From 78bcda14c4a0efc4a9d8c7713a7b9c34154164a4 Mon Sep 17 00:00:00 2001
From: Nick Treleaven <n@trelsoft.com>
Date: Tue, 13 Oct 2020 13:15:25 +0100
Subject: [PATCH] checker: infer T from parameter of type `[]T` (#6611)

---
 vlib/v/checker/check_types.v                  | 17 ++++++++++++++---
 vlib/v/tests/generic_fn_infer_modifier_test.v | 16 ++++++++++++++++
 2 files changed, 30 insertions(+), 3 deletions(-)
 create mode 100644 vlib/v/tests/generic_fn_infer_modifier_test.v

diff --git a/vlib/v/checker/check_types.v b/vlib/v/checker/check_types.v
index 8dbf5ad84b..7bc6896f87 100644
--- a/vlib/v/checker/check_types.v
+++ b/vlib/v/checker/check_types.v
@@ -376,15 +376,26 @@ pub fn (c &Checker) check_sumtype_compatibility(a, b table.Type) bool {
 pub fn (mut c Checker) infer_fn_types(f table.Fn, mut call_expr ast.CallExpr) {
 	gt_name := 'T'
 	mut typ := table.void_type
-	for i, arg in f.params {
-		if arg.type_source_name == gt_name {
-			typ = call_expr.args[i].typ
+	for i, param in f.params {
+		arg := call_expr.args[i]
+		if param.type_source_name == gt_name {
+			typ = arg.typ
+			break
+		}
+		arg_sym := c.table.get_type_symbol(arg.typ)
+		if arg_sym.kind == .array && param.type_source_name == '[]$gt_name' {
+			info := arg_sym.info as table.Array
+			typ = info.elem_type
 			break
 		}
 	}
 	if typ == table.void_type {
 		c.error('could not infer generic type `$gt_name` in call to `$f.name`', call_expr.pos)
 	} else {
+		if c.pref.is_verbose {
+			s := c.table.type_to_str(typ)
+			println('inferred `$f.name<$s>`')
+		}
 		c.table.register_fn_gen_type(f.name, typ)
 		call_expr.generic_type = typ
 	}
diff --git a/vlib/v/tests/generic_fn_infer_modifier_test.v b/vlib/v/tests/generic_fn_infer_modifier_test.v
new file mode 100644
index 0000000000..7e813b5b0f
--- /dev/null
+++ b/vlib/v/tests/generic_fn_infer_modifier_test.v
@@ -0,0 +1,16 @@
+fn f_array<T>(a []T) T {
+	return a[0]
+}
+
+fn g_array<T>(mut a []T) {
+	a[0] = a[1]
+}
+
+fn test_array() {
+	mut a := [7, 8]
+	r := f_array(a)
+	assert r == 7
+	
+	g_array(mut a)
+	assert a[0] == 8
+}