Fix parentheses in subtractions (StdStr generator)

Author: asterycs

src/stdstr.jl

@@ -40,7 +40,7 @@ function parenthesize(::ir.Sub, f, arg)
 end
 
 function parenthesize(::ir.Sub, f, arg::ir.Add)
-    return f(arg)
+    return "(" * f(arg) * ")"
 end
 
 function parenthesize(
@@ -126,9 +126,7 @@ function to_std_str(arg::ir.Add)
 end
 
 function to_std_str(arg::ir.Sub)
-    return parenthesize(arg, to_std_str, arg.l) *
-           " - " *
-           parenthesize(arg, to_std_str, arg.r)
+    return to_std_str(arg.l) * " - " * parenthesize(arg, to_std_str, arg.r)
 end
 
 function to_std_str(arg::ir.Product)

test/StdTest.jl

@@ -284,6 +284,15 @@ end
     @test to_std(sum(y)) == "sum(yᵀ)"
 end
 
+@testset "to_std output is correct with difference involving additive operations" begin
+    x = Variable("x", Upper(1))
+    y = Variable("y", Upper(1))
+    z = Variable("z", Upper(1))
+
+    @test to_std(x - (y + z)) == "x - (y + z)"
+    @test to_std((x + y) - z) == "x + y - z"
+end
+
 @testset "to_std output is correct with abs" begin
     x = Variable("x", Upper(2))
     A = Variable("A", Upper(1), Lower(2))