Fix parsing issue with negative literals as const generic arguments

src/libsyntax/parse/parser.rs

@@ -2319,7 +2319,8 @@ impl<'a> Parser<'a> {
         let ident = self.parse_path_segment_ident()?;
 
         let is_args_start = |token: &token::Token| match *token {
-            token::Lt | token::BinOp(token::Shl) | token::OpenDelim(token::Paren) => true,
+            token::Lt | token::BinOp(token::Shl) | token::OpenDelim(token::Paren)
+            | token::LArrow => true,
             _ => false,
         };
         let check_args_start = |this: &mut Self| {
@@ -6060,8 +6061,6 @@ impl<'a> Parser<'a> {
                         self.fatal("identifiers may currently not be used for const generics")
                     );
                 } else {
-                    // FIXME(const_generics): this currently conflicts with emplacement syntax
-                    // with negative integer literals.
                     self.parse_literal_maybe_minus()?
                 };
                 let value = AnonConst {

src/test/ui/const-generics/const-expression-parameter.rs

@@ -6,7 +6,7 @@ fn i32_identity<const X: i32>() -> i32 {
 }
 
 fn foo_a() {
-    i32_identity::<-1>(); //~ ERROR expected identifier, found `<-`
+    i32_identity::<-1>(); // ok
 }
 
 fn foo_b() {

src/test/ui/const-generics/const-expression-parameter.stderr

@@ -1,9 +1,3 @@
-error: expected identifier, found `<-`
-  --> $DIR/const-expression-parameter.rs:9:19
-   |
-LL |     i32_identity::<-1>();
-   |                   ^^ expected identifier
-
 error: expected one of `,` or `>`, found `+`
   --> $DIR/const-expression-parameter.rs:13:22
    |
@@ -16,5 +10,5 @@ warning: the feature `const_generics` is incomplete and may cause the compiler t
 LL | #![feature(const_generics)]
    |            ^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error