diff --git a/src/cmd/vet/bool.go b/src/cmd/vet/bool.go
index 67321c3df4..e8b73175aa 100644
--- a/src/cmd/vet/bool.go
+++ b/src/cmd/vet/bool.go
@@ -147,12 +147,18 @@ func hasSideEffects(f *File, e ast.Expr) bool {
 			// conversion as a *types.Named.
 			typVal := f.pkg.types[n.Fun]
 			_, isSig := typVal.Type.(*types.Signature)
-			if typVal.IsValue() && isSig {
+			switch {
+			case typVal.IsValue() && isSig:
 				// If we have a value of unnamed signature type,
-				// this CallExpr is a func call and not a type
-				// conversion. Conservatively assume that all
-				// function and method calls have side effects
-				// for now.
+				// this CallExpr is a non-builtin func call and
+				// not a type conversion. Conservatively assume
+				// that all function and method calls have side
+				// effects for now.
+				safe = false
+				return false
+			case typVal.IsBuiltin():
+				// For now, conservatively assume that all
+				// built-in functions have side effects.
 				safe = false
 				return false
 			}
diff --git a/src/cmd/vet/testdata/bool.go b/src/cmd/vet/testdata/bool.go
index be78caac18..80c44d25ca 100644
--- a/src/cmd/vet/testdata/bool.go
+++ b/src/cmd/vet/testdata/bool.go
@@ -10,12 +10,18 @@ import "io"
 
 type T int
 
+func (t T) Foo() int { return int(t) }
+
 type FT func() int
 
+var S []int
+
 func RatherStupidConditions() {
 	var f, g func() int
 	if f() == 0 || f() == 0 { // OK f might have side effects
 	}
+	var t T
+	_ = t.Foo() == 2 || t.Foo() == 2        // OK Foo might have side effects
 	if v, w := f(), g(); v == w || v == w { // ERROR "redundant or: v == w || v == w"
 	}
 	_ = f == nil || f == nil // ERROR "redundant or: f == nil || f == nil"
@@ -25,9 +31,10 @@ func RatherStupidConditions() {
 	_ = FT(f) == nil || FT(f) == nil // ERROR "redundant or: FT(f) == nil || FT(f) == nil"
 
 	_ = (func() int)(f) == nil || (func() int)(f) == nil // ERROR "redundant or: (func() int)(f) == nil || (func() int)(f) == nil"
+	_ = append(S, 3) == nil || append(S, 3) == nil       // OK append has side effects
 
 	var namedFuncVar FT
-	_ = namedFuncVar() == namedFuncVar() // OK; still func calls
+	_ = namedFuncVar() == namedFuncVar() // OK still func calls
 
 	var c chan int
 	_ = 0 == <-c || 0 == <-c                                  // OK subsequent receives may yield different values