Fix non-capturing closure return type coercion

Author: FabianWolff

compiler/rustc_typeck/src/check/coercion.rs

@@ -941,6 +941,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             exprs.len()
         );
 
+        if prev_ty == new_ty {
+            return Ok(prev_ty);
+        }
+
         // Special-case that coercion alone cannot handle:
         // Function items or non-capturing closures of differing IDs or InternalSubsts.
         let (a_sig, b_sig) = {

src/test/ui/coercion/issue-88097.rs

@@ -0,0 +1,31 @@
+// In #88097, the compiler attempted to coerce a closure type to itself via
+// a function pointer, which caused an unnecessary error. Check that this
+// behavior has been fixed.
+
+// check-pass
+
+fn peculiar() -> impl Fn(u8) -> u8 {
+    return |x| x + 1
+}
+
+fn peculiar2() -> impl Fn(u8) -> u8 {
+    return |x| x + 1;
+}
+
+fn peculiar3() -> impl Fn(u8) -> u8 {
+    let f = |x| x + 1;
+    return f
+}
+
+fn peculiar4() -> impl Fn(u8) -> u8 {
+    let f = |x| x + 1;
+    f
+}
+
+fn peculiar5() -> impl Fn(u8) -> u8 {
+    let f = |x| x + 1;
+    let g = |x| x + 2;
+    return if true { f } else { g }
+}
+
+fn main() {}