FIx no type error for interpreter

[cyangle]

Try to fix #16486

Reverting back to the original fix.

Summary

I've successfully fixed the Crystal interpreter bug where type narrowing failed when:

Inside an if block with a return statement
A union type is narrowed with a nil check and return
The narrowed variable is used in a method call

Root Cause

The interpreter's visit(node : Expressions) method didn't set the node.type. The semantic pass sets this via the bind_to method, but in the interpreter context this binding doesn't automatically happen. When an if statement's then/else branches (Expressions nodes) were used later, the interpreter would fail trying to access their type.

The Fix

Added code to the visit(node : Expressions) method to ensure the Expressions node has a type set. The fix:

• Only sets the type if it's not already set (to avoid overwriting)
• Sets it to the type of the last expression in the Expressions
• Sets it to nil_type if the Expressions is empty

This matches what the semantic pass does via its binding system.

[cyangle]

I have no idea. Maybe the first approach is the correct fix:

diff --git a/src/compiler/crystal/interpreter/compiler.cr b/src/compiler/crystal/interpreter/compiler.cr
index bba9d5093..da3b875a8 100644
--- a/src/compiler/crystal/interpreter/compiler.cr
+++ b/src/compiler/crystal/interpreter/compiler.cr
@@ -591,6 +591,17 @@ class Crystal::Repl::Compiler < Crystal::Visitor
 
     @wants_value = old_wants_value
 
+    # Ensure the Expressions node has a type. The semantic pass sets this via
+    # bind_to, but in some interpreter contexts it might not be set yet.
+    # Only set if not already set to avoid overwriting.
+    unless node.type?
+      if node.expressions.empty?
+        node.type = @context.program.nil_type
+      else
+        node.type = node.expressions.last.type?
+      end
+    end
+
     false
   end
 

There's a TODO comment acknowledging that there's something wrong with semantic pass:

crystal/src/compiler/crystal/interpreter/compiler.cr

Lines 1303 to 1306 in f6a1c53

	# TODO: for some reason the semantic pass might leave this as nil
	if @wants_value && (then_type = node.then.type?)
	upcast node.then, then_type, node.type
	end

[cyangle]

Alternative fix is not robust. It still fails for the non-reduced code.

The first fix works for both reduced and non-reduced code.