Require generic params for const generic params

compiler/rustc_ast_lowering/src/lib.rs

@@ -2147,7 +2147,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 ty_id,
                 &None,
                 path,
-                ParamMode::Optional,
+                ParamMode::Explicit,
                 AllowReturnTypeNotation::No,
                 // FIXME(mgca): update for `fn foo() -> Bar<FOO<impl Trait>>` support
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
@@ -2219,7 +2219,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 expr.id,
                 qself,
                 path,
-                ParamMode::Optional,
+                ParamMode::Explicit,
                 AllowReturnTypeNotation::No,
                 // FIXME(mgca): update for `fn foo() -> Bar<FOO<impl Trait>>` support
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),

tests/ui/const-generics/mgca/missing_generic_params.rs

@@ -0,0 +1,16 @@
+// This used to ICE: #137188
+// The missing parameter list on `N` was set to
+// "infer from use site" in ast lowering, which
+// caused later code to not emit a missing generic
+// param error. The missing param was then attempted
+// to be inferred, but inference of generic params
+// is only possible within bodies. So a delayed
+// bug was generated with no error ever reported.
+
+#![feature(min_generic_const_args)]
+#![allow(incomplete_features)]
+trait Trait {}
+impl Trait for [(); N] {}
+//~^ ERROR: missing generics for function `N`
+fn N<T>() {}
+pub fn main() {}

tests/ui/const-generics/mgca/missing_generic_params.stderr

@@ -0,0 +1,19 @@
+error[E0107]: missing generics for function `N`
+  --> $DIR/missing_generic_params.rs:13:21
+   |
+LL | impl Trait for [(); N] {}
+   |                     ^ expected 1 generic argument
+   |
+note: function defined here, with 1 generic parameter: `T`
+  --> $DIR/missing_generic_params.rs:15:4
+   |
+LL | fn N<T>() {}
+   |    ^ -
+help: add missing generic argument
+   |
+LL | impl Trait for [(); N<T>] {}
+   |                      +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0107`.