From 892971024e78cd0b3efd4e01ab0fb927a7d76525 Mon Sep 17 00:00:00 2001
From: Ruofan XU <47302112+SleepyRoy@users.noreply.github.com>
Date: Wed, 8 Sep 2021 10:54:15 +0800
Subject: [PATCH] parser: fix generic detection of `foo < bar<T>()` (#11434)

---
 vlib/v/parser/parser.v                      | 20 +++++++++++---------
 vlib/v/tests/generic_complex_sumtype_test.v |  2 +-
 vlib/v/tests/generics_test.v                | 14 ++++++++++++++
 3 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 80fefcdde6..eff7017105 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -1871,13 +1871,14 @@ fn (p &Parser) is_typename(t token.Token) bool {
 // heuristics to detect `func<T>()` from `var < expr`
 // 1. `f<[]` is generic(e.g. `f<[]int>`) because `var < []` is invalid
 // 2. `f<map[` is generic(e.g. `f<map[string]string>)
-// 3. `f<foo>` and `f<foo<` are generic because `v1 < foo > v2` and `v1 < foo < v2` are invalid syntax
-// 4. `f<Foo,` is generic when Foo is typename.
+// 3. `f<foo>` is generic because `v1 < foo > v2` is invalid syntax
+// 4. `f<foo<bar` is generic when bar is not generic T (f<foo<T>(), in contrast, is not generic!)
+// 5. `f<Foo,` is generic when Foo is typename.
 //	   otherwise it is not generic because it may be multi-value (e.g. `return f < foo, 0`).
-// 5. `f<mod.Foo>` is same as case 3
-// 6. `f<mod.Foo,` is same as case 4
-// 7. if there is a &, ignore the & and see if it is a type
-// 10. otherwise, it's not generic
+// 6. `f<mod.Foo>` is same as case 3
+// 7. `f<mod.Foo,` is same as case 5
+// 8. if there is a &, ignore the & and see if it is a type
+// 9. otherwise, it's not generic
 // see also test_generic_detection in vlib/v/tests/generics_test.v
 fn (p &Parser) is_generic_call() bool {
 	lit0_is_capital := p.tok.kind != .eof && p.tok.lit.len > 0 && p.tok.lit[0].is_capital()
@@ -1911,9 +1912,10 @@ fn (p &Parser) is_generic_call() bool {
 			return true
 		}
 		return match kind3 {
-			.gt, .lt { true } // case 3
-			.comma { p.is_typename(tok2) } // case 4
-			// case 5 and 6
+			.gt { true } // case 3
+			.lt { !(tok4.lit.len == 1 && tok4.lit[0].is_capital()) } // case 4
+			.comma { p.is_typename(tok2) } // case 5
+			// case 6 and 7
 			.dot { kind4 == .name && (kind5 == .gt || (kind5 == .comma && p.is_typename(tok4))) }
 			else { false }
 		}
diff --git a/vlib/v/tests/generic_complex_sumtype_test.v b/vlib/v/tests/generic_complex_sumtype_test.v
index 06f2789701..923145b9df 100644
--- a/vlib/v/tests/generic_complex_sumtype_test.v
+++ b/vlib/v/tests/generic_complex_sumtype_test.v
@@ -65,7 +65,7 @@ fn min<T>(tree Tree<T>) T {
 			1e100
 		}
 		Node<T> {
-			if tree.value < (min<T>(tree.left)) {
+			if tree.value < min<T>(tree.left) {
 				tree.value
 			} else {
 				min<T>(tree.left)
diff --git a/vlib/v/tests/generics_test.v b/vlib/v/tests/generics_test.v
index 7467d56fb4..01bd47ee3d 100644
--- a/vlib/v/tests/generics_test.v
+++ b/vlib/v/tests/generics_test.v
@@ -68,6 +68,7 @@ fn test_postfix_expr() {
 	assert minus_one(i16(-7)) == -8
 	assert minus_one(int(-6)) == -7
 	assert minus_one(i64(-5)) == -6
+
 	// the point is to see if it compiles, more than if the result
 	// is correct, so 1e-6 isn't necessarily the right value to do this
 	// but it's not important
@@ -370,6 +371,7 @@ fn test_generic_fn_with_variadics() {
 	s := 'abc'
 	i := 1
 	abc := Abc{1, 2, 3}
+
 	// these calls should all compile, and print the arguments,
 	// even though the arguments are all a different type and arity:
 	p(s)
@@ -457,18 +459,28 @@ fn test_generic_init() {
 	a << 'a'
 	assert a.len == 1
 	assert a[0] == 'a'
+
 	// map init
 	mut b := new<map[string]string>()
 	assert b.len == 0
 	b['b'] = 'b'
 	assert b.len == 1
 	assert b['b'] == 'b'
+
 	// struct init
 	mut c := new<User>()
 	c.name = 'c'
 	assert c.name == 'c'
 }
 
+fn return_one<T>(rec int, useless T) T {
+	// foo < bar<T>() should work
+	if rec == 0 || 0 < return_one<T>(rec - 1, useless) {
+		return T(1)
+	}
+	return T(0)
+}
+
 fn test_generic_detection() {
 	v1, v2 := -1, 1
 
@@ -482,9 +494,11 @@ fn test_generic_detection() {
 	assert multi_generic_args<int, string>(0, 's')
 	assert multi_generic_args<Foo1, Foo2>(Foo1{}, Foo2{})
 	assert multi_generic_args<Foo<int>, Foo<int> >(Foo<int>{}, Foo<int>{})
+
 	// TODO: assert multi_generic_args<Foo<int>, Foo<int>>(Foo1{}, Foo2{})
 	assert multi_generic_args<simplemodule.Data, int>(simplemodule.Data{}, 0)
 	assert multi_generic_args<int, simplemodule.Data>(0, simplemodule.Data{})
 	assert multi_generic_args<[]int, int>([]int{}, 0)
 	assert multi_generic_args<map[int]int, int>(map[int]int{}, 0)
+	assert 0 < return_one<int>(10, 0)
 }