[clang][Interp] Pass callee decl to null_callee diagnostics (#104426)

The callee is null, not the full call expression.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -2702,9 +2702,9 @@ inline bool CallPtr(InterpState &S, CodePtr OpPC, uint32_t ArgSize,
 
   const Function *F = FuncPtr.getFunction();
   if (!F) {
-    const Expr *E = S.Current->getExpr(OpPC);
+    const auto *E = cast<CallExpr>(S.Current->getExpr(OpPC));
     S.FFDiag(E, diag::note_constexpr_null_callee)
-        << const_cast<Expr *>(E) << E->getSourceRange();
+        << const_cast<Expr *>(E->getCallee()) << E->getSourceRange();
     return false;
   }
 

clang/test/AST/Interp/functions.cpp

@@ -221,6 +221,17 @@ namespace Comparison {
   static_assert(pg == &g, "");
 }
 
+  constexpr int Double(int n) { return 2 * n; }
+  constexpr int Triple(int n) { return 3 * n; }
+  constexpr int Twice(int (*F)(int), int n) { return F(F(n)); }
+  constexpr int Quadruple(int n) { return Twice(Double, n); }
+  constexpr auto Select(int n) -> int (*)(int) {
+    return n == 2 ? &Double : n == 3 ? &Triple : n == 4 ? &Quadruple : 0;
+  }
+  constexpr int Apply(int (*F)(int), int n) { return F(n); } // both-note {{'F' evaluates to a null function pointer}}
+
+  constexpr int Invalid = Apply(Select(0), 0); // both-error {{must be initialized by a constant expression}} \
+                                               // both-note {{in call to 'Apply(nullptr, 0)'}}
 }
 
 struct F {