Fiix fault on operations on uninitialized strings (#6520)

Signed-off-by: Artur Bieniek <abieniek@internships.antmicro.com>

src/V3AstNodeExpr.h

@@ -1266,6 +1266,9 @@ class AstExprStmt final : public AstNodeExpr {
     // resultp is evaluated AFTER the statement(s).
     // @astgen op1 := stmtsp : List[AstNode]
     // @astgen op2 := resultp : AstNodeExpr
+private:
+    bool m_hasResult = true;
+
 public:
     AstExprStmt(FileLine* fl, AstNode* stmtsp, AstNodeExpr* resultp)
         : ASTGEN_SUPER_ExprStmt(fl) {
@@ -1283,6 +1286,8 @@ public:
         return resultp()->isPure();
     }
     bool sameNode(const AstNode*) const override { return true; }
+    bool hasResult() { return m_hasResult; }
+    void hasResult(bool flag) { m_hasResult = flag; }
 };
 class AstFError final : public AstNodeExpr {
     // @astgen op1 := filep : AstNode

src/V3EmitCFunc.h

@@ -1227,6 +1227,11 @@ public:
         // GCC allows compound statements in expressions, but this is not standard.
         // So we use an immediate-evaluation lambda and comma operator
         putnbs(nodep, "([&]() {\n");
+        if (!nodep->hasResult()) {
+            iterateAndNextConstNull(nodep->stmtsp());
+            puts("}())");
+            return;
+        }
         iterateAndNextConstNull(nodep->stmtsp());
         puts("}(), ");
         iterateAndNextConstNull(nodep->resultp());

src/V3Task.cpp

@@ -374,6 +374,7 @@ class TaskVisitor final : public VNVisitor {
     AstScope* m_scopep = nullptr;  // Current scope
     AstNode* m_insStmtp = nullptr;  // Where to insert statement
     bool m_inSensesp = false;  // Are we under a senitem?
+    bool m_inNew = false;  // Are we under a constructor?
     int m_modNCalls = 0;  // Incrementing func # for making symbols
 
     // STATE - across all visitors
@@ -1476,6 +1477,11 @@ class TaskVisitor final : public VNVisitor {
         iterateChildren(nodep);
         UASSERT_OBJ(!m_insStmtp, nodep, "Didn't finish out last statement");
     }
+    void visit(AstCNew* nodep) override {
+        VL_RESTORER(m_inNew);
+        m_inNew = true;
+        iterateChildren(nodep);
+    }
     void visit(AstNodeFTaskRef* nodep) override {
         if (m_inSensesp && !nodep->isPure()) {
             nodep->v3warn(E_UNSUPPORTED,
@@ -1525,10 +1531,33 @@ class TaskVisitor final : public VNVisitor {
             UASSERT_OBJ(nodep->taskp()->isFunction(), nodep,
                         "funcref-like expression to non-function");
             AstVarRef* const outrefp = new AstVarRef{nodep->fileline(), outvscp, VAccess::READ};
-            beginp = new AstExprStmt{nodep->fileline(), beginp, outrefp};
+            AstExprStmt* lambdap = new AstExprStmt{nodep->fileline(), beginp, outrefp};
+
+            if (m_inNew) {
+                AstVar* varp = outvscp->varp();
+                varp->funcLocal(true);
+
+                // Create a new var that will be inside the lambda
+                AstVar* newvarp = varp->cloneTree(false);
+
+                // Replace all references so they point to the new var
+                lambdap->stmtsp()->foreachAndNext([varp, newvarp](AstVarRef* refp) {
+                    if (refp->varp() == varp) refp->varp(newvarp);
+                });
+
+                // Add variable initialization
+                lambdap->stmtsp()->addHereThisAsNext(newvarp);
+                lambdap->hasResult(false);
+
+                // Add return statement
+                AstCExpr* const exprp = new AstCExpr{nodep->fileline(), varp->name(), 0};
+                exprp->dtypeSetString();
+                lambdap->addStmtsp(new AstCReturn{nodep->fileline(), exprp});
+            }
+
             // AstExprStmt is currently treated as impure, so clear the cached purity of its
             // parents
-            nodep->replaceWith(beginp);
+            nodep->replaceWith(lambdap);
             VL_DO_DANGLING(nodep->deleteTree(), nodep);
             VIsCached::clearCacheTree();
         } else {  // VN_IS(nodep->backp(), StmtExpr)

test_regress/t/t_func_call_super_arg.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_func_call_super_arg.v

@@ -0,0 +1,28 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Antmicro.
+// SPDX-License-Identifier: CC0-1.0
+
+class base;
+  function new(string name);
+    $display(name);
+    if(name == "42") $finish;
+  endfunction
+
+  function string retstr();
+    return $sformatf("%0d", $c("42"));
+  endfunction
+endclass
+
+class derived extends base;
+  function new();
+    super.new(retstr());
+  endfunction
+endclass
+
+module t;
+  initial begin
+    derived test = new;
+  end
+endmodule