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

Assisted-by: DeepSeek V4.1 Flash

CHANGELOG.md

@@ -23,6 +23,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   every major version 2 module: imports change to
   `sourcedock.dev/petrbalvin/interpres/v2`.
 
+### Fixed
+
+- Decoding into a defined type whose underlying kind is string or bool, such
+  as `type Name string`, panicked instead of storing the value, because a
+  value of the predeclared type is not assignable to a defined type and the
+  decoder assigned it without a conversion.
+
 ## [1.1.0] - 2026-09-18
 
 ### Added

decode.go

@@ -202,7 +202,10 @@ func setBasic(dst, val reflect.Value, kind string) error {
 	if dst.Kind() != val.Kind() {
 		return fmt.Errorf("interpres: cannot assign %s to %s", kind, dst.Type())
 	}
-	dst.Set(val)
+	// Convert rather than assign: a value of the predeclared type is not
+	// assignable to a defined type of the same kind, so a plain Set panics on
+	// a destination such as `type Name string`.
+	dst.Set(val.Convert(dst.Type()))
 	return nil
 }
 

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")
+	}
+}