zasm: fix false truncation error on "-" expressions

apps/zasm/instr.asm

@@ -128,7 +128,7 @@ parseIXY:
 ; the value of that constant.
 parseArg:
 	call	strlen
-	cp	0
+	or	a
 	ret	z		; empty string? A already has our result: 0
 
 	push	bc
@@ -157,7 +157,7 @@ parseArg:
 	call	enterParens
 	jr	z, .withParens
 	; (HL) has no parens
-	call	parseExpr
+	call	.maybeParseExpr
 	jr	nz, .nomatch
 	; We have a proper number in no parens. Number in IX.
 	ld	a, 'N'
@@ -181,7 +181,7 @@ parseArg:
 .notY:
 	ld	c, 'x'
 .parseNumberInParens:
-	call	parseExpr
+	call	.maybeParseExpr
 	jr	nz, .nomatch
 	; We have a proper number in parens. Number in IX
 	ld	a, c	; M, x, or y
@@ -200,6 +200,16 @@ parseArg:
 	pop	bc
 	ret
 
+.maybeParseExpr:
+	; Before we try to parse expr in (HL), first check if we're in first
+	; pass if we are, skip parseExpr. Most of the time, that parse is
+	; harmless, but in some cases it causes false failures. For example,
+	; a "-" operator can cause is to falsely overflow and generate
+	; truncation error.
+	call	zasmIsFirstPass
+	ret	z
+	jp	parseExpr
+
 ; Returns, with Z, whether A is a groupId
 isGroupId:
 	cp	0xc	; max group id + 1

tools/tests/unit/test_expr.asm

@@ -19,6 +19,7 @@ s1:		.db "2+2", 0
 s2:		.db "0x4001+0x22", 0
 s3:		.db "FOO+BAR", 0
 s4:		.db "3*3", 0
+s5:		.db "FOO-3", 0
 
 sFOO:		.db "FOO", 0
 sBAR:		.db "BAR", 0
@@ -82,6 +83,17 @@ test:
 	jp	nz, fail
 	call	nexttest
 
+	ld	hl, s5
+	call	parseExpr
+	jp	nz, fail
+	ld	a, ixh
+	cp	0x3f
+	jp	nz, fail
+	ld	a, ixl
+	cp	0xfd
+	jp	nz, fail
+	call	nexttest
+
 	; success
 	xor	a
 	halt

tools/tests/zasm/test1.asm

@@ -19,3 +19,5 @@ label2: .dw	0x42
 	ld	(ix+1), l
 	ld	l, (ix+1)
 	ld	hl, 0x100
+.equ	baz	0x20
+	ld	b, baz-3