From 1658c4789f473536f4d68e30c3a72445302fe017 Mon Sep 17 00:00:00 2001
From: spaceface <spaceface7777@gmail.com>
Date: Mon, 22 Feb 2021 13:55:43 +0100
Subject: [PATCH] cgen: fix `match`ing interface types (#8889)

---
 vlib/v/gen/c/cgen.v                 | 13 ++++++-------
 vlib/v/tests/match_interface_test.v | 12 ++++++++++++
 2 files changed, 18 insertions(+), 7 deletions(-)
 create mode 100644 vlib/v/tests/match_interface_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 29e7c1bed0..e5e7091aa7 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -3610,16 +3610,15 @@ fn (mut g Gen) match_expr_sumtype(node ast.MatchExpr, is_expr bool, cond_var str
 					g.write('if (')
 				}
 				g.write(cond_var)
-				// branch_sym := g.table.get_type_symbol(branch.typ)
+				dot_or_ptr := if node.cond_type.is_ptr() { '->' } else { '.' }
 				if sym.kind == .sum_type {
-					dot_or_ptr := if node.cond_type.is_ptr() { '->' } else { '.' }
-					g.write(dot_or_ptr)
-					g.write('typ == ')
+					g.write('${dot_or_ptr}typ == ')
+					g.expr(branch.exprs[sumtype_index])
 				} else if sym.kind == .interface_ {
-					// g.write('._interface_idx == _${sym.name}_${branch_sym} ')
-					g.write('._interface_idx == ')
+					typ := branch.exprs[sumtype_index] as ast.Type
+					branch_sym := g.table.get_type_symbol(typ.typ)
+					g.write('${dot_or_ptr}_interface_idx == _${sym.cname}_${branch_sym.cname}_index')
 				}
-				g.expr(branch.exprs[sumtype_index])
 				if is_expr && tmp_var.len == 0 {
 					g.write(') ? ')
 				} else {
diff --git a/vlib/v/tests/match_interface_test.v b/vlib/v/tests/match_interface_test.v
new file mode 100644
index 0000000000..a33a7bb9e4
--- /dev/null
+++ b/vlib/v/tests/match_interface_test.v
@@ -0,0 +1,12 @@
+interface Animal { name string }
+struct Dog { name string }
+struct Cat { name string }
+
+fn test_interface_match() {
+    a := Animal(Dog{name: 'Jet'})
+    match a {
+        Dog { assert true }
+        Cat { assert false }
+        else { assert false }
+    }
+}