From 936ec197c737e45676ace89592f69b6500441c57 Mon Sep 17 00:00:00 2001
From: Supun Setunga <supun.setunga@gmail.com>
Date: Thu, 25 Jul 2024 15:42:30 -0700
Subject: [PATCH] Add tests for nil-coalesce type inference

---
 runtime/sema/checker.go                      |   6 +-
 runtime/tests/checker/type_inference_test.go | 113 +++++++++++++++++++
 2 files changed, 116 insertions(+), 3 deletions(-)

diff --git a/runtime/sema/checker.go b/runtime/sema/checker.go
index a03655e0ac..51d5204b44 100644
--- a/runtime/sema/checker.go
+++ b/runtime/sema/checker.go
@@ -2768,9 +2768,9 @@ func (checker *Checker) checkNativeModifier(isNative bool, position ast.HasPosit
 }
 
 func (checker *Checker) leastCommonSuperType(pos ast.HasPosition, types ...Type) Type {
-	elementType := LeastCommonSuperType(types...)
+	superType := LeastCommonSuperType(types...)
 
-	if elementType == InvalidType {
+	if superType == InvalidType {
 		checker.report(
 			&TypeAnnotationRequiredError{
 				Cause: "cannot infer type:",
@@ -2779,5 +2779,5 @@ func (checker *Checker) leastCommonSuperType(pos ast.HasPosition, types ...Type)
 		)
 	}
 
-	return elementType
+	return superType
 }
diff --git a/runtime/tests/checker/type_inference_test.go b/runtime/tests/checker/type_inference_test.go
index cf191c2879..99bc14702c 100644
--- a/runtime/tests/checker/type_inference_test.go
+++ b/runtime/tests/checker/type_inference_test.go
@@ -1375,3 +1375,116 @@ func TestCheckDeploymentResultInference(t *testing.T) {
 
 	assert.Equal(t, sema.DeploymentResultType, variableSizedType.Type)
 }
+
+func TestCheckNilCoalesceExpressionTypeInference(t *testing.T) {
+
+	t.Parallel()
+
+	t.Run("resource", func(t *testing.T) {
+		t.Parallel()
+
+		code := `
+            resource R {}
+
+            fun f(): @R? {
+                return <-create R()
+            }
+
+            let y <- f() ?? panic("no R")
+        `
+
+		checker, err := ParseAndCheckWithPanic(t, code)
+		require.NoError(t, err)
+
+		yType := RequireGlobalValue(t, checker.Elaboration, "y")
+		require.IsType(t, &sema.CompositeType{}, yType)
+		compositeType := yType.(*sema.CompositeType)
+
+		assert.Equal(t, "R", compositeType.Identifier)
+	})
+
+	t.Run("any resource", func(t *testing.T) {
+		t.Parallel()
+
+		code := `
+            resource R {}
+
+            fun f(): @AnyResource? {
+                return <-create R()
+            }
+
+            let y <- f() ?? panic("no R")
+        `
+
+		checker, err := ParseAndCheckWithPanic(t, code)
+		require.NoError(t, err)
+
+		yType := RequireGlobalValue(t, checker.Elaboration, "y")
+		assert.Equal(t, sema.AnyResourceType, yType)
+	})
+
+	t.Run("struct", func(t *testing.T) {
+		t.Parallel()
+
+		code := `
+            struct S {}
+
+            fun f(): S? {
+                return S()
+            }
+
+            let y = f() ?? panic("no S")
+        `
+
+		checker, err := ParseAndCheckWithPanic(t, code)
+		require.NoError(t, err)
+
+		yType := RequireGlobalValue(t, checker.Elaboration, "y")
+		require.IsType(t, &sema.CompositeType{}, yType)
+		compositeType := yType.(*sema.CompositeType)
+
+		assert.Equal(t, "S", compositeType.Identifier)
+	})
+
+	t.Run("any struct", func(t *testing.T) {
+		t.Parallel()
+
+		code := `
+            struct S {}
+
+            fun f(): AnyStruct? {
+                return S()
+            }
+
+            let y = f() ?? panic("no S")
+        `
+
+		checker, err := ParseAndCheckWithPanic(t, code)
+		require.NoError(t, err)
+
+		yType := RequireGlobalValue(t, checker.Elaboration, "y")
+		assert.Equal(t, sema.AnyStructType, yType)
+	})
+
+	t.Run("invalid type", func(t *testing.T) {
+		t.Parallel()
+
+		code := `
+            struct S {}
+            resource R {}
+
+            fun f(): @R? {
+                return <-create R()
+            }
+
+            let y <- f() ?? S()
+        `
+
+		_, err := ParseAndCheckWithPanic(t, code)
+
+		errs := RequireCheckerErrors(t, err, 1)
+
+		typeAnnotRequiredError := &sema.TypeAnnotationRequiredError{}
+		require.ErrorAs(t, errs[0], &typeAnnotRequiredError)
+	})
+}