diff --git a/vlib/v/ast/types.v b/vlib/v/ast/types.v
index 1702602e67..5a19f2a6b0 100644
--- a/vlib/v/ast/types.v
+++ b/vlib/v/ast/types.v
@@ -411,6 +411,9 @@ pub const (
 	error_type         = new_type(error_type_idx)
 	charptr_types      = [charptr_type, new_type(char_type_idx).set_nr_muls(1)]
 	byteptr_types      = [byteptr_type, new_type(byte_type_idx).set_nr_muls(1)]
+	cptr_or_bptr_types = [charptr_type, byteptr_type, new_type(char_type_idx).set_nr_muls(1),
+		new_type(byte_type_idx).set_nr_muls(1),
+	]
 )
 
 pub const (
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 47772a326b..23246c6f97 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -67,6 +67,7 @@ pub mut:
 	inside_anon_fn bool
 	inside_ref_lit bool
 	inside_fn_arg  bool // `a`, `b` in `a.f(b)`
+	inside_c_call  bool // true inside C.printf( param ) calls, but NOT in nested calls, unless they are also C.
 	skip_flags     bool // should `#flag` and `#include` be skipped
 mut:
 	files                            []ast.File
@@ -90,7 +91,6 @@ mut:
 	inside_println_arg       bool
 	inside_decl_rhs          bool
 	need_recheck_generic_fns bool // need recheck generic fns because there are cascaded nested generic fn
-	is_c_call                bool // remove once C.c_call("string") deprecation is removed
 }
 
 pub fn new_checker(table &ast.Table, pref &pref.Preferences) &Checker {
@@ -1694,6 +1694,11 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ ast.Type, call_exp
 }
 
 pub fn (mut c Checker) method_call(mut call_expr ast.CallExpr) ast.Type {
+	was_inside_c_call := c.inside_c_call
+	c.inside_c_call = call_expr.language == .c
+	defer {
+		c.inside_c_call = was_inside_c_call
+	}
 	left_type := c.expr(call_expr.left)
 	c.expected_type = left_type
 	mut is_generic := left_type.has_flag(.generic)
@@ -2176,10 +2181,10 @@ fn (mut c Checker) array_builtin_method_call(mut call_expr ast.CallExpr, left_ty
 }
 
 pub fn (mut c Checker) fn_call(mut call_expr ast.CallExpr) ast.Type {
-	was_c_call := c.is_c_call
-	c.is_c_call = call_expr.language == .c
+	was_inside_c_call := c.inside_c_call
+	c.inside_c_call = call_expr.language == .c
 	defer {
-		c.is_c_call = was_c_call
+		c.inside_c_call = was_inside_c_call
 	}
 	fn_name := call_expr.name
 	if fn_name == 'main' {
@@ -2506,6 +2511,10 @@ pub fn (mut c Checker) fn_call(mut call_expr ast.CallExpr) ast.Type {
 			c.warn('automatic referencing/dereferencing is deprecated and will be removed soon (got: $typ.nr_muls() references, expected: $param.typ.nr_muls() references)',
 				call_arg.pos)
 		}
+		if func.language == .c && typ == ast.string_type && param.typ in ast.cptr_or_bptr_types {
+			c.warn("automatic string to C-string conversion is deprecated and will be removed on 2021-06-19, use `c'<string_value>'` and set the C function parameter type to `&u8`",
+				call_arg.pos)
+		}
 	}
 	if func.generic_names.len != call_expr.concrete_types.len {
 		// no type arguments given in call, attempt implicit instantiation
@@ -4760,10 +4769,6 @@ pub fn (mut c Checker) expr(node ast.Expr) ast.Type {
 				// string literal starts with "c": `C.printf(c'hello')`
 				return ast.byte_type.set_nr_muls(1)
 			}
-			if node.language != .c && c.is_c_call {
-				c.warn("automatic string to C-string conversion is deprecated and will be removed on 2021-06-19, use `c'<string_value>'` and set the C function parameter type to `&u8`",
-					node.pos)
-			}
 			return ast.string_type
 		}
 		ast.StringInterLiteral {