v.parser: prohibit redeclaration of imported functions (#10564)

vlib/v/parser/fn.v

@@ -267,13 +267,20 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 				}
 			}
 		}
-		// cannot redefine buildin function
+		if !p.pref.is_fmt {
 			if !is_method && !p.builtin_mod && name in builtin_functions {
 				p.error_with_pos('cannot redefine builtin function `$name`', name_pos)
 				return ast.FnDecl{
 					scope: 0
 				}
 			}
+			if name in p.imported_symbols {
+				p.error_with_pos('cannot redefine imported function `$name`', name_pos)
+				return ast.FnDecl{
+					scope: 0
+				}
+			}
+		}
 	} else if p.tok.kind in [.plus, .minus, .mul, .div, .mod, .lt, .eq] && p.peek_tok.kind == .lpar {
 		name = p.tok.kind.str() // op_to_fn_name()
 		if rec.typ == ast.void_type {

vlib/v/parser/tests/redeclaration_of_imported_fn.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/redeclaration_of_imported_fn.vv:3:4: error: cannot redefine imported function `input`
+    1 | import os { input }
+    2 | 
+    3 | fn input() {}
+      |    ~~~~~
+    4 | 
+    5 | input()

vlib/v/parser/tests/redeclaration_of_imported_fn.vv

@@ -0,0 +1,5 @@
+import os { input }
+
+fn input() {}
+
+input()