Fix internal error on `dist` under implication operator in constraints (#7440) (#7442)

Fixes #7440

src/V3Randomize.cpp

@@ -3998,6 +3998,23 @@ class RandomizeVisitor final : public VNVisitor {
         }
     }
 
+    // Rewrite a LogIf-of-Dist chain into nested AstConstraintIf. The outermost
+    // AstLogIf shell is left for the caller's AstConstraintExpr to free; inner
+    // shells are deleted here once their children are transplanted.
+    AstConstraintIf* liftLogIfChainToConstraintIf(AstLogIf* logIfp) {
+        FileLine* const fl = logIfp->fileline();
+        AstNodeExpr* const condp = logIfp->lhsp()->unlinkFrBack();
+        AstNodeExpr* const rhsp = logIfp->rhsp()->unlinkFrBack();
+        AstNode* thenBodyp;
+        if (AstLogIf* const innerLogIfp = VN_CAST(rhsp, LogIf)) {
+            thenBodyp = liftLogIfChainToConstraintIf(innerLogIfp);
+            VL_DO_DANGLING(pushDeletep(innerLogIfp), innerLogIfp);
+        } else {
+            thenBodyp = new AstConstraintExpr{fl, rhsp};
+        }
+        return new AstConstraintIf{fl, condp, thenBodyp, nullptr};
+    }
+
     // Replace AstDist with weighted bucket selection via AstConstraintIf chain.
     // Supports both constant and variable weight expressions.
     void lowerDistConstraints(AstTask* taskp, AstNode* constrItemsp) {
@@ -4019,6 +4036,25 @@ class RandomizeVisitor final : public VNVisitor {
 
             AstConstraintExpr* const constrExprp = VN_CAST(itemp, ConstraintExpr);
             if (!constrExprp) continue;
+
+            // `cond -> x dist {...}` parses as ConstraintExpr(LogIf(cond, Dist)).
+            // This pass only scans ConstraintExpr/If/Foreach for Dist, so lift the
+            // LogIf chain into nested ConstraintIf first. Chains like
+            // `a -> b -> dist` nest accordingly.
+            if (AstLogIf* const topLogIfp = VN_CAST(constrExprp->exprp(), LogIf)) {
+                AstNode* chainEndp = topLogIfp->rhsp();
+                while (AstLogIf* const innerp = VN_CAST(chainEndp, LogIf)) {
+                    chainEndp = innerp->rhsp();
+                }
+                if (VN_IS(chainEndp, Dist)) {
+                    AstConstraintIf* const liftedp = liftLogIfChainToConstraintIf(topLogIfp);
+                    constrExprp->replaceWith(liftedp);
+                    VL_DO_DANGLING(pushDeletep(constrExprp), constrExprp);
+                    lowerDistConstraints(taskp, liftedp->thensp());
+                    continue;
+                }
+            }
+
             AstDist* const distp = VN_CAST(constrExprp->exprp(), Dist);
             if (!distp) continue;
 

test_regress/t/t_randomize_dist_implication.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_randomize_dist_implication.v

@@ -0,0 +1,84 @@
+// 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);
+// verilog_format: on
+
+class DistScalar;
+  rand bit [7:0] x;
+  rand bit mode;
+  rand bit [1:0] mode2;
+  constraint force_cond { mode == 1'b0; mode2 == 2'b00; }
+  constraint c {
+    if (mode) {
+      x dist { 8'd0 := 1, 8'd255 := 3 };
+    } else {
+      (mode2 == '0) -> x dist { 8'd0 := 3, 8'd255 := 1 };
+    }
+  }
+endclass
+
+class DistChain;
+  rand bit [7:0] x;
+  rand bit a;
+  rand bit b;
+  constraint force_ab { a == 1'b1; b == 1'b1; }
+  constraint c {
+    a -> b -> x dist { 8'd0 := 3, 8'd255 := 1 };
+  }
+endclass
+
+class DistForeachImpl;
+  rand bit [7:0] arr [4];
+  rand bit enb;
+  constraint force_enb { enb == 1'b1; }
+  constraint c {
+    foreach (arr[i]) {
+      enb -> arr[i] dist { 8'd10 := 1, 8'd20 := 1, 8'd30 := 1 };
+    }
+  }
+endclass
+
+module t;
+  DistScalar obj;
+  DistChain ch;
+  DistForeachImpl fa;
+
+  initial begin
+    int p;
+    obj = new;
+    ch = new;
+    fa = new;
+
+    repeat (20) begin
+      p = obj.randomize();
+      `checkd(p, 1);
+      // Implication is forced to fire by force_cond; x must be a bucket value.
+      `checkd(obj.mode, 1'b0);
+      `checkd(obj.mode2, 2'b00);
+      `checkd((obj.x == 8'd0) || (obj.x == 8'd255), 1'b1);
+
+      p = ch.randomize();
+      `checkd(p, 1);
+      `checkd(ch.a, 1'b1);
+      `checkd(ch.b, 1'b1);
+      `checkd((ch.x == 8'd0) || (ch.x == 8'd255), 1'b1);
+
+      p = fa.randomize();
+      `checkd(p, 1);
+      `checkd(fa.enb, 1'b1);
+      foreach (fa.arr[i]) begin
+        `checkd((fa.arr[i] == 8'd10) || (fa.arr[i] == 8'd20)
+                || (fa.arr[i] == 8'd30), 1'b1);
+      end
+    end
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule