Rollup merge of #93892 - compiler-errors:issue-92917, r=jackh726,nikomatsakis

Only mark projection as ambiguous if GAT substs are constrained

A slightly more targeted version of #92917, where we only give up with ambiguity if we infer something about the GATs substs when probing for a projection candidate.

fixes #93874
also note (but like the previous PR, does not fix) #91762

r? `@jackh726`
cc `@nikomatsakis` who reviewed #92917

src/test/ui/generic-associated-types/issue-74824.rs

@@ -17,7 +17,6 @@ impl<T> UnsafeCopy for T {}
 fn main() {
     let b = Box::new(42usize);
     let copy = <()>::copy(&b);
-    //~^ type annotations needed
 
     let raw_b = Box::deref(&b) as *const _;
     let raw_copy = Box::deref(&copy) as *const _;

src/test/ui/generic-associated-types/issue-74824.stderr

@@ -27,13 +27,6 @@ help: consider restricting type parameter `T`
 LL |     type Copy<T: std::clone::Clone>: Copy = Box<T>;
    |                +++++++++++++++++++
 
-error[E0282]: type annotations needed
-  --> $DIR/issue-74824.rs:19:16
-   |
-LL |     let copy = <()>::copy(&b);
-   |                ^^^^^^^^^^ cannot infer type for type parameter `T` declared on the associated function `copy`
+error: aborting due to 2 previous errors
 
-error: aborting due to 3 previous errors
-
-Some errors have detailed explanations: E0277, E0282.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/generic-associated-types/issue-93874.rs

@@ -0,0 +1,35 @@
+// check-pass
+
+#![feature(generic_associated_types)]
+
+pub trait Build {
+    type Output<O>;
+    fn build<O>(self, input: O) -> Self::Output<O>;
+}
+
+pub struct IdentityBuild;
+impl Build for IdentityBuild {
+    type Output<O> = O;
+    fn build<O>(self, input: O) -> Self::Output<O> {
+        input
+    }
+}
+
+fn a() {
+    let _x: u8 = IdentityBuild.build(10);
+}
+
+fn b() {
+    let _x: Vec<u8> = IdentityBuild.build(Vec::new());
+}
+
+fn c() {
+    let mut f = IdentityBuild.build(|| ());
+    (f)();
+}
+
+pub fn main() {
+    a();
+    b();
+    c();
+}