fix(decode): decode into a defined string or bool type

Assisted-by: DeepSeek V4.1 Flash

decode_test.go

@@ -725,3 +725,25 @@ func TestDecodeErrorOnMapDestination(t *testing.T) {
 		t.Fatalf("Path = %v", de.Path)
 	}
 }
+
+func TestUnmarshalIntoDefinedScalarTypes(t *testing.T) {
+	// A defined type whose underlying kind is string or bool takes the value.
+	// A bare reflect Set panics on such a type, because a string is not
+	// assignable to a defined string type without a conversion.
+	type Name string
+	type Flag bool
+	type Cfg struct {
+		N Name `toml:"n"`
+		F Flag `toml:"f"`
+	}
+	var cfg Cfg
+	if err := Unmarshal([]byte("n = \"x\"\nf = true\n"), &cfg); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if cfg.N != "x" {
+		t.Errorf("N = %q, want \"x\"", cfg.N)
+	}
+	if !cfg.F {
+		t.Error("F = false, want true")
+	}
+}