Add regression tests

tests/ui/impl-trait/auto-trait-selection-freeze.next.stderr

@@ -0,0 +1,22 @@
+error[E0283]: type annotations needed
+  --> $DIR/auto-trait-selection-freeze.rs:20:16
+   |
+LL |     if false { is_trait(foo()) } else { Default::default() }
+   |                ^^^^^^^^ ----- type must be known at this point
+   |                |
+   |                cannot infer type of the type parameter `T` declared on the function `is_trait`
+   |
+   = note: cannot satisfy `_: Trait<_>`
+note: required by a bound in `is_trait`
+  --> $DIR/auto-trait-selection-freeze.rs:12:16
+   |
+LL | fn is_trait<T: Trait<U>, U: Default>(_: T) -> U {
+   |                ^^^^^^^^ required by this bound in `is_trait`
+help: consider specifying the generic arguments
+   |
+LL |     if false { is_trait::<T, U>(foo()) } else { Default::default() }
+   |                        ++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.

tests/ui/impl-trait/auto-trait-selection-freeze.rs

@@ -0,0 +1,24 @@
+//! This test shows how we fail selection in a way that can influence
+//! selection in a code path that succeeds.
+
+//@ revisions: next old
+//@[next] compile-flags: -Znext-solver
+//@[old]check-pass
+
+#![feature(freeze)]
+
+use std::marker::Freeze;
+
+fn is_trait<T: Trait<U>, U: Default>(_: T) -> U {
+    Default::default()
+}
+
+trait Trait<T> {}
+impl<T: Freeze> Trait<u32> for T {}
+impl<T> Trait<i32> for T {}
+fn foo() -> impl Sized {
+    if false { is_trait(foo()) } else { Default::default() }
+    //[next]~^ ERROR: type annotations needed
+}
+
+fn main() {}

tests/ui/impl-trait/auto-trait-selection.next.stderr

@@ -0,0 +1,22 @@
+error[E0283]: type annotations needed
+  --> $DIR/auto-trait-selection.rs:16:16
+   |
+LL |     if false { is_trait(foo()) } else { Default::default() }
+   |                ^^^^^^^^ ----- type must be known at this point
+   |                |
+   |                cannot infer type of the type parameter `T` declared on the function `is_trait`
+   |
+   = note: cannot satisfy `_: Trait<_>`
+note: required by a bound in `is_trait`
+  --> $DIR/auto-trait-selection.rs:8:16
+   |
+LL | fn is_trait<T: Trait<U>, U: Default>(_: T) -> U {
+   |                ^^^^^^^^ required by this bound in `is_trait`
+help: consider specifying the generic arguments
+   |
+LL |     if false { is_trait::<T, U>(foo()) } else { Default::default() }
+   |                        ++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.

tests/ui/impl-trait/auto-trait-selection.rs

@@ -0,0 +1,20 @@
+//! This test shows how we fail selection in a way that can influence
+//! selection in a code path that succeeds.
+
+//@ revisions: next old
+//@[next] compile-flags: -Znext-solver
+//@[old]check-pass
+
+fn is_trait<T: Trait<U>, U: Default>(_: T) -> U {
+    Default::default()
+}
+
+trait Trait<T> {}
+impl<T: Send> Trait<u32> for T {}
+impl<T> Trait<i32> for T {}
+fn foo() -> impl Sized {
+    if false { is_trait(foo()) } else { Default::default() }
+    //[next]~^ ERROR: type annotations needed
+}
+
+fn main() {}