checker: fix generic lambda with late concrete type inference (fix vl…

…ang#22497) (vlang#22509)

vlib/v/checker/str.v

@@ -75,7 +75,7 @@ fn (mut c Checker) string_inter_lit(mut node ast.StringInterLiteral) ast.Type {
 		if fmt == `_` { // set default representation for type if none has been given
 			fmt = c.get_default_fmt(ftyp, typ)
 			if fmt == `_` {
-				if typ != ast.void_type {
+				if typ != ast.void_type && !(c.inside_lambda && typ.has_flag(.generic)) {
 					c.error('no known default format for type `${c.table.get_type_name(ftyp)}`',
 						node.fmt_poss[i])
 				}

vlib/v/comptime/comptimeinfo.v

@@ -287,6 +287,9 @@ fn (mut ct ComptimeInfo) comptime_get_kind_var(var ast.Ident) ?ast.ComptimeForKi
 
 pub fn (mut ct ComptimeInfo) unwrap_generic_expr(expr ast.Expr, default_typ ast.Type) ast.Type {
 	match expr {
+		ast.StringLiteral, ast.StringInterLiteral {
+			return ast.string_type
+		}
 		ast.ParExpr {
 			return ct.unwrap_generic_expr(expr.expr, default_typ)
 		}

vlib/v/tests/generics/generic_lambda_inference_test.v

@@ -0,0 +1,21 @@
+const result = ['0: a', '1: b', '2: c', '3: d']
+
+fn mapi[T, U](arr []T, callback fn (int, T) U) []U {
+	mut mapped := []U{}
+	for i, el in arr {
+		mapped << callback(i, el)
+	}
+	return mapped
+}
+
+fn test_main() {
+	arr := [`a`, `b`, `c`, `d`]
+	arr_1 := mapi(arr, |i, e| '${i}: ${e}')
+	assert arr_1 == result
+	arr_2 := mapi[rune, string](arr, |i, e| '${i}: ${e}')
+	assert arr_2 == result
+	arr_3 := mapi(arr, fn (i int, e rune) string {
+		return '${i}: ${e}'
+	})
+	assert arr_3 == result
+}