fix: panic of if command as a constant expr by bringing back Type::Block (#16122)

Fixes #16110. Alternative to #16120 

# Description

# User-Facing Changes

no more panic

# Tests + Formatting

+1

# After Submitting

crates/nu-cmd-lang/src/core_commands/if_.rs

@@ -60,11 +60,13 @@ impl Command for If {
     ) -> Result<PipelineData, ShellError> {
         let call = call.assert_ast_call()?;
         let cond = call.positional_nth(0).expect("checked through parser");
-        let then_block = call
+        let then_expr = call.positional_nth(1).expect("checked through parser");
-            .positional_nth(1)
+        let then_block = then_expr
-            .expect("checked through parser")
             .as_block()
-            .expect("internal error: missing block");
+            .ok_or_else(|| ShellError::TypeMismatch {
+                err_message: "expected block".into(),
+                span: then_expr.span,
+            })?;
         let else_case = call.positional_nth(2);
 
         if eval_constant(working_set, cond)?.as_bool()? {

crates/nu-command/src/database/commands/into_sqlite.rs

@@ -353,6 +353,7 @@ fn nu_value_to_sqlite_type(val: &Value) -> Result<&'static str, ShellError> {
 
         // intentionally enumerated so that any future types get handled
         Type::Any
+        | Type::Block
         | Type::CellPath
         | Type::Closure
         | Type::Custom(_)

crates/nu-parser/src/parse_keywords.rs

@@ -2487,7 +2487,7 @@ pub fn parse_module(
         working_set,
         Expr::Block(block_id),
         block_expr_span,
-        Type::Any,
+        Type::Block,
     );
 
     let module_decl_id = working_set

crates/nu-parser/src/parser.rs

@@ -4768,7 +4768,7 @@ pub fn parse_block_expression(working_set: &mut StateWorkingSet, span: Span) ->
 
     let block_id = working_set.add_block(Arc::new(output));
 
-    Expression::new(working_set, Expr::Block(block_id), span, Type::Any)
+    Expression::new(working_set, Expr::Block(block_id), span, Type::Block)
 }
 
 pub fn parse_match_block_expression(working_set: &mut StateWorkingSet, span: Span) -> Expression {

crates/nu-parser/src/type_check.rs

@@ -88,6 +88,7 @@ pub fn type_compatible(lhs: &Type, rhs: &Type) -> bool {
         (Type::Int, Type::Number) => true,
         (Type::Number, Type::Float) => true,
         (Type::Float, Type::Number) => true,
+        (Type::Closure, Type::Block) => true,
         (Type::Any, _) => true,
         (_, Type::Any) => true,
         (Type::Record(lhs), Type::Record(rhs)) | (Type::Table(lhs), Type::Table(rhs)) => {

crates/nu-protocol/src/syntax_shape.rs

@@ -143,7 +143,7 @@ impl SyntaxShape {
 
         match self {
             SyntaxShape::Any => Type::Any,
-            SyntaxShape::Block => Type::Any,
+            SyntaxShape::Block => Type::Block,
             SyntaxShape::Closure(_) => Type::Closure,
             SyntaxShape::Binary => Type::Binary,
             SyntaxShape::CellPath => Type::Any,

crates/nu-protocol/src/ty.rs

@@ -9,6 +9,7 @@ use strum_macros::EnumIter;
 pub enum Type {
     Any,
     Binary,
+    Block,
     Bool,
     CellPath,
     Closure,
@@ -110,6 +111,7 @@ impl Type {
             Type::Range => SyntaxShape::Range,
             Type::Bool => SyntaxShape::Boolean,
             Type::String => SyntaxShape::String,
+            Type::Block => SyntaxShape::Block, // FIXME needs more accuracy
             Type::Closure => SyntaxShape::Closure(None), // FIXME needs more accuracy
             Type::CellPath => SyntaxShape::CellPath,
             Type::Duration => SyntaxShape::Duration,
@@ -134,6 +136,7 @@ impl Type {
         match self {
             Type::Closure => String::from("closure"),
             Type::Bool => String::from("bool"),
+            Type::Block => String::from("block"),
             Type::CellPath => String::from("cell-path"),
             Type::Date => String::from("datetime"),
             Type::Duration => String::from("duration"),
@@ -159,6 +162,7 @@ impl Type {
 impl Display for Type {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
+            Type::Block => write!(f, "block"),
             Type::Closure => write!(f, "closure"),
             Type::Bool => write!(f, "bool"),
             Type::CellPath => write!(f, "cell-path"),

tests/const_/mod.rs

@@ -410,6 +410,15 @@ fn if_const() {
     assert_eq!(actual.out, "no!");
 }
 
+#[rstest]
+#[case(&"const x = if true ()", "expected block, found nothing")]
+#[case(&"const x = if true {foo: bar}", "expected block, found record")]
+#[case(&"const x = if true {1: 2}", "expected block")]
+fn if_const_error(#[case] inp: &str, #[case] expect: &str) {
+    let actual = nu!(inp);
+    assert!(actual.err.contains(expect));
+}
+
 #[test]
 fn const_glob_type() {
     let actual = nu!("const x: glob = 'aa'; $x | describe");