Fix (const) ref default task argument handling (#7698)

src/V3Task.cpp

@@ -119,6 +119,8 @@ class TaskStateVisitor final : public VNVisitor {
     V3Graph m_callGraph;  // Task call graph
     TaskBaseVertex* m_curVxp;  // Current vertex we're adding to
     std::vector<AstInitialAutomatic*> m_initialps;  // Initial blocks to move
+    bool m_underPortVar = false;  // Visiting under a port AstVar; any expression there
+                                  // is a default value, evaluated at call sites only
 
 public:
     // METHODS
@@ -289,11 +291,14 @@ private:
         }
     }
     void visit(AstVar* nodep) override {
+        VL_RESTORER(m_underPortVar);
+        if (nodep->isIO()) m_underPortVar = true;
         iterateChildren(nodep);
         nodep->user4p(m_curVxp);  // Remember what task it's under
     }
     void visit(AstVarRef* nodep) override {
         iterateChildren(nodep);
+        if (m_underPortVar) return;
         AstVar* const varp = nodep->varp();
         if (varp->user4u().toGraphVertex() != m_curVxp) {
             if (m_curVxp->pure() && !varp->isXTemp() && !varp->isParam()) m_curVxp->impure(nodep);
@@ -1404,6 +1409,7 @@ class TaskVisitor final : public VNVisitor {
                         // Move it to new function
                         unlinkAndClone(nodep, portp, false);
                         portp->funcLocal(true);
+                        if (portp->valuep()) pushDeletep(portp->valuep()->unlinkFrBack());
                         cfuncp->addArgsp(portp);
                         // Pass inputs to DPI import wrappers by reference, unless fits in register
                         if (cfuncp->dpiImportWrapper() && portp->isReadOnly()) {
@@ -2139,6 +2145,14 @@ AstNodeFTask* V3Task::taskConnectWrapNew(AstNodeFTask* taskp, const string& newn
             newTaskp->addStmtsp(newPortp);
         } else {  // Defaulting arg
             AstNodeExpr* const valuep = VN_AS(portp->valuep(), NodeExpr);
+            if ((portp->isRef() || portp->isConstRef()) && VN_IS(valuep, VarRef)) {
+                const VAccess refAccess = portp->isWritable() ? VAccess::WRITE : VAccess::READ;
+                AstVarRef* const refp = VN_AS(valuep->cloneTree(false), VarRef);
+                refp->access(refAccess);
+                AstArg* const newArgp = new AstArg{portp->fileline(), portp->name(), refp};
+                newCallp->addArgsp(newArgp);
+                continue;
+            }
             // Create local temporary
             newPortp = new AstVar{portp->fileline(), VVarType::BLOCKTEMP, portp->name(),
                                   portp->dtypep()};

test_regress/t/t_func_ref_arg_default.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# 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-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile(verilator_flags2=["--binary"])
+
+test.execute()
+
+test.passes()

test_regress/t/t_func_ref_arg_default.v

@@ -0,0 +1,52 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+// A task argument that defaults to a plain variable must alias that variable,
+// not a copy of it: a write through a 'ref' default must propagate back, and a
+// 'const ref' default must observe later updates to the variable.
+
+module t;
+  int shared = 5;
+  logic flag = 0;
+  logic done = 0;
+
+  // writable 'ref' default: a write through it must reach 'shared'
+  task automatic incr(ref int r = shared);
+`ifdef TEST_NOINLINE
+    // verilator no_inline_task
+`endif
+    r = r + 10;
+  endtask
+
+  // 'const ref' default: must observe a later update to 'flag'
+  task automatic waitflag(output logic odone, const ref logic r = flag);
+`ifdef TEST_NOINLINE
+    // verilator no_inline_task
+`endif
+    while (!r) #1;
+    odone = 1'b1;
+  endtask
+
+  initial begin
+    incr();
+    if (shared !== 15) begin
+      $write("%%Error: write through default 'ref' lost (shared=%0d)\n", shared);
+      $stop;
+    end
+    fork
+      waitflag(done);
+    join_none
+    #5;
+    flag = 1'b1;
+    #5;
+    if (done !== 1'b1) begin
+      $write("%%Error: default 'const ref' did not observe update\n");
+      $stop;
+    end
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule

test_regress/t/t_func_ref_arg_default_noinl.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# 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-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+test.top_filename = "t/t_func_ref_arg_default.v"
+
+test.compile(verilator_flags2=["--binary"], v_flags2=["+define+TEST_NOINLINE"])
+
+test.execute()
+
+test.passes()