Fix rand variable used as array index in constraint evaluated as constant (#7238) (#7247)

src/V3Randomize.cpp

@@ -1672,10 +1672,31 @@ class ConstraintExprVisitor final : public VNVisitor {
         if (editFormat(nodep)) return;
         FileLine* const fl = nodep->fileline();
         VNRelinker handle;
-        AstNodeExpr* const indexp
-            = new AstSFormatF{fl, "#x%8x", false, nodep->bitp()->unlinkFrBack(&handle)};
-        handle.relink(indexp);
-        editSMT(nodep, nodep->fromp(), indexp);
+        // Check if index actually references a rand variable (not just user1,
+        // which can be over-marked in sum/with expansion contexts)
+        bool indexIsRand = false;
+        nodep->bitp()->foreach([&](const AstNodeVarRef* vrefp) {
+            if (vrefp->varp()->rand().isRandomizable()) indexIsRand = true;
+        });
+        if (indexIsRand) {
+            // Index depends on rand variable -- keep as SMT symbol.
+            // Array index sort is 32-bit, so zero-extend narrower indices.
+            AstNodeExpr* indexp = nodep->bitp()->unlinkFrBack(&handle);
+            if (indexp->width() < 32) {
+                AstExtend* const extendp = new AstExtend{fl, indexp, 32};
+                extendp->dtypeSetLogicSized(32, VSigning::UNSIGNED);
+                extendp->user1(true);
+                indexp = extendp;
+            }
+            handle.relink(indexp);
+            editSMT(nodep, nodep->fromp(), indexp);
+        } else {
+            // Index is constant or non-rand -- format as hex literal
+            AstNodeExpr* const indexp
+                = new AstSFormatF{fl, "#x%8x", false, nodep->bitp()->unlinkFrBack(&handle)};
+            handle.relink(indexp);
+            editSMT(nodep, nodep->fromp(), indexp);
+        }
     }
     void visit(AstMemberSel* nodep) override {
         // Check if rootVar is globalConstrained

test_regress/t/t_constraint_rand_array_index.py

@@ -0,0 +1,21 @@
+#!/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')
+
+if not test.have_solver:
+    test.skip("No constraint solver installed")
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_constraint_rand_array_index.v

@@ -0,0 +1,41 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 PlanV GmbH
+// SPDX-License-Identifier: CC0-1.0
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
+`define check_range(gotv,minv,maxv) do if ((gotv) < (minv) || (gotv) > (maxv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d-%0d\n", `__FILE__,`__LINE__, (gotv), (minv), (maxv)); `stop; end while(0);
+// verilog_format: on
+
+// Test that rand variables used as array indices in constraints are treated
+// as symbolic in the solver, not evaluated at C++ constraint setup time.
+
+class RandArrayIndexTest;
+  rand bit [1:0] idx;
+  rand bit [7:0] data [4];
+  rand bit [7:0] selected_value;
+
+  constraint solve_order { solve idx before selected_value; }
+  constraint value_match { selected_value == data[idx]; }
+  constraint data_values {
+    foreach (data[i]) {
+      data[i] inside {[8'd10:8'd50]};
+    }
+  }
+endclass
+
+module t;
+  initial begin
+    static RandArrayIndexTest obj = new();
+    repeat (20) begin
+      `checkd(obj.randomize(), 1)
+      `checkd(obj.selected_value, obj.data[obj.idx])
+      `check_range(obj.selected_value, 8'd10, 8'd50)
+    end
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule