Fix incorrect assumption in V3DfgDecomposition (#6215)

Due to SCC merging the deleted assumption/assertion can be violated.
Fixes #6211.

src/V3DfgDecomposition.cpp

@@ -345,16 +345,19 @@ class ExtractCyclicComponents final {
     void fixEdges(DfgVertexVar& vtx) {
         const size_t component = state(vtx).component;
 
-        // All variable vertices have at most a single source, and only variable
-        // vertices can have multiple sinks, therefore the source must be either:
-        // - in the same component as the variable vertex
-        // - be a variable vertex itself, which might be in a different component
-        // The later case will be fixed up when handling the source variable
-        DfgVertex* const srcp = vtx.srcp();
-        UASSERT_OBJ(!srcp || srcp->is<DfgVertexVar>() || state(*srcp).component == component, &vtx,
-                    "Driver of DfgVertexVar must be in the same component");
+        // Fix up sources in a different component
+        vtx.forEachSourceEdge([&](DfgEdge& edge, size_t) {
+            DfgVertex* const srcp = edge.sourcep();
+            if (!srcp) return;
+            const size_t sourceComponent = state(*srcp).component;
+            // Same component is OK
+            if (sourceComponent == component) return;
+            // Relink the source to write the clone
+            edge.unlinkSource();
+            getClone(vtx, sourceComponent).srcp(srcp);
+        });
 
-        // Fix up sinks in a differetn component
+        // Fix up sinks in a different component
         vtx.forEachSinkEdge([&](DfgEdge& edge) {
             const size_t sinkComponent = state(*edge.sinkp()).component;
             // Same component is OK

test_regress/t/t_dfg_circular_merged_scc.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('vlt')
+
+test.compile()
+
+test.passes()

test_regress/t/t_dfg_circular_merged_scc.v

@@ -0,0 +1,27 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Geza Lore.
+// SPDX-License-Identifier: CC0-1.0
+
+module mul (input [8:0] A, input [16:0] B, output [25:0] Y);
+  assign Y = $signed(A) * $signed(B);
+endmodule
+
+module A;
+  wire [26:0] C;
+  wire [26:0] D;
+  wire [8:0]  E;
+
+  // This yields a circular DFG with a fairly special form that used to trip
+  // decomposition.
+  mul mul (
+    .A(9'd10),
+    .B(17'h0cccd),
+    .Y({ C[26], C[9:0], D[15:1] })
+  );
+
+  assign E = { C[8:0] };
+  assign C[25:10] = {16{C[26]}};
+
+endmodule