[clang][ExprConst] Don't try to evaluate value-dependent DeclRefExprs (#67778)

The Expression here migth be value dependent, which makes us run into an
assertion later on. Just bail out early.

Fixes #67690

Author: tbaederr

clang/docs/ReleaseNotes.rst

@@ -339,6 +339,9 @@ Bug Fixes in This Version
   (`#64462 <https://github.com/llvm/llvm-project/issues/64462>`_)
 - Fixes a regression where the ``UserDefinedLiteral`` was not properly preserved
   while evaluating consteval functions. (`#63898 <https://github.com/llvm/llvm-project/issues/63898>`_).
+- Fix a crash when evaluating value-dependent structured binding
+  variables at compile time.
+  Fixes (`#67690 <https://github.com/llvm/llvm-project/issues/67690>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -3357,6 +3357,9 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
     return false;
   }
 
+  if (E->isValueDependent())
+    return false;
+
   // Dig out the initializer, and use the declaration which it's attached to.
   // FIXME: We should eventually check whether the variable has a reachable
   // initializing declaration.

clang/test/SemaCXX/constant-expression-cxx1z.cpp

@@ -177,3 +177,16 @@ namespace LambdaCallOp {
     p();
   }
 }
+
+// This used to crash due to an assertion failure,
+// see gh#67690
+namespace {
+  struct C {
+    int x;
+  };
+
+  template <const C *p> void f() {
+    const auto &[c] = *p;
+    &c; // expected-warning {{expression result unused}}
+  }
+}