From 7e13518cc2a1cd24be0c83cca9285c1127c9268c Mon Sep 17 00:00:00 2001
From: Swastik Baranwal <swstkbaranwal@gmail.com>
Date: Sat, 3 Oct 2020 19:00:58 +0530
Subject: [PATCH] parser: disallow match expr at certain places (#6490)

---
 doc/docs.md                                |  2 ++
 vlib/v/checker/tests/for_match_err.out     |  7 +++++
 vlib/v/checker/tests/for_match_err.vv      | 15 +++++++++
 vlib/v/checker/tests/if_match_expr_err.out |  7 +++++
 vlib/v/checker/tests/if_match_expr_err.vv  | 13 ++++++++
 vlib/v/parser/for.v                        |  3 ++
 vlib/v/parser/if_match.v                   |  6 ++++
 vlib/v/tests/for_loops_2_test.v            | 36 +++++++++++-----------
 8 files changed, 71 insertions(+), 18 deletions(-)
 create mode 100644 vlib/v/checker/tests/for_match_err.out
 create mode 100644 vlib/v/checker/tests/for_match_err.vv
 create mode 100644 vlib/v/checker/tests/if_match_expr_err.out
 create mode 100644 vlib/v/checker/tests/if_match_expr_err.vv

diff --git a/doc/docs.md b/doc/docs.md
index 236f735078..b2cf74ed9b 100644
--- a/doc/docs.md
+++ b/doc/docs.md
@@ -933,6 +933,8 @@ 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.
 
+Note: `match` as an expression is not usable in `for` loop and `if` statements.
+
 ### Defer
 
 A defer statement defers the execution of a block of statements until the surrounding function returns.
diff --git a/vlib/v/checker/tests/for_match_err.out b/vlib/v/checker/tests/for_match_err.out
new file mode 100644
index 0000000000..4e2aa9083d
--- /dev/null
+++ b/vlib/v/checker/tests/for_match_err.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/for_match_err.vv:3:7: error: cannot use `match` in `for` loop
+    1 | fn main() {
+    2 |   mut a := 2
+    3 |   for match a {
+      |       ~~~~~
+    4 |         2 {
+    5 |             println('a == 2')
diff --git a/vlib/v/checker/tests/for_match_err.vv b/vlib/v/checker/tests/for_match_err.vv
new file mode 100644
index 0000000000..910f39fb4b
--- /dev/null
+++ b/vlib/v/checker/tests/for_match_err.vv
@@ -0,0 +1,15 @@
+fn main() {
+  mut a := 2
+  for match a {
+		2 {
+			println('a == 2')
+			a = 0
+		}
+		0 {
+			println('a == 0')	
+		}
+		else {
+			println('unexpected branch')
+		}
+	}
+}
diff --git a/vlib/v/checker/tests/if_match_expr_err.out b/vlib/v/checker/tests/if_match_expr_err.out
new file mode 100644
index 0000000000..25b13cb189
--- /dev/null
+++ b/vlib/v/checker/tests/if_match_expr_err.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/if_match_expr_err.vv:3:8: error: cannot use `match` with `if` statements
+    1 | fn main() {
+    2 |     a := 0
+    3 |     if match a {
+      |        ~~~~~
+    4 |         0 {
+    5 |             println('a is zero')
diff --git a/vlib/v/checker/tests/if_match_expr_err.vv b/vlib/v/checker/tests/if_match_expr_err.vv
new file mode 100644
index 0000000000..511bbc7b65
--- /dev/null
+++ b/vlib/v/checker/tests/if_match_expr_err.vv
@@ -0,0 +1,13 @@
+fn main() {
+    a := 0
+    if match a {
+        0 {
+            println('a is zero')
+        }
+        else {
+            println('unreachable branch')
+        }
+    } 5 < 7 {
+        println('5 is less than 7')
+    }
+}
diff --git a/vlib/v/parser/for.v b/vlib/v/parser/for.v
index 8dfdb66632..fb93886207 100644
--- a/vlib/v/parser/for.v
+++ b/vlib/v/parser/for.v
@@ -11,6 +11,9 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 	pos := p.tok.position()
 	p.open_scope()
 	p.inside_for = true
+	if p.tok.kind == .key_match {
+		p.error('cannot use `match` in `for` loop')
+	}
 	// defer { p.close_scope() }
 	// Infinite loop
 	if p.tok.kind == .lcbr {
diff --git a/vlib/v/parser/if_match.v b/vlib/v/parser/if_match.v
index 20c0da30b2..4fc0358cc0 100644
--- a/vlib/v/parser/if_match.v
+++ b/vlib/v/parser/if_match.v
@@ -34,6 +34,9 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 			comments << p.eat_comments()
 			p.check(.key_else)
 			comments << p.eat_comments()
+			if p.tok.kind == .key_match {
+				p.error('cannot use `match` with `if` statements')
+			}
 			if p.tok.kind == .lcbr {
 				// else {
 				has_else = true
@@ -72,6 +75,9 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 		}
 		// `if` or `else if`
 		p.check(.key_if)
+		if p.tok.kind == .key_match {
+			p.error('cannot use `match` with `if` statements')
+		}
 		comments << p.eat_comments()
 		// `if mut name is T`
 		mut mut_name := false
diff --git a/vlib/v/tests/for_loops_2_test.v b/vlib/v/tests/for_loops_2_test.v
index 8db5c284c8..5def00b15d 100644
--- a/vlib/v/tests/for_loops_2_test.v
+++ b/vlib/v/tests/for_loops_2_test.v
@@ -1,24 +1,24 @@
 fn test_for_match() {
 	mut a := 2
 	mut b := 0
-	for match a {
-		2 {
-			println('a == 2')
-			a = 0
-			true
+	for {
+		match a {
+			2 {
+				println('a == 2')
+				a = 0
+				continue
+			}
+			0 {
+				println('a == 0')
+				a = 5
+				b++
+				break
+			}
+			else {
+				println('unexpected branch')
+				break
+			}
 		}
-		0 {
-			println('a == 0')
-			a = 5
-			false
-		}
-		else {
-			println('unexpected branch')
-			false
-		}
-	} {
-		b++
-		println('${b}. run')
 	}
 	assert a == 5
 	assert b == 1
@@ -34,7 +34,7 @@ fn test_for_select() {
 		x := <-ch1 {
 			a += x
 		}
-		y := <- ch2 {
+		y := <-ch2 {
 			a += int(y)
 		}
 	} {