Refine error spans for const args in hir typeck

compiler/rustc_middle/src/ty/mod.rs

@@ -1051,6 +1051,21 @@ impl<'tcx> TermKind<'tcx> {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum ParamTerm {
+    Ty(ParamTy),
+    Const(ParamConst),
+}
+
+impl ParamTerm {
+    pub fn index(self) -> usize {
+        match self {
+            ParamTerm::Ty(ty) => ty.index as usize,
+            ParamTerm::Const(ct) => ct.index as usize,
+        }
+    }
+}
+
 /// This kind of predicate has no *direct* correspondent in the
 /// syntax, but it roughly corresponds to the syntactic forms:
 ///