write_verilog: don't `assign` to a `reg`.

Fixes #2035.

backends/verilog/verilog_backend.cc

@@ -2014,9 +2014,22 @@ void dump_sync_effect(std::ostream &f, std::string indent, const RTLIL::SigSpec
 
 void dump_conn(std::ostream &f, std::string indent, const RTLIL::SigSpec &left, const RTLIL::SigSpec &right)
 {
-	if (simple_lhs) {
+	bool all_chunks_wires = true;
+	for (auto &chunk : left.chunks())
+		if (chunk.is_wire() && reg_wires.count(chunk.wire->name))
+			all_chunks_wires = false;
+	if (!simple_lhs && all_chunks_wires) {
+		f << stringf("%s" "assign ", indent.c_str());
+		dump_sigspec(f, left);
+		f << stringf(" = ");
+		dump_sigspec(f, right);
+		f << stringf(";\n");
+	} else {
 		int offset = 0;
 		for (auto &chunk : left.chunks()) {
+			if (chunk.is_wire() && reg_wires.count(chunk.wire->name))
+				f << stringf("%s" "always%s\n%s  ", indent.c_str(), systemverilog ? "_comb" : " @*", indent.c_str());
+			else
 				f << stringf("%s" "assign ", indent.c_str());
 			dump_sigspec(f, chunk);
 			f << stringf(" = ");
@@ -2024,12 +2037,6 @@ void dump_conn(std::ostream &f, std::string indent, const RTLIL::SigSpec &left,
 			f << stringf(";\n");
 			offset += GetSize(chunk);
 		}
-	} else {
-		f << stringf("%s" "assign ", indent.c_str());
-		dump_sigspec(f, left);
-		f << stringf(" = ");
-		dump_sigspec(f, right);
-		f << stringf(";\n");
 	}
 }
 

tests/simple/.gitignore

@@ -1,2 +1,3 @@
 *.log
 *.out
+*.err

tests/verilog/.gitignore

@@ -1,6 +1,10 @@
 /*.log
 /*.out
+/*.err
 /run-test.mk
 /const_arst.v
 /const_sr.v
 /doubleslash.v
+/roundtrip_proc_1.v
+/roundtrip_proc_2.v
+/assign_to_reg.v

tests/verilog/assign_to_reg.ys

@@ -0,0 +1,22 @@
+# https://github.com/yosyshq/yosys/issues/2035
+
+read_ilang <<END
+module \top
+  wire width 1 input 0 \halfbrite
+  wire width 2 output 1 \r_on
+  process $1
+    assign \r_on [1:0] 2'00
+    assign \r_on [1:0] 2'11
+    switch \halfbrite [0]
+      case 1'1
+        assign \r_on [1] 1'0
+    end
+  end
+end
+END
+proc_prune
+write_verilog assign_to_reg.v
+design -reset
+
+logger -expect-no-warnings
+read_verilog assign_to_reg.v