[clang] Fix const eval of constexpr-unknown relational comparisons.

[efriedma-quic]

Like in other places, ignore the reference type of the base. (It might make sense to refactor this at some point.)

Fixes #150015.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Eli Friedman (efriedma-quic)

Changes

Like in other places, ignore the reference type of the base. (It might make sense to refactor this at some point.)

Fixes #150015.

---

Full diff: https://github.com/llvm/llvm-project/pull/150088.diff

2 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+4-3)
• (modified) clang/test/SemaCXX/constant-expression-p2280r4.cpp (+13)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 8797eaddd0e18..106ad30adf1e5 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -14631,8 +14631,9 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
     // - Otherwise pointer comparisons are unspecified.
     if (!LHSDesignator.Invalid && !RHSDesignator.Invalid && IsRelational) {
       bool WasArrayIndex;
-      unsigned Mismatch = FindDesignatorMismatch(
-          getType(LHSValue.Base), LHSDesignator, RHSDesignator, WasArrayIndex);
+      unsigned Mismatch =
+          FindDesignatorMismatch(getType(LHSValue.Base).getNonReferenceType(),
+                                 LHSDesignator, RHSDesignator, WasArrayIndex);
       // At the point where the designators diverge, the comparison has a
       // specified value if:
       //  - we are comparing array indices
@@ -14676,7 +14677,7 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
     // compare pointers within the object in question; otherwise, the result
     // depends on where the object is located in memory.
     if (!LHSValue.Base.isNull() && IsRelational) {
-      QualType BaseTy = getType(LHSValue.Base);
+      QualType BaseTy = getType(LHSValue.Base).getNonReferenceType();
       if (BaseTy->isIncompleteType())
         return Error(E);
       CharUnits Size = Info.Ctx.getTypeSizeInChars(BaseTy);
diff --git a/clang/test/SemaCXX/constant-expression-p2280r4.cpp b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
index 16f5f823d26c1..3fb7bf28b1e8b 100644
--- a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
+++ b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
@@ -383,3 +383,16 @@ namespace enable_if_2 {
   }
 }
 }
+
+namespace GH150015 {
+  extern int (& c)[8]; // interpreter-note {{declared here}}
+  constexpr int x = c <= c+8; // interpreter-error {{constexpr variable 'x' must be initialized by a constant expression}} \
+                              // interpreter-note {{initializer of 'c' is unknown}}
+
+  struct X {};
+  struct Y {};
+  struct Z : X, Y {};
+  extern Z z;
+  constexpr int bases = (void*)(X*)&z <= (Y*)&z; // nointerpreter-error {{constexpr variable 'bases' must be initialized by a constant expression}} \
+                                                 // nointerpreter-note {{comparison of addresses of subobjects of different base classes has unspecified value}}
+}

[shafik]

LGTM

[shafik]

I think it makes sense to create a local and set it to the result of getType(LHSValue.Base).getNonReferenceType(), comment it and then use that in the two locations you just modified.

[efriedma-quic]

The problem with that is that it presumes LHSValue.Base is non-null. Which I don't think we actually guarantee at the point we'd want to declare the variable, so the the best I can do is something like auto GetBaseType = [&]{return getType(LHSValue.Base).getNonReferenceType(); };.