[clang] [clang][bytecode] Get the right definition before compiling functions (PR #201105)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

<details>
<summary>Changes</summary>

This broke libc++'s
std/ranges/range.adaptors/range.concat/iterator/arithmetic.pass.cpp.

The (reduced via cvise but not enough) function looks like this:

```c++
  friend constexpr unsigned
  operator-(const __iterator &__x, const __iterator &__y)
    {
      (void)-(__y - __x);
      return 0;
    }
```

When evaluating the binary operator for overflow, we will compile the operator- (_this_ function) to bytecode. At that point, ::isThisDeclarationADefiniton() will return true and ::getDefiniton() returns the function itself. However, all this is happening while the function is being instantiated, which means the function doesn't have a body yet and the bytecode ends up being just a NoRet op. This will of course later fail.

Fix this by querying the body before trying to compile a function.

Unfortunately I wasn't able to create a reproducer of reasonable size.

---
Full diff: https://github.com/llvm/llvm-project/pull/201105.diff


1 Files Affected:

- (modified) clang/lib/AST/ByteCode/Interp.cpp (+3-1) 


``````````diff
diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 6bcebf3ee892b..9d363191f8008 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -1725,7 +1725,9 @@ bool CheckBitCast(InterpState &S, CodePtr OpPC, const Type *TargetType,
 }
 
 static void compileFunction(InterpState &S, const Function *Func) {
-  const FunctionDecl *Definition = Func->getDecl()->getDefinition();
+  const FunctionDecl *Definition;
+  if (!Func->getDecl()->getBody(Definition))
+    return;
   if (!Definition)
     return;
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/201105