Improve MissingColonColon diagnostic.

crates/cairo-lang-semantic/src/diagnostic.rs

@@ -810,6 +810,7 @@ impl DiagnosticEntry for SemanticDiagnostic {
                     )
                 }
             }
+            SemanticDiagnosticKind::MissingColonColon => "Are you missing a `::`?.".into(),
         }
     }
 
@@ -1141,6 +1142,7 @@ pub enum SemanticDiagnosticKind {
     DerefCycle {
         deref_chain: String,
     },
+    MissingColonColon,
 }
 
 /// The kind of an expression with multiple possible return types.

crates/cairo-lang-semantic/src/diagnostic_test_data/tests

@@ -730,3 +730,36 @@ error: Cannot have array of type "()" that is zero sized.
  --> lib.cairo:3:15
   let _fail = array![()];
               ^********^
+
+//! > ==========================================================================
+
+//! > Test missing '::' in path.
+
+//! > test_runner_name
+test_expr_diagnostics(expect_diagnostics: true)
+
+//! > expr_code
+{
+  let _fail = bar<felt252>(1);
+}
+
+//! > module_code
+fn bar<T>(_a: T) {}
+
+//! > function_body
+
+//! > expected_diagnostics
+error: Expected variable or constant, found function.
+ --> lib.cairo:4:15
+  let _fail = bar<felt252>(1);
+              ^*^
+
+error: Are you missing a `::`?.
+ --> lib.cairo:4:18
+  let _fail = bar<felt252>(1);
+                 ^
+
+error: Type annotations needed. Failed to infer ?0.
+ --> lib.cairo:4:15
+  let _fail = bar<felt252>(1);
+              ^*************^

crates/cairo-lang-semantic/src/expr/compute.rs

@@ -16,7 +16,7 @@ use cairo_lang_defs::ids::{
 use cairo_lang_diagnostics::{skip_diagnostic, Maybe, ToOption};
 use cairo_lang_filesystem::ids::{FileKind, FileLongId, VirtualFile};
 use cairo_lang_syntax::node::ast::{
-    BlockOrIf, ExprPtr, PatternListOr, PatternStructParam, UnaryOperator,
+    BinaryOperator, BlockOrIf, ExprPtr, PatternListOr, PatternStructParam, UnaryOperator,
 };
 use cairo_lang_syntax::node::db::SyntaxGroup;
 use cairo_lang_syntax::node::helpers::{GetIdentifier, PathSegmentEx};
@@ -596,7 +596,15 @@ fn call_core_binary_op(
         })?;
 
     let mut lexpr = compute_expr_semantic(ctx, lhs_syntax);
+
+    if let (Expr::Missing(_), BinaryOperator::LT(_)) = (&lexpr.expr, &binary_op) {
+        return Err(ctx
+            .diagnostics
+            .report(binary_op.stable_ptr(), SemanticDiagnosticKind::MissingColonColon));
+    }
+
     let mut rexpr = compute_expr_semantic(ctx, rhs_syntax);
+
     ctx.reduce_ty(lexpr.ty()).check_not_missing(db)?;
     ctx.reduce_ty(rexpr.ty()).check_not_missing(db)?;