Do not suggest replacing an expression by an ambiguous type name

An expression such as `<_>::default()` should not be replaced by an
ambiguous type name such as `_` even if the inferred type in the
original expression is a singleton.

clippy_lints/src/default_constructed_unit_structs.rs

@@ -70,6 +70,8 @@ impl LateLintPass<'_> for DefaultConstructedUnitStructs {
             && let var @ ty::VariantDef { ctor: Some((hir::def::CtorKind::Const, _)), .. } = def.non_enum_variant()
             && !var.is_field_list_non_exhaustive()
             && !expr.span.from_expansion() && !qpath.span().from_expansion()
+            // do not suggest replacing an expression by a type name with placeholders
+            && !base.is_suggestable_infer_ty()
         {
             span_lint_and_sugg(
                 cx,

tests/ui/default_constructed_unit_structs.fixed

@@ -161,3 +161,12 @@ fn main() {
 
     let _ = <struct_from_macro!()>::default();
 }
+
+fn issue12654() {
+    #[derive(Default)]
+    struct G;
+
+    fn f(_g: G) {}
+
+    f(<_>::default());
+}

tests/ui/default_constructed_unit_structs.rs

@@ -161,3 +161,12 @@ fn main() {
 
     let _ = <struct_from_macro!()>::default();
 }
+
+fn issue12654() {
+    #[derive(Default)]
+    struct G;
+
+    fn f(_g: G) {}
+
+    f(<_>::default());
+}