[clang] [clang][ExprConst] Add diagnostics for invalid binary arithmetic (PR #118475)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/118475

... between unrelated declarations or literals.

Leaving this small (I haven't run the whole test suite locally) to get some feedback on the wording and implementation first.

The output of the sample in https://github.com/llvm/llvm-project/issues/117409 is now:
```console
./array.cpp:57:6: warning: expression result unused [-Wunused-value]
   57 |   am - aj.af();
      |   ~~ ^ ~~~~~~~
./array.cpp:70:8: error: call to consteval function 'L::L<bx>' is not a constant expression
   70 |   q(0, [] {
      |        ^
./array.cpp:57:6: note: arithmetic on addresses of literals has unspecified value
   57 |   am - aj.af();
      |      ^
./array.cpp:62:5: note: in call to 'al(&""[0], {&""[0]})'
   62 |     al(bp.af(), k);
      |     ^~~~~~~~~~~~~~
./array.cpp:70:8: note: in call to 'L<bx>({})'
   70 |   q(0, [] {
      |        ^~~~
   71 |     struct bx {
      |     ~~~~~~~~~~~
   72 |       constexpr operator ab<g<l<decltype(""[0])>::e>::e>() { return t(""); }
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   73 |     };
      |     ~~
   74 |     return bx();
      |     ~~~~~~~~~~~~
   75 |   }());
      |   ~~~
```

The output for 
```c++
int a, b;
constexpr int n = &b - &a
```

is now:
```console
./array.cpp:80:15: error: constexpr variable 'n' must be initialized by a constant expression
   80 | constexpr int n = &b - &a;
      |               ^   ~~~~~~~
./array.cpp:80:22: note: arithmetic involving '&b' and '&a' has unspecified value
   80 | constexpr int n = &b - &a;
      |                      ^
1 error generated.

```

>From ce52d3d0c04fc53b814debdcd2c5019f488eddc0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Tue, 3 Dec 2024 11:54:07 +0100
Subject: [PATCH] [clang][ExprConst] Add diagnostics for invalid binary
 arithmetic

... between unrelated declarations or literals.
---
 clang/include/clang/Basic/DiagnosticASTKinds.td |  4 ++++
 clang/lib/AST/ExprConstant.cpp                  | 17 +++++++++++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticASTKinds.td b/clang/include/clang/Basic/DiagnosticASTKinds.td
index f630698757c5fb..4d078fc9ca6bb7 100644
--- a/clang/include/clang/Basic/DiagnosticASTKinds.td
+++ b/clang/include/clang/Basic/DiagnosticASTKinds.td
@@ -91,11 +91,15 @@ def note_constexpr_pointer_subtraction_zero_size : Note<
   "subtraction of pointers to type %0 of zero size">;
 def note_constexpr_pointer_comparison_unspecified : Note<
   "comparison between '%0' and '%1' has unspecified value">;
+def note_constexpr_pointer_arith_unspecified : Note<
+  "arithmetic involving '%0' and '%1' has unspecified value">;
 def note_constexpr_pointer_constant_comparison : Note<
   "comparison of numeric address '%0' with pointer '%1' can only be performed "
   "at runtime">;
 def note_constexpr_literal_comparison : Note<
   "comparison of addresses of literals has unspecified value">;
+def note_constexpr_literal_arith : Note<
+  "arithmetic on addresses of literals has unspecified value">;
 def note_constexpr_opaque_call_comparison : Note<
   "comparison against opaque constant address '%0' can only be performed at "
   "runtime">;
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 6b5b95aee35522..2aefcd870ebaf8 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -14548,8 +14548,21 @@ bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
         return Error(E);
       const Expr *LHSExpr = LHSValue.Base.dyn_cast<const Expr *>();
       const Expr *RHSExpr = RHSValue.Base.dyn_cast<const Expr *>();
-      if (!LHSExpr || !RHSExpr)
-        return Error(E);
+      if (!LHSExpr || !RHSExpr) {
+        std::string LHS = LHSValue.toString(Info.Ctx, E->getLHS()->getType());
+        std::string RHS = RHSValue.toString(Info.Ctx, E->getRHS()->getType());
+        Info.FFDiag(E, diag::note_constexpr_pointer_arith_unspecified)
+            << LHS << RHS;
+        return false;
+      }
+
+      if (ArePotentiallyOverlappingStringLiterals(Info, LHSValue, RHSValue)) {
+        std::string LHS = LHSValue.toString(Info.Ctx, E->getLHS()->getType());
+        std::string RHS = RHSValue.toString(Info.Ctx, E->getRHS()->getType());
+        Info.FFDiag(E, diag::note_constexpr_literal_arith) << LHS << RHS;
+        return false;
+      }
+
       const AddrLabelExpr *LHSAddrExpr = dyn_cast<AddrLabelExpr>(LHSExpr);
       const AddrLabelExpr *RHSAddrExpr = dyn_cast<AddrLabelExpr>(RHSExpr);
       if (!LHSAddrExpr || !RHSAddrExpr)