoldableAddrLabelDiff2() { static short x = (long)&&a-(long)&&b; a:b:return;}
+
+
 // But make sure we do fold this.
 // CHECK: @_ZZ21FoldableAddrLabelDiffvE1x = internal global i64 sub (i64 ptrtoint (ptr blockaddress(@_Z21FoldableAddrLabelDiffv
 void FoldableAddrLabelDiff() { static long x = (long)&&a-(long)&&b; a:b:return;}