fix(derive): Type check default_value_t

Fixes #2064

clap_derive/src/attrs.rs

@@ -340,27 +340,28 @@ impl Attrs {
                 VerbatimDocComment(ident) => self.verbatim_doc_comment = Some(ident),
 
                 DefaultValueT(ident, expr) => {
+                    let ty = if let Some(ty) = self.ty.as_ref() {
+                        ty
+                    } else {
+                        abort!(
+                            ident,
+                            "#[clap(default_value_t)] (without an argument) can be used \
+                            only on field level";
+
+                            note = "see \
+                                https://docs.rs/structopt/0.3.5/structopt/#magical-methods")
+                    };
+
                     let val = if let Some(expr) = expr {
                         quote!(#expr)
                     } else {
-                        let ty = if let Some(ty) = self.ty.as_ref() {
-                            ty
-                        } else {
-                            abort!(
-                                ident,
-                                "#[clap(default_value_t)] (without an argument) can be used \
-                                only on field level";
-
-                                note = "see \
-                                    https://docs.rs/structopt/0.3.5/structopt/#magical-methods")
-                        };
                         quote!(<#ty as ::std::default::Default>::default())
                     };
 
                     let val = quote_spanned!(ident.span()=> {
                         clap::lazy_static::lazy_static! {
                             static ref DEFAULT_VALUE: &'static str = {
-                                let val = #val;
+                                let val: #ty = #val;
                                 let s = ::std::string::ToString::to_string(&val);
                                 ::std::boxed::Box::leak(s.into_boxed_str())
                             };

clap_derive/tests/ui/default_value_t_invalid.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 Guillaume Pinot (@TeXitoi) <texitoi@texitoi.eu>
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use clap::Clap;
+
+#[derive(Clap, Debug)]
+#[clap(name = "basic")]
+struct Opt {
+    #[clap(default_value_t = -10)]
+    value: u32,
+}
+
+fn main() {
+    let opt = Opt::parse();
+    println!("{:?}", opt);
+}

clap_derive/tests/ui/default_value_t_invalid.stderr

@@ -0,0 +1,7 @@
+error[E0600]: cannot apply unary operator `-` to type `u32`
+  --> $DIR/default_value_t_invalid.rs:14:30
+   |
+14 |     #[clap(default_value_t = -10)]
+   |                              ^^^ cannot apply unary operator `-`
+   |
+   = note: unsigned values cannot be negated