Fix attempt to construct an abstract AST node (#6905)

Work on #6892

The function `tryInferLoopMaxIterations` in `slang-check-stmt.cpp` was
doing:

    auto litExpr = m_astBuilder->create<LiteralExpr>();

but the declaration of `LiteralExpr` in `slang-ast-expr.h` had been
marked as abstract, during the switch to using the `slang-fiddle` tool
to generate code:

    FIDDLE(abstract)
    class LiteralExpr : public Expr
    { ... }

In this case, the intention of the AST design is that `LiteralExpr`
should be kept abstract, and only the concrete subclasses should
ever be instantiated.

Because of some historical design choices, the `ASTNodeType`
enumeration includes both the concrete and abstract AST classes, so
the code ended up constructing a `LiteralExpr` that had the tag
`ASTNodeType::LiteralExpr`. Then attempts to use the `ASTNodeDispatcher`
code on such a node caused crashes, because the dispatcher code only
included `case` statements for the non-abstract `ASTNodeType`s.

The quick fix here was to change the `tryInferLoopMaxIterations`
function to instead do:

    auto litExpr = m_astBuilder->create<IntegerLiteralExpr>();

A test case was added to help catch any future regressions on this
specific issue.

A more long-term fix should involve introducing code that statically
and/or dynamically prohibits the creation of instances of AST node
classes that have been marked abstract.

1 files changed, 1 insertions, 1 deletions

diff --git a/source/slang/slang-check-stmt.cpp b/source/slang/slang-check-stmt.cpp
index 8a914c60b..9525f71c9 100644
--- a/source/slang/slang-check-stmt.cpp
+++ b/source/slang/slang-check-stmt.cpp
@@ -880,7 +880,7 @@ void SemanticsStmtVisitor::tryInferLoopMaxIterations(ForStmt* stmt)
     // if the loop body modifies the induction variable.
     //
     auto maxItersAttr = m_astBuilder->create<InferredMaxItersAttribute>();
-    auto litExpr = m_astBuilder->create<LiteralExpr>();
+    auto litExpr = m_astBuilder->create<IntegerLiteralExpr>();
     litExpr->type.type = m_astBuilder->getIntType();
     litExpr->token.setName(getNamePool()->getName(String(iterations)));
     maxItersAttr->args.add(litExpr);