[Sema] make printf a reserved function

Author: isuckatcs

include/lexer.h

@@ -34,6 +34,7 @@ enum class TokenKind : char {
   KwWhile,
   KwReturn,
   KwStruct,
+  KwPrintf,
 
   Eof = singleCharTokens[0],
   Lpar = singleCharTokens[1],

src/sema.cpp

@@ -666,6 +666,10 @@ Sema::resolveFunctionDecl(const FunctionDecl &function) {
     if (!function.params.empty())
       return report(function.location,
                     "'main' function is expected to take no arguments");
+  } else if (function.identifier == "printf") {
+    return report(function.location,
+                  "'printf' is a reserved function name and cannot be used for "
+                  "user-defined functions");
   }
 
   std::vector<std::unique_ptr<ResolvedParamDecl>> resolvedParams;

test/sema/printf_reserved.yl

@@ -0,0 +1,10 @@
+// RUN: compiler %s -res-dump 2>&1 | filecheck %s
+
+// CHECK: [[# @LINE + 1 ]]:1: error: 'printf' is a reserved function name and cannot be used for user-defined functions
+fn printf(): number {
+  return 12345;
+}
+
+fn main(): void {
+  println(printf());
+}