compiler: fix generation of default .str() methods in interpolation

vlib/compiler/tests/string_interpolation_variadic_test.v

@@ -0,0 +1,34 @@
+// This file tests whether V can generate a convenience default .str() method
+// for var args of a custom type, when the developer has NOT defined one.
+// Although similar to string_interpolation_struct_test.v, they should not be
+// merged.
+
+struct Man {
+	name string
+	age int
+	interests []string
+}
+
+fn my_variadic_function(x ...Man) string {
+	return '$x' // this interpolation should generate .str() methods for Man
+}
+
+fn test_vargs_string_interpolation(){
+	man := Man{'Me', 38, ['programming', 'reading', 'hiking']}
+	superman := Man{'Superman', 30, ['flying','fighting evil','being nice']}
+
+	results := my_variadic_function(superman, man)
+
+	assert results.contains('Man {')
+	//
+	assert results.contains('name: Superman')
+	assert results.contains('age: 30')
+	assert results.contains('}, Man {')
+	//
+	assert results.contains('interests: ["programming"')
+	assert results.contains('name: Me')
+	//
+	assert results.contains('}]')
+	//
+	println(results)
+}