[PATCH] D130058: [Clang] Diagnose ill-formed constant expression when setting a non fixed enum to a value outside the range of the enumeration values

[Shafik Yaghmour via Phabricator via cfe-commits]

shafik added a comment.

Note I took my range generation code from `CGExpr.cpp` function `getRangeForType(...)` I was considering perhaps making the code common, maybe this could be a helper in `EnumDecl`? Does that makes sense?



================
Comment at: clang/lib/AST/ExprConstant.cpp:13512
 
+    if (Info.Ctx.getLangOpts().CPlusPlus20)
+      if (const EnumType *ET = dyn_cast<EnumType>(DestType)) {
----------------
erichkeane wrote:
> Why the check for C++20?  This is UB as a Defect Report, which applies to all versions of C++.
I did not realized we apply defects back, I can remove the C++20 check.


================
Comment at: clang/lib/AST/ExprConstant.cpp:13533
+
+          if (NumNegativeBits) {
+            unsigned NumBits = std::max(NumNegativeBits, NumPositiveBits + 1);
----------------
erichkeane wrote:
> erichkeane wrote:
> > So my reading of 'within the range of the enumeration values' is different here.  Given:
> > 
> > `enum F { -4, 4};`, the valid values for assignment I would say are -4, -3, -2, -1, 0, 1, 2, 3, 4.  This error seems to be a bit more permissive here by making sure it is represent-able by the number of bits required to represent the enum.
> > 
> > 
> Aaron pointed out off-line that I'm incorrect in what 'range of enumeration values' means, and that the values comes from https://eel.is/c++draft/dcl.enum#8.sentence-2.
> 
> So the logic here needs to be to find the smallest integer (regardless of the power-of-2-ness of its bit-size) that can represent all of the values (including a 1 bit value for empty I think?), and diagnose based on that.
So I used ubsan to validate my checks e.g. https://godbolt.org/z/1j43465K7 but perhaps ubsan is getting it wrong as well?


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Comment at: clang/test/SemaCXX/constant-expression-cxx2a.cpp:1477
+
+enum E { e1=-4, e2=4};
+void testValueInRangeOfEnumerationValues() {
----------------
erichkeane wrote:
> erichkeane wrote:
> > Theres likely quite a few more tests that need to be done.  We should have one where it validates against a fixed underlying type, AND probably a pair of ones to check against scoped enums.  
> > 
> > I wouldn't expect a scoped enum to diagnose, but a test to make sure we're checking the right thing is likely valuable. 
> Another good test is the empty-enum case.
Good point on the tests, yes we should not diagnose on a scoped enum.


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  https://reviews.llvm.org/D130058/new/

https://reviews.llvm.org/D130058