all: add support for const ident in match range (#16526)

CHANGELOG.md

@@ -2,7 +2,8 @@
 *Not yet released*
 - `go foo()` has been replaced with `spawn foo()` (launches an OS thread, `go` will be used for
   upcoming coroutines instead).
-- vfmt now supports `// vfmt off` and `// vfmt on` for turning off the formatting locally for *short* snippets of code. Useful for keeping your carefully arranged matrices in tact.
+- vfmt now supports `// vfmt off` and `// vfmt on` for turning off the formatting locally for *short* snippets of code. Useful for keeping your carefully arranged matrices intact.
+- support match branch range expressions with consts: `match x { const1...const2 {} }`
 
 ## V 0.3.2
 *31 Oct 2022*

doc/docs.md

@@ -1676,6 +1676,26 @@ Note that the ranges use `...` (three dots) rather than `..` (two dots). This is
 because the range is *inclusive* of the last element, rather than exclusive
 (as `..` ranges are). Using `..` in a match branch will throw an error.
 
+```v
+const start = 1
+
+const end = 10
+
+c := 2
+num := match c {
+	start...end {
+		1000
+	}
+	else {
+		0
+	}
+}
+println(num)
+// 1000
+```
+
+Constants can also be used in the range branch expressions.
+
 Note: `match` as an expression is not usable in `for` loop and `if` statements.
 
 ### In operator

vlib/v/ast/ast.v

@@ -1325,6 +1325,7 @@ pub:
 pub mut:
 	low  Expr
 	high Expr
+	typ  Type // filled in by checker; the type of `0...1` is `int` for example, while `a`...`z` is `rune` etc
 }
 
 [minify]

vlib/v/checker/check_types.v

@@ -12,8 +12,19 @@ fn (mut c Checker) check_types(got ast.Type, expected ast.Type) bool {
 	}
 	got_is_ptr := got.is_ptr()
 	exp_is_ptr := expected.is_ptr()
+	got_is_int := got.is_int()
+	exp_is_int := expected.is_int()
+
+	exp_is_pure_int := expected.is_pure_int()
+	got_is_pure_int := got.is_pure_int()
+	// allow int literals where any kind of real integers are expected:
+	if (exp_is_pure_int && got == ast.int_literal_type)
+		|| (got_is_pure_int && expected == ast.int_literal_type) {
+		return true
+	}
+
 	if c.pref.translated {
-		if expected.is_int() && got.is_int() {
+		if exp_is_int && got_is_int {
 			return true
 		}
 		if expected == ast.byteptr_type {
@@ -22,8 +33,8 @@ fn (mut c Checker) check_types(got ast.Type, expected ast.Type) bool {
 		if expected == ast.voidptr_type || expected == ast.nil_type {
 			return true
 		}
-		if (expected == ast.bool_type && (got.is_any_kind_of_pointer() || got.is_int()))
-			|| ((expected.is_any_kind_of_pointer() || expected.is_int()) && got == ast.bool_type) {
+		if (expected == ast.bool_type && (got.is_any_kind_of_pointer() || got_is_int))
+			|| ((expected.is_any_kind_of_pointer() || exp_is_int) && got == ast.bool_type) {
 			return true
 		}
 
@@ -38,8 +49,7 @@ fn (mut c Checker) check_types(got ast.Type, expected ast.Type) bool {
 		}
 
 		// allow rune -> any int and vice versa
-		if (expected == ast.rune_type && got.is_int())
-			|| (got == ast.rune_type && expected.is_int()) {
+		if (expected == ast.rune_type && got_is_int) || (got == ast.rune_type && exp_is_int) {
 			return true
 		}
 		got_sym := c.table.sym(got)
@@ -546,6 +556,9 @@ fn (mut c Checker) promote_keeping_aliases(left_type ast.Type, right_type ast.Ty
 }
 
 fn (mut c Checker) promote(left_type ast.Type, right_type ast.Type) ast.Type {
+	if left_type == right_type {
+		return left_type // strings, self defined operators
+	}
 	if left_type.is_any_kind_of_pointer() {
 		if right_type.is_int() || c.pref.translated {
 			return left_type
@@ -559,9 +572,6 @@ fn (mut c Checker) promote(left_type ast.Type, right_type ast.Type) ast.Type {
 			return ast.void_type
 		}
 	}
-	if left_type == right_type {
-		return left_type // strings, self defined operators
-	}
 	if right_type.is_number() && left_type.is_number() {
 		return c.promote_num(left_type, right_type)
 	} else if left_type.has_flag(.optional) != right_type.has_flag(.optional) {

vlib/v/checker/checker.v

@@ -2452,8 +2452,20 @@ pub fn (mut c Checker) expr(node_ ast.Expr) ast.Type {
 			return c.expr(node.expr)
 		}
 		ast.RangeExpr {
-			// never happens
-			return ast.void_type
+			// branch range expression of `match x { a...b {} }`, or: `a#[x..y]`:
+			ltyp := c.expr(node.low)
+			htyp := c.expr(node.high)
+			if !c.check_types(ltyp, htyp) {
+				lstype := c.table.type_to_str(ltyp)
+				hstype := c.table.type_to_str(htyp)
+				c.add_error_detail('')
+				c.add_error_detail(' low part type: ${lstype}')
+				c.add_error_detail('high part type: ${hstype}')
+				c.error('the low and high parts of a range expression, should have matching types',
+					node.pos)
+			}
+			node.typ = c.promote(ltyp, htyp)
+			return ltyp
 		}
 		ast.SelectExpr {
 			return c.select_expr(mut node)

vlib/v/checker/match.v

@@ -139,66 +139,98 @@ fn (mut c Checker) check_match_branch_last_stmt(last_stmt ast.ExprStmt, ret_type
 	}
 }
 
+fn (mut c Checker) get_comptime_number_value(mut expr ast.Expr) ?i64 {
+	if mut expr is ast.CharLiteral {
+		return expr.val[0]
+	}
+	if mut expr is ast.IntegerLiteral {
+		return expr.val.i64()
+	}
+	if mut expr is ast.Ident {
+		if mut obj := c.table.global_scope.find_const(expr.name) {
+			if obj.typ == 0 {
+				obj.typ = c.expr(obj.expr)
+			}
+			return c.get_comptime_number_value(mut obj.expr)
+		}
+	}
+	return none
+}
+
 fn (mut c Checker) match_exprs(mut node ast.MatchExpr, cond_type_sym ast.TypeSymbol) {
+	c.expected_type = node.expected_type
+	cond_sym := c.table.sym(node.cond_type)
 	// branch_exprs is a histogram of how many times
 	// an expr was used in the match
 	mut branch_exprs := map[string]int{}
 	for branch_i, _ in node.branches {
 		mut branch := node.branches[branch_i]
 		mut expr_types := []ast.TypeNode{}
-		for k, expr in branch.exprs {
+		for k, mut expr in branch.exprs {
 			mut key := ''
-			if expr is ast.RangeExpr {
+			// TODO: investigate why enums are different here:
+			if expr !is ast.EnumVal {
+				// ensure that the sub expressions of the branch are actually checked, before anything else:
+				_ := c.expr(expr)
+			}
+			if mut expr is ast.RangeExpr {
+				// Allow for `match enum_value { 4..5 { } }`, even though usually int and enum values,
+				// are considered incompatible outside unsafe{}, and are not allowed to be compared directly
+				if cond_sym.kind != .enum_ && !c.check_types(expr.typ, node.cond_type) {
+					mcstype := c.table.type_to_str(node.cond_type)
+					brstype := c.table.type_to_str(expr.typ)
+					c.add_error_detail('')
+					c.add_error_detail('match condition type: ${mcstype}')
+					c.add_error_detail('          range type: ${brstype}')
+					c.error('the range type and the match condition type should match',
+						expr.pos)
+				}
+				mut low_value_higher_than_high_value := false
 				mut low := i64(0)
 				mut high := i64(0)
-				c.expected_type = node.expected_type
-				low_expr := expr.low
-				high_expr := expr.high
-				final_cond_sym := c.table.final_sym(node.cond_type)
-				if low_expr is ast.IntegerLiteral {
-					if high_expr is ast.IntegerLiteral
-						&& (final_cond_sym.is_int() || final_cond_sym.info is ast.Enum) {
-						low = low_expr.val.i64()
-						high = high_expr.val.i64()
-						if low > high {
-							c.error('start value is higher than end value', branch.pos)
+				mut both_low_and_high_are_known := false
+				if low_value := c.get_comptime_number_value(mut expr.low) {
+					low = low_value
+					if high_value := c.get_comptime_number_value(mut expr.high) {
+						high = high_value
+						both_low_and_high_are_known = true
+						if low_value > high_value {
+							low_value_higher_than_high_value = true
 						}
 					} else {
-						c.error('mismatched range types - ${expr.low} is an integer, but ${expr.high} is not',
-							low_expr.pos)
-					}
-				} else if low_expr is ast.CharLiteral {
-					if high_expr is ast.CharLiteral && final_cond_sym.kind in [.u8, .char, .rune] {
-						low = low_expr.val[0]
-						high = high_expr.val[0]
-						if low > high {
-							c.error('start value is higher than end value', branch.pos)
+						if expr.high !is ast.EnumVal {
+							c.error('match branch range expressions need the end value to be known at compile time (only enums, const or literals are supported)',
+								expr.high.pos())
 						}
-					} else {
-						typ := c.table.type_to_str(c.expr(node.cond))
-						c.error('mismatched range types - trying to match `${node.cond}`, which has type `${typ}`, against a range of `rune`',
-							low_expr.pos)
 					}
 				} else {
-					typ := c.table.type_to_str(c.expr(expr.low))
-					c.error('cannot use type `${typ}` in match range', branch.pos)
+					if expr.low !is ast.EnumVal {
+						c.error('match branch range expressions need the start value to be known at compile time (only enums, const or literals are supported)',
+							expr.low.pos())
+					}
 				}
-				high_low_cutoff := 1000
-				if high - low > high_low_cutoff {
-					c.warn('more than ${high_low_cutoff} possibilities (${low} ... ${high}) in match range',
+				if low_value_higher_than_high_value {
+					c.error('the start value `${low}` should be lower than the end value `${high}`',
 						branch.pos)
 				}
-				for i in low .. high + 1 {
-					key = i.str()
-					val := if key in branch_exprs { branch_exprs[key] } else { 0 }
-					if val == 1 {
-						c.error('match case `${key}` is handled more than once', branch.pos)
+				if both_low_and_high_are_known {
+					high_low_cutoff := 1000
+					if high - low > high_low_cutoff {
+						c.warn('more than ${high_low_cutoff} possibilities (${low} ... ${high}) in match range',
+							branch.pos)
+					}
+					for i in low .. high + 1 {
+						key = i.str()
+						val := if key in branch_exprs { branch_exprs[key] } else { 0 }
+						if val == 1 {
+							c.error('match case `${key}` is handled more than once', branch.pos)
+						}
+						branch_exprs[key] = val + 1
 					}
-					branch_exprs[key] = val + 1
 				}
 				continue
 			}
-			match expr {
+			match mut expr {
 				ast.TypeNode {
 					key = c.table.type_to_str(expr.typ)
 					expr_types << expr
@@ -207,7 +239,7 @@ fn (mut c Checker) match_exprs(mut node ast.MatchExpr, cond_type_sym ast.TypeSym
 					key = expr.val
 				}
 				else {
-					key = expr.str()
+					key = (*expr).str()
 				}
 			}
 			val := if key in branch_exprs { branch_exprs[key] } else { 0 }

vlib/v/checker/tests/const_expr_match_range_invalid_err.out

@@ -0,0 +1,20 @@
+vlib/v/checker/tests/const_expr_match_range_invalid_err.vv:6:2: error: the range type and the match condition type should match
+    4 | 
+    5 | match 5 {
+    6 |     start...`1` {
+      |     ~~~~~~~~~~~
+    7 |         println(start)
+    8 |     }
+Details: 
+match condition type: int
+          range type: rune
+vlib/v/checker/tests/const_expr_match_range_invalid_err.vv:9:2: error: the low and high parts of a range expression, should have matching types
+    7 |         println(start)
+    8 |     }
+    9 |     'str'...end {
+      |     ~~~~~~~~~~~
+   10 |         println(end)
+   11 |     }
+Details: 
+ low part type: string
+high part type: int literal

vlib/v/checker/tests/const_expr_match_range_invalid_err.vv

@@ -0,0 +1,13 @@
+const start = 1
+
+const end = 20
+
+match 5 {
+	start...`1` {
+		println(start)
+	}
+	'str'...end {
+		println(end)
+	}
+	else {}
+}

vlib/v/checker/tests/const_match_invalid_type_range_err.out

@@ -0,0 +1,20 @@
+vlib/v/checker/tests/const_match_invalid_type_range_err.vv:6:2: error: the range type and the match condition type should match
+    4 | 
+    5 | match 5 {
+    6 |     start...end {
+      |     ~~~~~~~~~~~
+    7 |         println(start)
+    8 |     }
+Details: 
+match condition type: int
+          range type: rune
+vlib/v/checker/tests/const_match_invalid_type_range_err.vv:13:2: error: the range type and the match condition type should match
+   11 | 
+   12 | b := match 5 {
+   13 |     start...end {
+      |     ~~~~~~~~~~~
+   14 |         3
+   15 |     }
+Details: 
+match condition type: int
+          range type: rune

vlib/v/checker/tests/const_match_invalid_type_range_err.vv

@@ -0,0 +1,20 @@
+const start = `a`
+
+const end = 2
+
+match 5 {
+	start...end {
+		println(start)
+	}
+	else {}
+}
+
+b := match 5 {
+	start...end {
+		3
+	}
+	else {
+		2
+	}
+}
+println(b)

vlib/v/checker/tests/const_match_mismatch_end_range_err.out

@@ -0,0 +1,40 @@
+vlib/v/checker/tests/const_match_mismatch_end_range_err.vv:6:2: error: the range type and the match condition type should match
+    4 | 
+    5 | match 5 {
+    6 |     0...end {
+      |     ~~~~~~~
+    7 |         println(start)
+    8 |     }
+Details: 
+match condition type: int
+          range type: rune
+vlib/v/checker/tests/const_match_mismatch_end_range_err.vv:9:2: error: the range type and the match condition type should match
+    7 |         println(start)
+    8 |     }
+    9 |     `a`...start {
+      |     ~~~~~~~~~~~
+   10 |         println(end)
+   11 |     }
+Details: 
+match condition type: int
+          range type: rune
+vlib/v/checker/tests/const_match_mismatch_end_range_err.vv:16:2: error: the range type and the match condition type should match
+   14 | 
+   15 | a := match 5 {
+   16 |     0...end {
+      |     ~~~~~~~
+   17 |         1
+   18 |     }
+Details: 
+match condition type: int
+          range type: rune
+vlib/v/checker/tests/const_match_mismatch_end_range_err.vv:19:2: error: the range type and the match condition type should match
+   17 |         1
+   18 |     }
+   19 |     `a`...start {
+      |     ~~~~~~~~~~~
+   20 |         `a`
+   21 |     }
+Details: 
+match condition type: int
+          range type: rune

vlib/v/checker/tests/const_match_mismatch_end_range_err.vv

@@ -0,0 +1,26 @@
+const start = 12
+
+const end = `a`
+
+match 5 {
+	0...end {
+		println(start)
+	}
+	`a`...start {
+		println(end)
+	}
+	else {}
+}
+
+a := match 5 {
+	0...end {
+		1
+	}
+	`a`...start {
+		`a`
+	}
+	else {
+		2
+	}
+}
+println(a)

vlib/v/checker/tests/const_match_range_duplicate_case_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/const_match_range_duplicate_case_err.vv:9:2: error: match case `2` is handled more than once
+    7 |         println(1)
+    8 |     }
+    9 |     start...end {
+      |     ~~~~~~~~~~~
+   10 |         println(3)
+   11 |     }
+vlib/v/checker/tests/const_match_range_duplicate_case_err.vv:21:2: error: match case `2` is handled more than once
+   19 |         1
+   20 |     }
+   21 |     start...end {
+      |     ~~~~~~~~~~~
+   22 |         3
+   23 |     }

vlib/v/checker/tests/const_match_range_duplicate_case_err.vv

@@ -0,0 +1,28 @@
+const start = 2
+
+const end = 12
+
+match 5 {
+	start...end {
+		println(1)
+	}
+	start...end {
+		println(3)
+	}
+	else {
+		println(2)
+	}
+}
+
+a := match 5 {
+	start...end {
+		1
+	}
+	start...end {
+		3
+	}
+	else {
+		2
+	}
+}
+println(a)

vlib/v/checker/tests/const_match_range_invalid_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/const_match_range_invalid_err.vv:6:2: error: the start value `10` should be lower than the end value `1`
+    4 | 
+    5 | match 5 {
+    6 |     end...start {
+      |     ~~~~~~~~~~~
+    7 |         println(start)
+    8 |     }
+vlib/v/checker/tests/const_match_range_invalid_err.vv:13:2: error: the start value `10` should be lower than the end value `1`
+   11 | 
+   12 | a := match 5 {
+   13 |     end...start {
+      |     ~~~~~~~~~~~
+   14 |         4
+   15 |     }

vlib/v/checker/tests/const_match_range_invalid_err.vv

@@ -0,0 +1,20 @@
+const end = 10
+
+const start = 1
+
+match 5 {
+	end...start {
+		println(start)
+	}
+	else {}
+}
+
+a := match 5 {
+	end...start {
+		4
+	}
+	else {
+		3
+	}
+}
+println(a)

vlib/v/checker/tests/const_match_type_mismatch_range_err.out

@@ -0,0 +1,20 @@
+vlib/v/checker/tests/const_match_type_mismatch_range_err.vv:6:2: error: the low and high parts of a range expression, should have matching types
+    4 | 
+    5 | match 5 {
+    6 |     start...end {
+      |     ~~~~~~~~~~~
+    7 |         println(start)
+    8 |     }
+Details: 
+ low part type: string
+high part type: int literal
+vlib/v/checker/tests/const_match_type_mismatch_range_err.vv:13:2: error: the low and high parts of a range expression, should have matching types
+   11 | 
+   12 | z := match 5 {
+   13 |     start...end {
+      |     ~~~~~~~~~~~
+   14 |         2
+   15 |     }
+Details: 
+ low part type: string
+high part type: int literal

vlib/v/checker/tests/const_match_type_mismatch_range_err.vv

@@ -0,0 +1,20 @@
+const end = 10
+
+const start = 'str'
+
+match 5 {
+	start...end {
+		println(start)
+	}
+	else {}
+}
+
+z := match 5 {
+	start...end {
+		2
+	}
+	else {
+		5
+	}
+}
+println(z)

vlib/v/checker/tests/invalid_const_expr_match_range_err.out

@@ -0,0 +1,28 @@
+vlib/v/checker/tests/invalid_const_expr_match_range_err.vv:5:2: error: match branch range expressions need the start value to be known at compile time (only enums, const or literals are supported)
+    3 | 
+    4 | match 5 {
+    5 |     start...10 {
+      |     ~~~~~
+    6 |         println(start)
+    7 |     }
+vlib/v/checker/tests/invalid_const_expr_match_range_err.vv:8:7: error: match branch range expressions need the end value to be known at compile time (only enums, const or literals are supported)
+    6 |         println(start)
+    7 |     }
+    8 |     10...end {
+      |          ~~~
+    9 |         println(end)
+   10 |     }
+vlib/v/checker/tests/invalid_const_expr_match_range_err.vv:15:2: error: match branch range expressions need the start value to be known at compile time (only enums, const or literals are supported)
+   13 | 
+   14 | a := match 5 {
+   15 |     start...10 {
+      |     ~~~~~
+   16 |         2
+   17 |     }
+vlib/v/checker/tests/invalid_const_expr_match_range_err.vv:18:7: error: match branch range expressions need the end value to be known at compile time (only enums, const or literals are supported)
+   16 |         2
+   17 |     }
+   18 |     10...end {
+      |          ~~~
+   19 |         3
+   20 |     }

vlib/v/checker/tests/invalid_const_expr_match_range_err.vv

@@ -0,0 +1,25 @@
+start := 1
+end := 20
+
+match 5 {
+	start...10 {
+		println(start)
+	}
+	10...end {
+		println(end)
+	}
+	else {}
+}
+
+a := match 5 {
+	start...10 {
+		2
+	}
+	10...end {
+		3
+	}
+	else {
+		1
+	}
+}
+println(a)

vlib/v/checker/tests/match_expr_range_low_higher_than_high.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/match_expr_range_low_higher_than_high.vv:5:3: error: start value is higher than end value
+vlib/v/checker/tests/match_expr_range_low_higher_than_high.vv:5:3: error: the start value `20` should be lower than the end value `10`
     3 |     value := 10
     4 |     match value {
     5 |         20...10 {}

vlib/v/checker/tests/match_range_mismatch_type_err.out

@@ -1,14 +1,20 @@
-vlib/v/checker/tests/match_range_mismatch_type_err.vv:4:3: error: mismatched range types - trying to match `x`, which has type `string`, against a range of `rune`
+vlib/v/checker/tests/match_range_mismatch_type_err.vv:4:3: error: the range type and the match condition type should match
     2 |     x := '1'
     3 |     match x {
     4 |         `0`...`9` {
-      |         ~~~
+      |         ~~~~~~~~~
     5 |             println('0-9')
     6 |         }
-vlib/v/checker/tests/match_range_mismatch_type_err.vv:16:3: error: mismatched range types - 0 is an integer, but `9` is not
+Details: 
+match condition type: string
+          range type: rune
+vlib/v/checker/tests/match_range_mismatch_type_err.vv:16:3: error: the range type and the match condition type should match
    14 |     x := 1
    15 |     match x {
    16 |         0...`9` {}
-      |         ^
+      |         ~~~~~~~
    17 |         else {}
    18 |     }
+Details: 
+match condition type: int
+          range type: rune

vlib/v/checker/tests/non_const_match_range_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/non_const_match_range_err.vv:6:2: error: match branch range expressions need the start value to be known at compile time (only enums, const or literals are supported)
+    4 | 
+    5 | match 5 {
+    6 |     start...end {
+      |     ~~~~~
+    7 |         println(start)
+    8 |     }
+vlib/v/checker/tests/non_const_match_range_err.vv:13:2: error: match branch range expressions need the start value to be known at compile time (only enums, const or literals are supported)
+   11 | 
+   12 | a := match 5 {
+   13 |     start...end {
+      |     ~~~~~
+   14 |         0
+   15 |     }

vlib/v/checker/tests/non_const_match_range_err.vv

@@ -0,0 +1,20 @@
+const end = 10
+
+start := 1
+
+match 5 {
+	start...end {
+		println(start)
+	}
+	else {}
+}
+
+a := match 5 {
+	start...end {
+		0
+	}
+	else {
+		1
+	}
+}
+println(a)

vlib/v/gen/c/index.v

@@ -8,7 +8,7 @@ import v.util
 
 fn (mut g Gen) index_expr(node ast.IndexExpr) {
 	if node.index is ast.RangeExpr {
-		g.range_expr(node, node.index)
+		g.index_range_expr(node, node.index)
 	} else {
 		sym := g.table.final_sym(g.unwrap_generic(node.left_type))
 		if sym.kind == .array {
@@ -57,7 +57,7 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 	}
 }
 
-fn (mut g Gen) range_expr(node ast.IndexExpr, range ast.RangeExpr) {
+fn (mut g Gen) index_range_expr(node ast.IndexExpr, range ast.RangeExpr) {
 	sym := g.table.final_sym(node.left_type)
 	mut tmp_opt := ''
 	mut cur_line := ''

vlib/v/gen/c/match.v

@@ -252,15 +252,8 @@ fn (mut g Gen) match_expr_switch(node ast.MatchExpr, is_expr bool, cond_var stri
 							g.write(' || ')
 						}
 						if expr is ast.RangeExpr {
-							// if type is unsigned and low is 0, check is unneeded
-							mut skip_low := false
-							if expr.low is ast.IntegerLiteral {
-								if node_cond_type_unsigned && expr.low.val == '0' {
-									skip_low = true
-								}
-							}
 							g.write('(')
-							if !skip_low {
+							if g.should_check_low_bound_in_range_expr(expr, node_cond_type_unsigned) {
 								g.write('${cond_var} >= ')
 								g.expr(expr.low)
 								g.write(' && ')
@@ -324,15 +317,8 @@ fn (mut g Gen) match_expr_switch(node ast.MatchExpr, is_expr bool, cond_var stri
 					g.write(' || ')
 				}
 				if expr is ast.RangeExpr {
-					// if type is unsigned and low is 0, check is unneeded
-					mut skip_low := false
-					if expr.low is ast.IntegerLiteral {
-						if node_cond_type_unsigned && expr.low.val == '0' {
-							skip_low = true
-						}
-					}
 					g.write('(')
-					if !skip_low {
+					if g.should_check_low_bound_in_range_expr(expr, node_cond_type_unsigned) {
 						g.write('${cond_var} >= ')
 						g.expr(expr.low)
 						g.write(' && ')
@@ -359,6 +345,28 @@ fn (mut g Gen) match_expr_switch(node ast.MatchExpr, is_expr bool, cond_var stri
 	g.writeln('}')
 }
 
+fn (mut g Gen) should_check_low_bound_in_range_expr(expr ast.RangeExpr, node_cond_type_unsigned bool) bool {
+	// if the type is unsigned, and the low bound of the range expression is 0,
+	// checking it at runtime is not needed:
+	mut should_check_low_bound := true
+	if node_cond_type_unsigned {
+		if expr.low is ast.IntegerLiteral {
+			if expr.low.val == '0' {
+				should_check_low_bound = false
+			}
+		} else if expr.low is ast.Ident {
+			if mut obj := g.table.global_scope.find_const(expr.low.name) {
+				if mut obj.expr is ast.IntegerLiteral {
+					if obj.expr.val == '0' {
+						should_check_low_bound = false
+					}
+				}
+			}
+		}
+	}
+	return should_check_low_bound
+}
+
 fn (mut g Gen) match_expr_classic(node ast.MatchExpr, is_expr bool, cond_var string, tmp_var string) {
 	node_cond_type_unsigned := node.cond_type in [ast.u16_type, ast.u32_type, ast.u64_type]
 	type_sym := g.table.sym(node.cond_type)
@@ -431,15 +439,8 @@ fn (mut g Gen) match_expr_classic(node ast.MatchExpr, is_expr bool, cond_var str
 					}
 					else {
 						if expr is ast.RangeExpr {
-							// if type is unsigned and low is 0, check is unneeded
-							mut skip_low := false
-							if expr.low is ast.IntegerLiteral {
-								if node_cond_type_unsigned && expr.low.val == '0' {
-									skip_low = true
-								}
-							}
 							g.write('(')
-							if !skip_low {
+							if g.should_check_low_bound_in_range_expr(expr, node_cond_type_unsigned) {
 								g.write('${cond_var} >= ')
 								g.expr(expr.low)
 								g.write(' && ')

vlib/v/parser/if_match.v

@@ -267,6 +267,7 @@ fn (mut p Parser) match_expr() ast.MatchExpr {
 			// Expression match
 			for {
 				p.inside_match_case = true
+				mut range_pos := p.tok.pos()
 				expr := p.expr(0)
 				ecmnts << p.eat_comments()
 				p.inside_match_case = false
@@ -276,13 +277,15 @@ fn (mut p Parser) match_expr() ast.MatchExpr {
 					return ast.MatchExpr{}
 				} else if p.tok.kind == .ellipsis {
 					p.next()
+					p.inside_match_case = true
 					expr2 := p.expr(0)
+					p.inside_match_case = false
 					exprs << ast.RangeExpr{
 						low: expr
 						high: expr2
 						has_low: true
 						has_high: true
-						pos: p.tok.pos()
+						pos: range_pos.extend(p.prev_tok.pos())
 					}
 				} else {
 					exprs << expr

vlib/v/tests/match_const_range_test.v

@@ -0,0 +1,65 @@
+const (
+	start        = 1
+	start_2      = 4
+	end          = 3
+	end_2        = 8
+	//
+	start_rune   = `a`
+	start_2_rune = `d`
+	end_rune     = `c`
+	end_2_rune   = `i`
+)
+
+fn test_match_int_const_ranges() {
+	mut results := []int{}
+	for x in 0 .. 10 {
+		match x {
+			start...end { results << 1 }
+			start_2...5 { results << 2 }
+			6...end_2 { results << 3 }
+			else { results << 4 }
+		}
+	}
+	assert results == [4, 1, 1, 1, 2, 2, 3, 3, 3, 4]
+}
+
+fn test_match_rune_const_ranges() {
+	mut results := []int{}
+	for x in `a` .. `l` {
+		match x {
+			start_rune...end_rune { results << 1 }
+			start_2_rune...`e` { results << 2 }
+			`f`...end_2_rune { results << 3 }
+			else { results << 4 }
+		}
+	}
+	assert results == [1, 1, 1, 2, 2, 3, 3, 3, 3, 4, 4]
+}
+
+fn test_match_expr_int_const_ranges() {
+	mut results := []int{}
+	for x in 0 .. 10 {
+		result := match x {
+			start...end { 1 }
+			start_2...5 { 2 }
+			6...end_2 { 3 }
+			else { 4 }
+		}
+		results << result
+	}
+	assert results == [4, 1, 1, 1, 2, 2, 3, 3, 3, 4]
+}
+
+fn test_match_expr_rune_const_ranges() {
+	mut results := []int{}
+	for x in `a` .. `l` {
+		result := match x {
+			start_rune...end_rune { 1 }
+			start_2_rune...`e` { 2 }
+			`f`...end_2_rune { 3 }
+			else { 4 }
+		}
+		results << result
+	}
+	assert results == [1, 1, 1, 2, 2, 3, 3, 3, 3, 4, 4]
+}