diagnostics: add test case for already-solved issue

Fixes #70082

tests/ui/inference/issue-70082.rs

@@ -0,0 +1,10 @@
+fn main() {
+    // this closure is fine, and should not get any error annotations
+    let em = |v: f64| -> f64 { v };
+
+    let x: f64 = em(1i16.into());
+
+    let y: f64 = 0.01f64 * 1i16.into();
+    //~^ ERROR type annotations needed
+    //~| HELP try using a fully qualified path
+}

tests/ui/inference/issue-70082.stderr

@@ -0,0 +1,17 @@
+error[E0284]: type annotations needed
+  --> $DIR/issue-70082.rs:7:33
+   |
+LL |     let y: f64 = 0.01f64 * 1i16.into();
+   |                          -      ^^^^
+   |                          |
+   |                          type must be known at this point
+   |
+   = note: cannot satisfy `<f64 as Mul<_>>::Output == f64`
+help: try using a fully qualified path to specify the expected types
+   |
+LL |     let y: f64 = 0.01f64 * <i16 as Into<T>>::into(1i16);
+   |                            +++++++++++++++++++++++    ~
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0284`.