From 01ccaefe2f7db6d9d08401449ebe4d2ab51b2ed4 Mon Sep 17 00:00:00 2001
From: Evgenii Stratonikov <evgeniy@nspcc.ru>
Date: Mon, 19 Oct 2020 10:43:45 +0300
Subject: [PATCH] compiler: refactor void calls processing

Call result is not used only if it occurs inside a `ExprStmt`.
Otherwise (`IfStmt`, `BinExpr`...) it is used.

Fix #1495.
---
 pkg/compiler/func_scope.go         | 70 ++----------------------------
 pkg/compiler/function_call_test.go |  9 ++++
 2 files changed, 13 insertions(+), 66 deletions(-)

diff --git a/pkg/compiler/func_scope.go b/pkg/compiler/func_scope.go
index 0b72c0aed..f7923fe95 100644
--- a/pkg/compiler/func_scope.go
+++ b/pkg/compiler/func_scope.go
@@ -90,73 +90,11 @@ func (c *codegen) getFuncNameFromDecl(pkgPath string, decl *ast.FuncDecl) string
 // analyzeVoidCalls checks for functions that are not assigned
 // and therefore we need to cleanup the return value from the stack.
 func (c *funcScope) analyzeVoidCalls(node ast.Node) bool {
-	switch n := node.(type) {
-	case *ast.AssignStmt:
-		for i := 0; i < len(n.Rhs); i++ {
-			switch n.Rhs[i].(type) {
-			case *ast.CallExpr:
-				return false
-			}
-		}
-	case *ast.ReturnStmt:
-		if len(n.Results) > 0 {
-			switch n.Results[0].(type) {
-			case *ast.CallExpr:
-				return false
-			}
-		}
-	case *ast.TypeAssertExpr:
-		ce, ok := n.X.(*ast.CallExpr)
+	est, ok := node.(*ast.ExprStmt)
+	if ok {
+		ce, ok := est.X.(*ast.CallExpr)
 		if ok {
-			c.voidCalls[ce] = false
-		}
-	case *ast.BinaryExpr:
-		return false
-	case *ast.RangeStmt:
-		ce, ok := n.X.(*ast.CallExpr)
-		if ok {
-			c.voidCalls[ce] = false
-		}
-	case *ast.UnaryExpr:
-		ce, ok := n.X.(*ast.CallExpr)
-		if ok {
-			c.voidCalls[ce] = false
-		}
-	case *ast.IfStmt:
-		// we can't just return `false`, because we still need to process body
-		ce, ok := n.Cond.(*ast.CallExpr)
-		if ok {
-			c.voidCalls[ce] = false
-		}
-	case *ast.SwitchStmt:
-		ce, ok := n.Tag.(*ast.CallExpr)
-		if ok {
-			c.voidCalls[ce] = false
-		}
-	case *ast.CaseClause:
-		for _, e := range n.List {
-			ce, ok := e.(*ast.CallExpr)
-			if ok {
-				c.voidCalls[ce] = false
-			}
-		}
-	case *ast.CallExpr:
-		_, ok := c.voidCalls[n]
-		if !ok {
-			c.voidCalls[n] = true
-		}
-		return false
-	case *ast.CompositeLit:
-		for _, e := range n.Elts {
-			switch val := e.(type) {
-			case *ast.CallExpr: // slice
-				c.voidCalls[val] = false
-			case *ast.KeyValueExpr: // struct and map
-				ce, ok := val.Value.(*ast.CallExpr)
-				if ok {
-					c.voidCalls[ce] = false
-				}
-			}
+			c.voidCalls[ce] = true
 		}
 	}
 	return true
diff --git a/pkg/compiler/function_call_test.go b/pkg/compiler/function_call_test.go
index 1de9f3ae4..2f8d6ea01 100644
--- a/pkg/compiler/function_call_test.go
+++ b/pkg/compiler/function_call_test.go
@@ -86,6 +86,15 @@ func TestNotAssignedFunctionCall(t *testing.T) {
 		}`
 		eval(t, src, big.NewInt(42))
 	})
+	t.Run("VarDecl", func(t *testing.T) {
+		src := `package foo
+		func foo() []int { return []int{1} }
+		func Main() int {
+			var x = foo()
+			return len(x)
+		}`
+		eval(t, src, big.NewInt(1))
+	})
 }
 
 func TestMultipleFunctionCalls(t *testing.T) {