Fix lambda coroutines (#6106) (#7135)

src/V3EmitCFunc.h

@@ -1199,7 +1199,7 @@ public:
         if (VN_IS(nodep->exprp()->dtypep()->skipRefp(), VoidDType)) {
             putns(nodep, "");
         } else {
-            putns(nodep, "(void)");  // Prevent unused expression warning in C
+            putns(nodep, "std::ignore = ");
         }
         iterateConst(nodep->exprp());
         puts(";\n");
@@ -1293,6 +1293,22 @@ public:
     }
     void visit(AstExprStmt* nodep) override {
         VL_RESTORER(m_createdScopeHash);
+        const bool containsAwait = nodep->exists([](AstCAwait*) -> bool { return true; });
+        if (containsAwait) {
+            UASSERT_OBJ(m_cfuncp && m_cfuncp->isCoroutine(), nodep,
+                        "AstExprStmt with CAwait must be in coroutine");
+            putnbs(nodep, "(co_await ([&]() -> VlCoroutine {\n");
+            iterateAndNextConstNull(nodep->stmtsp());
+            puts("co_return;\n");
+            if (!nodep->hasResult()) {
+                puts("}()))");
+                return;
+            }
+            puts("}()), ");
+            iterateAndNextConstNull(nodep->resultp());
+            puts(")");
+            return;
+        }
         // GCC allows compound statements in expressions, but this is not standard.
         // So we use an immediate-evaluation lambda and comma operator
         putnbs(nodep, "([&]() {\n");

src/V3Width.cpp

@@ -4504,7 +4504,7 @@ class WidthVisitor final : public VNVisitor {
                     = new AstCMethodHard{nodep->fileline(), nodep->fromp()->unlinkFrBack(),
                                          VCMethod::RNG_SET_RANDSTATE, exprp->unlinkFrBack()};
                 newp->usePtr(true);
-                newp->dtypeSetString();
+                newp->dtypeSetVoid();
                 nodep->replaceWith(newp);
                 VL_DO_DANGLING(pushDeletep(nodep), nodep);
                 return;
@@ -6994,7 +6994,7 @@ class WidthVisitor final : public VNVisitor {
                     = new AstCExpr{nodep->fileline(), "__Vm_rng.set_randstate(", 1};
                 newp->add(exprp->unlinkFrBack());
                 newp->add(")");
-                newp->dtypeSetString();
+                newp->dtypeSetVoid();
                 nodep->replaceWith(newp);
                 VL_DO_DANGLING(pushDeletep(nodep), nodep);
                 return;

test_regress/t/t_coroutine_lambda.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Coroutine call inside AstExprStmt lambda
+#
+# 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",
+    "--timing",
+    "-Wno-FUNCTIMECTL",
+    "-Wno-WIDTHTRUNC",
+])
+
+test.passes()

test_regress/t/t_coroutine_lambda.v

@@ -0,0 +1,83 @@
+// DESCRIPTION: Verilator: Coroutine call inside AstExprStmt lambda test module
+//
+// 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
+interface cpu_if(input logic clk);
+endinterface
+
+package p;
+
+virtual class WriterIf;
+  virtual function void write(input int t);
+  endfunction
+endclass
+
+class BlockingWriter;
+  virtual cpu_if vif;
+  task write(int t);
+    @(posedge vif.clk);
+  endtask
+endclass
+
+class WriterAdapter extends WriterIf;
+  BlockingWriter m_impl;
+  function new(BlockingWriter impl);
+    m_impl = impl;
+  endfunction
+  function void write(input int t);
+    m_impl.write(t);  // function -> task path
+  endfunction
+endclass
+
+class QueueLike;
+  WriterIf sink;
+  mailbox #(int) m;
+  function bit try_get(output int t);
+    if (!m.try_get(t)) begin
+    end
+    sink.write(t);  // can become coroutine call
+  endfunction
+endclass
+
+class DriverLike;
+  QueueLike reqq;
+  function void item_done();
+    int t;
+    if (reqq.try_get(t) == 0) begin
+    end
+  endfunction
+endclass
+
+endpackage
+
+module t;
+  import p::*;
+
+  logic clk = 0;
+  cpu_if vif(clk);
+
+  always #1 clk = ~clk;
+
+  initial begin
+    BlockingWriter writer;
+    WriterAdapter adapter;
+    QueueLike reqq;
+    DriverLike drv;
+
+    writer = new();
+    writer.vif = vif;
+    adapter = new(writer);
+
+    reqq = new();
+    reqq.sink = adapter;
+
+    drv = new();
+    drv.reqq = reqq;
+    drv.item_done();
+
+    #2 $finish;
+  end
+endmodule