[clang][Interp] Ignore incomplete records when visiting lambdas

We need them to be complete so we have knowledge about all the
lambda captures.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeEmitter.cpp

@@ -93,6 +93,11 @@ Function *ByteCodeEmitter::compileFunc(const FunctionDecl *FuncDecl) {
 
     // Set up lambda capture to closure record field mapping.
     if (isLambdaCallOperator(MD)) {
+      // The parent record needs to be complete, we need to know about all
+      // the lambda captures.
+      if (!MD->getParent()->isCompleteDefinition())
+        return nullptr;
+
       const Record *R = P.getOrCreateRecord(MD->getParent());
       llvm::DenseMap<const ValueDecl *, FieldDecl *> LC;
       FieldDecl *LTC;

clang/lib/AST/Interp/Context.cpp

@@ -31,6 +31,9 @@ bool Context::isPotentialConstantExpr(State &Parent, const FunctionDecl *FD) {
   if (!Func || !Func->hasBody())
     Func = Compiler<ByteCodeEmitter>(*this, *P).compileFunc(FD);
 
+  if (!Func)
+    return false;
+
   APValue DummyResult;
   if (!Run(Parent, Func, DummyResult))
     return false;

clang/test/AST/Interp/lambda.cpp

@@ -280,3 +280,9 @@ namespace InvalidCapture {
     } ();
   }
 }
+
+constexpr int fn() {
+  int Capture = 42;
+  return [=]() constexpr { return Capture; }();
+}
+static_assert(fn() == 42, "");