Add check for automatic variable in continuous assignment LHS

src/V3AstNodeStmt.h

@@ -1332,6 +1332,20 @@ public:
         return new AstAssign{fileline(), lhsp, rhsp, controlp};
     }
 };
+class AstAssignCont final : public AstNodeAssign {
+    // Continuous procedural 'assign'.  See AstAssignW for non-procedural version.
+public:
+    AstAssignCont(FileLine* fl, AstNodeExpr* lhsp, AstNodeExpr* rhsp,
+                  AstNode* timingControlp = nullptr)
+        : ASTGEN_SUPER_AssignCont(fl, lhsp, rhsp, timingControlp) {
+        dtypeFrom(lhsp);
+    }
+    ASTGEN_MEMBERS_AstAssignCont;
+    AstNodeAssign* cloneType(AstNodeExpr* lhsp, AstNodeExpr* rhsp) override {
+        AstNode* const controlp = timingControlp() ? timingControlp()->cloneTree(false) : nullptr;
+        return new AstAssignCont{fileline(), lhsp, rhsp, controlp};
+    }
+};
 class AstAssignDly final : public AstNodeAssign {
 public:
     AstAssignDly(FileLine* fl, AstNodeExpr* lhsp, AstNodeExpr* rhsp,

src/V3WidthCommit.cpp

@@ -48,8 +48,8 @@ class WidthCommitVisitor final : public VNVisitor {
     AstNodeFTask* m_ftaskp = nullptr;  // Current function/task
     AstNodeModule* m_modp = nullptr;  // Current module
     std::string m_contNba;  // In continuous- or non-blocking assignment
-    bool m_dynsizedelem
-        = false;  // Writing a dynamically-sized array element, not the array itself
+    bool m_contReads = false;  // Check read continuous automatic variables
+    bool m_dynsizedelem = false;  // Writing dynamically-sized array element, not the array itself
     VMemberMap m_memberMap;  // Member names cached for fast lookup
     bool m_taskRefWarn = true;  // Allow task reference warnings
     bool m_underSel = false;  // Under AstMemberSel or AstSel
@@ -390,7 +390,7 @@ private:
         iterateChildren(nodep);
         editDType(nodep);
         classEncapCheck(nodep, nodep->varp(), VN_CAST(nodep->classOrPackagep(), Class));
-        if (nodep->access().isWriteOrRW()) varLifetimeCheck(nodep, nodep->varp());
+        if (nodep->access().isWriteOrRW() || m_contReads) varLifetimeCheck(nodep, nodep->varp());
         if (VN_IS(nodep, VarRef))
             nodep->name("");  // Clear to save memory; nodep->name() will work via nodep->varp()
     }
@@ -416,12 +416,31 @@ private:
             }
         }
     }
+    void visit(AstAssignCont* nodep) override {
+        iterateAndNextNull(nodep->timingControlp());
+        {
+            VL_RESTORER(m_contNba);
+            VL_RESTORER(m_contReads);
+            m_contNba = "continuous";
+            m_contReads = true;
+            iterateAndNextNull(nodep->lhsp());
+            iterateAndNextNull(nodep->rhsp());
+        }
+        editDType(nodep);
+        AstNode* const controlp
+            = nodep->timingControlp() ? nodep->timingControlp()->unlinkFrBack() : nullptr;
+        nodep->replaceWith(new AstAssign{nodep->fileline(), nodep->lhsp()->unlinkFrBack(),
+                                         nodep->rhsp()->unlinkFrBack(), controlp});
+        VL_DO_DANGLING(pushDeletep(nodep), nodep);
+    }
     void visit(AstAssignDly* nodep) override {
         iterateAndNextNull(nodep->timingControlp());
         iterateAndNextNull(nodep->rhsp());
         {
             VL_RESTORER(m_contNba);
+            VL_RESTORER(m_contReads);
             m_contNba = "nonblocking";
+            m_contReads = false;
             iterateAndNextNull(nodep->lhsp());
         }
         editDType(nodep);
@@ -431,7 +450,9 @@ private:
         iterateAndNextNull(nodep->rhsp());
         {
             VL_RESTORER(m_contNba);
+            VL_RESTORER(m_contReads);
             m_contNba = "continuous";
+            m_contReads = false;
             iterateAndNextNull(nodep->lhsp());
         }
         editDType(nodep);
@@ -470,7 +491,7 @@ private:
             iterateChildren(nodep);
         }
         editDType(nodep);
-        if (auto* const classrefp = VN_CAST(nodep->fromp()->dtypep(), ClassRefDType)) {
+        if (AstClassRefDType* const classrefp = VN_CAST(nodep->fromp()->dtypep(), ClassRefDType)) {
             classEncapCheck(nodep, nodep->varp(), classrefp->classp());
         }  // else might be struct, etc
         varLifetimeCheck(nodep, nodep->varp());

src/verilog.y

@@ -3527,7 +3527,7 @@ statement_item<nodeStmtp>:          // IEEE: statement_item
                         { $$ = new AstAssignDly{$2, $1, $4, $3}; }
         //UNSUP cycle_delay fexprLvalue yP_LTE ';'      { UNSUP }
         |       yASSIGN idClassSel '=' delay_or_event_controlE expr ';'
-                        { $$ = new AstAssign{$1, $2, $5, $4}; }
+                        { $$ = new AstAssignCont{$1, $2, $5, $4}; }
         |       yDEASSIGN variable_lvalue ';'
                         { $$ = nullptr; BBUNSUP($1, "Unsupported: Verilog 1995 deassign"); DEL($2); }
         |       yFORCE variable_lvalue '=' expr ';'

test_regress/t/t_assign_cont_automatic_bad.out

@@ -0,0 +1,6 @@
+%Error: t/t_assign_cont_automatic_bad.v:14:26: Automatic lifetime variable not allowed in continuous assignment (IEEE 1800-2023 6.21): 'l'
+                                             : ... note: In instance 't'
+   14 |       assign g = signed'(l);   
+      |                          ^
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: Exiting due to

test_regress/t/t_assign_cont_automatic_bad.py

@@ -0,0 +1,16 @@
+#!/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('linter')
+
+test.lint(fails=True, expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_assign_cont_automatic_bad.v

@@ -0,0 +1,20 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2026 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+
+  reg g;
+
+  task automatic tsk;
+    reg l;
+    begin: cont_block
+      assign g = signed'(l);  // <--- BAD: using automatic in cont assignment
+    end
+  endtask
+
+  initial $stop;
+
+endmodule