alias syntax fixes, improvements and tests (#21671)

* alias syntax fixes, improvements and tests

* even better, cannot use alias syntax with generics

* more type tests, improve comment

* fix again

* consistent error message + make t5167_5 work

* more comments, remove {.noalias.}

3 files changed, 91 insertions, 16 deletions

diff --git a/tests/template/t13515.nim b/tests/template/t13515.nim
deleted file mode 100644
index ffebedcbe..000000000
--- a/tests/template/t13515.nim
+++ /dev/null
@@ -1,16 +0,0 @@
-discard """
-  action: compile
-"""
-
-template test: bool = true
-
-# compiles:
-if not test:
-  echo "wtf"
-
-# does not compile:
-template x =
-  if not test:
-    echo "wtf"
-
-x
diff --git a/tests/template/taliassyntax.nim b/tests/template/taliassyntax.nim
new file mode 100644
index 000000000..969a9d144
--- /dev/null
+++ b/tests/template/taliassyntax.nim
@@ -0,0 +1,63 @@
+type Foo = object
+  bar: int
+
+var foo = Foo(bar: 10)
+template bar: int = foo.bar
+doAssert bar == 10
+bar = 15
+doAssert bar == 15
+var foo2 = Foo(bar: -10)
+doAssert bar == 15
+# works in generics
+proc genericProc[T](x: T): string =
+  $(x, bar)
+doAssert genericProc(true) == "(true, 15)"
+# redefine
+template bar: int {.redefine.} = foo2.bar
+doAssert bar == -10
+
+block: # subscript
+  var bazVal = @[1, 2, 3]
+  template baz: seq[int] = bazVal
+  doAssert baz[1] == 2
+  proc genericProc2[T](x: T): string =
+    result = $(x, baz[1])
+    baz[1] = 7
+  doAssert genericProc2(true) == "(true, 2)"
+  doAssert baz[1] == 7
+  baz[1] = 14
+  doAssert baz[1] == 14
+
+block: # type alias
+  template Int2: untyped = int
+  let x: Int2 = 123
+  proc generic[T](): string =
+    template U: untyped = T
+    var x: U
+    result = $typeof(x)
+    doAssert result == $U
+    doAssert result == $T
+  doAssert generic[int]() == "int"
+  doAssert generic[Int2]() == "int"
+  doAssert generic[string]() == "string"
+  doAssert generic[seq[int]]() == "seq[int]"
+  doAssert generic[seq[Int2]]() == "seq[int]"
+  discard generic[123]()
+  proc genericStatic[X; T: static[X]](): string =
+    template U: untyped = T
+    result = $U
+    doAssert result == $T
+  doAssert genericStatic[int, 123]() == "123"
+  doAssert genericStatic[Int2, 123]() == "123"
+  doAssert genericStatic[(string, bool), ("a", true)]() == "(\"a\", true)"
+
+block: # issue #13515
+  template test: bool = true
+  # compiles:
+  if not test:
+    doAssert false
+  # does not compile:
+  template x =
+    if not test:
+      doAssert false
+  x
diff --git a/tests/template/taliassyntaxerrors.nim b/tests/template/taliassyntaxerrors.nim
new file mode 100644
index 000000000..f16acaf91
--- /dev/null
+++ b/tests/template/taliassyntaxerrors.nim
@@ -0,0 +1,28 @@
+discard """
+  cmd: "nim check --hints:off $file"
+"""
+
+block: # with params
+  type Foo = object
+    bar: int
+
+  var foo = Foo(bar: 10)
+  template bar(x: int): int = x + foo.bar
+  let a = bar #[tt.Error
+      ^ invalid type: 'template (x: int): int' for let. Did you mean to call the template with '()'?]#
+  bar = 15 #[tt.Error
+  ^ 'bar' cannot be assigned to]#
+
+block: # generic template
+  type Foo = object
+    bar: int
+
+  var foo = Foo(bar: 10)
+  template bar[T]: T = T(foo.bar)
+  let a = bar #[tt.Error
+      ^ invalid type: 'template (): T' for let. Did you mean to call the template with '()'?; tt.Error
+          ^ 'bar' has unspecified generic parameters]#
+  let b = bar[float]()
+  doAssert b == 10.0
+  bar = 15 #[tt.Error
+  ^ 'bar' cannot be assigned to]#