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, 9 insertions, 0 deletions

diff --git a/tests/bugs/serialization/gh-6892.slang b/tests/bugs/serialization/gh-6892.slang
new file mode 100644
index 000000000..1235c292c
--- /dev/null
+++ b/tests/bugs/serialization/gh-6892.slang
@@ -0,0 +1,9 @@
+//TEST:COMPILE: tests/bugs/serialization/gh-6892.slang -o tests/bugs/serialization/gh-6892.slang-module
+
+// assertion failure when compiling to a slang module
+
+void f()
+{
+    for (int i = 0; i < 1; ++i)
+        ;
+}