diff --git a/vlib/builtin/builtin.v b/vlib/builtin/builtin.v
index 17b9d9981c..92a768e370 100644
--- a/vlib/builtin/builtin.v
+++ b/vlib/builtin/builtin.v
@@ -4,8 +4,8 @@
 module builtin
 
 __global (
-	g_m2_buf byteptr
-	g_m2_ptr byteptr
+	g_m2_buf byteptr 
+	g_m2_ptr byteptr 
 )
 
 // isnil returns true if an object is nil (only for C objects).
@@ -29,14 +29,31 @@ pub fn print_backtrace() {
 	print_backtrace_skipping_top_frames(2)
 }
 
+struct VCastTypeIndexName {
+	tindex int
+	tname  string
+}
+
 __global (
-	total_m    = i64(0)
-	nr_mallocs = int(0)
+	total_m              = i64(0)
+	nr_mallocs           = int(0)
+	// will be filled in cgen
+	as_cast_type_indexes   []VCastTypeIndexName 
 )
 
 fn __as_cast(obj voidptr, obj_type int, expected_type int) voidptr {
 	if obj_type != expected_type {
-		panic('as cast: cannot cast $obj_type to $expected_type')
+		mut obj_name := as_cast_type_indexes[0].tname
+		mut expected_name := as_cast_type_indexes[0].tname
+		for x in as_cast_type_indexes {
+			if x.tindex == obj_type {
+				obj_name = x.tname
+			}
+			if x.tindex == expected_type {
+				expected_name = x.tname
+			}
+		}
+		panic('as cast: cannot cast `$obj_name` to `$expected_name`')
 	}
 	return obj
 }
@@ -55,15 +72,15 @@ pub:
 	lvalue  string // the stringified *actual value* of the left side of a failed assertion
 	rvalue  string // the stringified *actual value* of the right side of a failed assertion
 }
+
 fn __print_assert_failure(i &VAssertMetaInfo) {
-	eprintln('${i.fpath}:${i.line_nr+1}: FAIL: fn ${i.fn_name}: assert ${i.src}')
+	eprintln('$i.fpath:${i.line_nr + 1}: FAIL: fn $i.fn_name: assert $i.src')
 	if i.op.len > 0 && i.op != 'call' {
-		eprintln('   left value: ${i.llabel} = ${i.lvalue}')
+		eprintln('   left value: $i.llabel = $i.lvalue')
 		if i.rlabel == i.rvalue {
 			eprintln('  right value: $i.rlabel')
-		}
-		else {
-			eprintln('  right value: ${i.rlabel} = ${i.rvalue}')
+		} else {
+			eprintln('  right value: $i.rlabel = $i.rvalue')
 		}
 	}
 }
diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index 207f02110a..a891a2d014 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -146,6 +146,7 @@ mut:
 	branch_parent_pos  int    // used in BranchStmt (continue/break) for autofree stop position
 	timers             &util.Timers = util.new_timers(false)
 	force_main_console bool // true when [console] used on fn main()
+	as_cast_type_names map[string]string // table for type name lookup in runtime (for __as_cast)
 }
 
 pub fn cgen(files []ast.File, table &table.Table, pref &pref.Preferences) string {
@@ -201,7 +202,6 @@ pub fn cgen(files []ast.File, table &table.Table, pref &pref.Preferences) string
 	}
 	g.init()
 	g.timers.show('cgen init')
-	//
 	mut tests_inited := false
 	mut autofree_used := false
 	for file in files {
@@ -258,7 +258,6 @@ pub fn cgen(files []ast.File, table &table.Table, pref &pref.Preferences) string
 			g.definitions.writeln('int _v_type_idx_${typ.cname}() { return $idx; };')
 		}
 	}
-	// g.write_str_definitions()
 	// v files are finished, what remains is pure C code
 	g.gen_vlines_reset()
 	if g.pref.build_mode != .build_module {
@@ -4984,6 +4983,12 @@ fn (mut g Gen) write_init_function() {
 		g.writeln('g_m2_buf = malloc(50 * 1000 * 1000);')
 		g.writeln('g_m2_ptr = g_m2_buf;')
 	}
+	// NB: the as_cast table should be *before* the other constant initialize calls,
+	// because it may be needed during const initialization of builtin and during
+	// calling module init functions too, just in case they do fail...
+	g.write('\tas_cast_type_indexes = ')
+	g.writeln(g.as_cast_name_table())
+	//
 	g.writeln('\tbuiltin_init();')
 	g.writeln('\tvinit_string_literals();')
 	//
@@ -4999,7 +5004,6 @@ fn (mut g Gen) write_init_function() {
 			}
 		}
 	}
-	//
 	g.writeln('}')
 	if g.pref.printfn_list.len > 0 && '_vinit' in g.pref.printfn_list {
 		println(g.out.after(fn_vinit_start_pos))
@@ -5014,6 +5018,7 @@ fn (mut g Gen) write_init_function() {
 			g.writeln(g.cleanups[mod_name].str())
 		}
 		// g.writeln('\tfree(g_str_buf);')
+		g.writeln('\tarray_free(&as_cast_type_indexes);')
 		g.writeln('}')
 		if g.pref.printfn_list.len > 0 && '_vcleanup' in g.pref.printfn_list {
 			println(g.out.after(fn_vcleanup_start_pos))
@@ -5848,22 +5853,50 @@ fn (mut g Gen) as_cast(node ast.AsCast) {
 	styp := g.typ(node.typ)
 	sym := g.table.get_type_symbol(node.typ)
 	expr_type_sym := g.table.get_type_symbol(node.expr_type)
-	if expr_type_sym.kind == .sum_type {
+	if expr_type_sym.info is table.SumType {
 		dot := if node.expr_type.is_ptr() { '->' } else { '.' }
-		g.write('/* as */ *($styp*)__as_cast((')
+		g.write('/* as */ *($styp*)__as_cast(')
+		g.write('(')
 		g.expr(node.expr)
 		g.write(')')
 		g.write(dot)
-		g.write('_$sym.cname, (')
+		g.write('_$sym.cname,')
+		g.write('(')
 		g.expr(node.expr)
 		g.write(')')
 		g.write(dot)
 		// g.write('typ, /*expected:*/$node.typ)')
 		sidx := g.type_sidx(node.typ)
-		g.write('typ, /*expected:*/$sidx)')
+		expected_sym := g.table.get_type_symbol(node.typ)
+		g.write('typ, $sidx) /*expected idx: $sidx, name: $expected_sym.name */ ')
+
+		// fill as cast name table
+		for variant in expr_type_sym.info.variants {
+			idx := variant.str()
+			if idx in g.as_cast_type_names {
+				continue
+			}
+			variant_sym := g.table.get_type_symbol(variant)
+			g.as_cast_type_names[idx] = variant_sym.name
+		}
 	}
 }
 
+fn (g Gen) as_cast_name_table() string {
+	if g.as_cast_type_names.len == 0 {
+		return 'new_array_from_c_array(1, 1, sizeof(VCastTypeIndexName), _MOV((VCastTypeIndexName[1]){(VCastTypeIndexName){.tindex = 0,.tname = _SLIT("unknown")}}));'
+	}
+	mut name_table := strings.new_builder(1024)
+	casts_len := g.as_cast_type_names.len + 1
+	name_table.writeln('new_array_from_c_array($casts_len, $casts_len, sizeof(VCastTypeIndexName), _MOV((VCastTypeIndexName[$casts_len]){')
+	name_table.writeln('\t\t  (VCastTypeIndexName){.tindex = 0, .tname = _SLIT("unknown")}')
+	for key, value in g.as_cast_type_names {
+		name_table.writeln('\t\t, (VCastTypeIndexName){.tindex = $key, .tname = _SLIT("$value")}')
+	}
+	name_table.writeln('\t}));')
+	return name_table.str()
+}
+
 fn (mut g Gen) is_expr(node ast.InfixExpr) {
 	eq := if node.op == .key_is { '==' } else { '!=' }
 	g.write('(')
diff --git a/vlib/v/tests/inout/bad_st_as.out b/vlib/v/tests/inout/bad_st_as.out
index a1baab65af..0282290301 100644
--- a/vlib/v/tests/inout/bad_st_as.out
+++ b/vlib/v/tests/inout/bad_st_as.out
@@ -2,4 +2,4 @@ Foo
 ================ V panic ================
    module: builtin
  function: __as_cast()
-  message: as cast: cannot cast
+  message: as cast: cannot cast `main.Struct` to `main.Interface`