drop tf block edge case

- The deleted chunk in TFBlock has been unreachable since 57ef23ef. The
  RemoveComments change would make it reachable by dropping the leading
  Null in a task or function body.
- The optimization in TFBlock was unsound because the nested block could
  shadow identifiers used (but not defined) in the tail of the outer
  block.
- The optimization is not clearly necessary, so I'm dropping it for now.
- Expand tf_block test coverage.

src/Convert/RemoveComments.hs

@@ -61,4 +61,6 @@ convertDecls = filter (not . isCommentDecl)
         isCommentDecl _ = False
 
 convertStmts :: [Stmt] -> [Stmt]
-convertStmts = map $ traverseNestedStmts convertStmt
+convertStmts =
+    filter (/= Null) .
+    map (traverseNestedStmts convertStmt)

src/Convert/TFBlock.hs

@@ -47,11 +47,6 @@ flattenOuterBlocks (Block Seq "" declsA [stmt]) =
         then (declsA ++ declsB, stmtsB)
         else (declsA, [stmt])
     where (declsB, stmtsB) = flattenOuterBlocks stmt
-flattenOuterBlocks (Block Seq "" declsA (Block Seq name declsB stmtsA : stmtsB)) =
-    if canCombine declsA declsB
-        then flattenOuterBlocks $
-                Block Seq name (declsA ++ declsB) (stmtsA ++ stmtsB)
-        else (declsA, Block Seq name declsB stmtsA : stmtsB)
 flattenOuterBlocks (Block Seq name decls stmts)
     | null name = (decls, stmts)
     | otherwise = ([], [Block Seq name decls stmts])

test/core/tf_block.sv

@@ -11,4 +11,66 @@ module top;
     initial t();
     initial $display("f(0) = ", f(0));
     initial $display("f(1) = ", f(1));
+
+    task t1;
+        localparam X = 1;
+        begin
+            localparam X = 2;
+            $display("t1", X);
+        end
+    endtask
+    task t2;
+        localparam X = 10;
+        begin
+            localparam X = 20;
+            $display("t2", X);
+        end
+        $display("t2", X);
+    endtask
+    task t3;
+        localparam X = 100;
+        begin
+            localparam Y = 200;
+            $display("t3", Y);
+        end
+        $display("t3", X);
+    endtask
+    task t4;
+        begin
+            begin
+                localparam Y = 99;
+                $display("t4", Y);
+            end
+            $display("t4", 123);
+        end
+    endtask
+    initial begin
+        t1;
+        t2;
+        t3;
+        t4;
+    end
+
+    localparam X = 1;
+    task a;
+        localparam X = 2;
+        begin
+            begin : blk
+                localparam X = 3;
+                $display("a", X);
+            end
+            $display("a", X);
+        end
+    endtask
+    task b;
+        begin : blk
+            localparam X = 5;
+            $display("b", X);
+        end
+        $display("b", X);
+    endtask
+    initial begin
+        a;
+        b;
+    end
 endmodule

test/core/tf_block.v

@@ -12,4 +12,59 @@ module top;
     initial t();
     initial $display("f(0) = ", f(0));
     initial $display("f(1) = ", f(1));
+
+    task t1;
+        localparam X = 2;
+        $display("t1", X);
+    endtask
+    task t2;
+        localparam X = 10;
+        localparam Y = 20;
+        begin
+            $display("t2", Y);
+            $display("t2", X);
+        end
+    endtask
+    task t3;
+        localparam X = 100;
+        localparam Y = 200;
+        begin
+            $display("t3", Y);
+            $display("t3", X);
+        end
+    endtask
+    task t4;
+        localparam Y = 99;
+        begin
+            $display("t4", Y);
+            $display("t4", 123);
+        end
+    endtask
+    initial begin
+        t1;
+        t2;
+        t3;
+        t4;
+    end
+
+    localparam X = 1;
+    task a;
+        localparam X = 2;
+        localparam Y = 3;
+        begin
+            $display("a", Y);
+            $display("a", X);
+        end
+    endtask
+    task b;
+        localparam Y = 5;
+        begin
+            $display("b", Y);
+            $display("b", X);
+        end
+    endtask
+    initial begin
+        a;
+        b;
+    end
 endmodule