Add some regression tests for opaque types and const generics

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -429,7 +429,8 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 // as the cause of an overflow.
                 ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
                     match self.selcx.infcx.at(&obligation.cause, obligation.param_env).eq(
-                        DefineOpaqueTypes::No,
+                        // Only really excercised by generic_const_exprs
+                        DefineOpaqueTypes::Yes,
                         ct.ty(),
                         ty,
                     ) {

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -982,7 +982,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
                 ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
                     match self.infcx.at(&obligation.cause, obligation.param_env).eq(
-                        DefineOpaqueTypes::No,
+                        // Only really excercised by generic_const_exprs
+                        DefineOpaqueTypes::Yes,
                         ct.ty(),
                         ty,
                     ) {

tests/ui/const-generics/opaque_types.rs

@@ -0,0 +1,13 @@
+#![feature(type_alias_impl_trait)]
+
+type Foo = impl Sized;
+//~^ ERROR: cycle
+//~| ERROR: cycle
+
+fn foo<const C: Foo>() {}
+//~^ ERROR: `Foo` is forbidden as the type of a const generic parameter
+
+fn main() {
+    foo::<42>();
+    //~^ ERROR: mismatched types
+}

tests/ui/const-generics/opaque_types.stderr

@@ -0,0 +1,125 @@
+error[E0308]: mismatched types
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL | type Foo = impl Sized;
+   |            ---------- the expected opaque type
+...
+LL |     foo::<42>();
+   |           ^^ expected opaque type, found integer
+   |
+   = note: expected opaque type `Foo`
+                     found type `{integer}`
+
+error[E0391]: cycle detected when computing type of `Foo::{opaque#0}`
+  --> $DIR/opaque_types.rs:3:12
+   |
+LL | type Foo = impl Sized;
+   |            ^^^^^^^^^^
+   |
+note: ...which requires computing type of opaque `Foo::{opaque#0}`...
+  --> $DIR/opaque_types.rs:3:12
+   |
+LL | type Foo = impl Sized;
+   |            ^^^^^^^^^^
+note: ...which requires type-checking `main`...
+  --> $DIR/opaque_types.rs:10:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
+note: ...which requires evaluating type-level constant...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires const-evaluating + checking `main::{constant#0}`...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires caching mir of `main::{constant#0}` for CTFE...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires elaborating drops for `main::{constant#0}`...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+   = note: ...which requires normalizing `Foo`...
+   = note: ...which again requires computing type of `Foo::{opaque#0}`, completing the cycle
+note: cycle used when checking that `Foo::{opaque#0}` is well-formed
+  --> $DIR/opaque_types.rs:3:12
+   |
+LL | type Foo = impl Sized;
+   |            ^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: `Foo` is forbidden as the type of a const generic parameter
+  --> $DIR/opaque_types.rs:7:17
+   |
+LL | fn foo<const C: Foo>() {}
+   |                 ^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error[E0391]: cycle detected when computing type of opaque `Foo::{opaque#0}`
+  --> $DIR/opaque_types.rs:3:12
+   |
+LL | type Foo = impl Sized;
+   |            ^^^^^^^^^^
+   |
+note: ...which requires type-checking `main`...
+  --> $DIR/opaque_types.rs:10:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
+note: ...which requires evaluating type-level constant...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires const-evaluating + checking `main::{constant#0}`...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires caching mir of `main::{constant#0}` for CTFE...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires elaborating drops for `main::{constant#0}`...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires borrow-checking `main::{constant#0}`...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires promoting constants in MIR for `main::{constant#0}`...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+note: ...which requires const checking `main::{constant#0}`...
+  --> $DIR/opaque_types.rs:11:11
+   |
+LL |     foo::<42>();
+   |           ^^
+   = note: ...which requires computing whether `Foo` is freeze...
+   = note: ...which requires evaluating trait selection obligation `Foo: core::marker::Freeze`...
+   = note: ...which again requires computing type of opaque `Foo::{opaque#0}`, completing the cycle
+note: cycle used when computing type of `Foo::{opaque#0}`
+  --> $DIR/opaque_types.rs:3:12
+   |
+LL | type Foo = impl Sized;
+   |            ^^^^^^^^^^
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0308, E0391.
+For more information about an error, try `rustc --explain E0308`.

tests/ui/const-generics/opaque_types2.rs

@@ -0,0 +1,17 @@
+#![feature(type_alias_impl_trait)]
+
+type Foo = impl Sized;
+
+fn foo<const C: u32>() {}
+
+const C: Foo = 42;
+
+fn bar()
+where
+    Foo:,
+{
+    foo::<C>();
+    //~^ ERROR: mismatched types
+}
+
+fn main() {}

tests/ui/const-generics/opaque_types2.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/opaque_types2.rs:13:11
+   |
+LL | type Foo = impl Sized;
+   |            ---------- the found opaque type
+...
+LL |     foo::<C>();
+   |           ^ expected `u32`, found opaque type
+   |
+   = note:     expected type `u32`
+           found opaque type `Foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.