[clang] Fix const eval of constexpr-unknown relational comparisons. (#150088)

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

Fixes #150015.

(cherry picked from commit bba8467)

Author: efriedma-quic

clang/lib/AST/ExprConstant.cpp

@@ -14566,7 +14566,9 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
     if (!LHSDesignator.Invalid && !RHSDesignator.Invalid && IsRelational) {
       bool WasArrayIndex;
       unsigned Mismatch = FindDesignatorMismatch(
-          getType(LHSValue.Base), LHSDesignator, RHSDesignator, WasArrayIndex);
+          LHSValue.Base.isNull() ? QualType()
+                                 : 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
@@ -14610,7 +14612,7 @@ 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);

clang/test/SemaCXX/constant-expression-p2280r4.cpp

@@ -383,3 +383,17 @@ 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; // interpreter-note{{declared here}}
+  constexpr int bases = (void*)(X*)&z <= (Y*)&z; // expected-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}} \
+                                                 // interpreter-note {{initializer of 'z' is unknown}}
+}