Don't reject uses of void-returning consteval functions.

Author: zygoloid

clang/lib/AST/ExprConstant.cpp

@@ -2299,6 +2299,10 @@ static bool
 CheckConstantExpression(EvalInfo &Info, SourceLocation DiagLoc, QualType Type,
                         const APValue &Value,
                         Expr::ConstExprUsage Usage = Expr::EvaluateForCodeGen) {
+  // Nothing to check for a constant expression of type 'cv void'.
+  if (Type->isVoidType())
+    return true;
+
   CheckedTemporaries CheckedTemps;
   return CheckEvaluationResult(CheckEvaluationResultKind::ConstantExpression,
                                Info, DiagLoc, Type, Value, Usage,

clang/test/SemaCXX/consteval-return-void.cpp

@@ -18,3 +18,12 @@ template <typename T> constexpr E operator*(E,E);
 template <typename T> consteval E operator/(E,E);
 template <> constexpr E operator*<int>(E,E) { return; } // expected-error {{non-void constexpr function 'operator*<int>' should return a value}}
 template <> consteval E operator/<int>(E,E) { return; } // expected-error {{non-void consteval function 'operator/<int>' should return a value}}
+
+consteval void no_return() {}
+consteval void with_return() { return; }
+consteval void with_return_void() { return void(); }
+void use_void_fn() {
+  no_return();
+  with_return();
+  with_return_void();
+}