Consider Scalar to be a bool only if its unsigned

compiler/rustc_target/src/abi/call/mod.rs

@@ -103,7 +103,12 @@ impl ArgAttributes {
     }
 
     pub fn ext(&mut self, ext: ArgExtension) -> &mut Self {
-        assert!(self.arg_ext == ArgExtension::None || self.arg_ext == ext);
+        assert!(
+            self.arg_ext == ArgExtension::None || self.arg_ext == ext,
+            "cannot set {:?} when {:?} is already set",
+            ext,
+            self.arg_ext
+        );
         self.arg_ext = ext;
         self
     }

compiler/rustc_target/src/abi/mod.rs

@@ -682,7 +682,7 @@ pub struct Scalar {
 
 impl Scalar {
     pub fn is_bool(&self) -> bool {
-        if let Int(I8, _) = self.value { self.valid_range == (0..=1) } else { false }
+        matches!(self.value, Int(I8, false)) && self.valid_range == (0..=1)
     }
 
     /// Returns the valid range as a `x..y` range.

src/test/codegen/abi-repr-ext.rs

@@ -0,0 +1,13 @@
+#![crate_type="lib"]
+
+#[repr(i8)]
+pub enum Type {
+    Type1 = 0,
+    Type2 = 1
+}
+
+// CHECK: define signext i8 @test()
+#[no_mangle]
+pub extern "C" fn test() -> Type {
+    Type::Type1
+}