fix #13502 a and b now short-circuits semcheck in VM

compiler/semexprs.nim

@@ -2153,6 +2153,15 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags): PNode =
     result[0] = newSymNode(s, n[0].info)
     result[1] = semAddrArg(c, n[1], s.name.s == "unsafeAddr")
     result.typ = makePtrType(c, result[1].typ)
+  of mBitandI: # somehow, not mAnd
+    if c.inStaticContext > 0:
+      result = newTree(nkCall)
+      result.add newIdentNode(getIdent(c.cache, "nimInternalAndVM"), n.info)
+      result.add n[1]
+      result.add n[2]
+      result = semExpr(c, result)
+    else:
+      result = semDirectOp(c, n, flags)
   of mTypeOf:
     markUsed(c, n.info, s)
     result = semTypeOf(c, n)

lib/system/basic_types.nim

@@ -50,6 +50,13 @@ proc `and`*(x, y: bool): bool {.magic: "And", noSideEffect.}
   ## are true).
   ##
   ## Evaluation is lazy: if ``x`` is false, ``y`` will not even be evaluated.
+  ## Semantic check is lazy in VM, to allow `when declared(Foo) and T is Foo`
+
+template nimInternalAndVM*(x: bool, y: untyped): bool =
+  ## Internal lowering used by `and` in VM to enable shortcuiting semcheck
+  when x: y
+  else: false
+
 proc `or`*(x, y: bool): bool {.magic: "Or", noSideEffect.}
   ## Boolean ``or``; returns true if ``not (not x and not y)`` (if any of
   ## the arguments is true).

tests/system/tsystem_misc.nim

@@ -210,3 +210,18 @@ block: # Ordinal
   # doAssert enum is Ordinal # fails
   # doAssert Ordinal is SomeOrdinal
   # doAssert SomeOrdinal is Ordinal
+
+block: # and VM shortcircuits semcheck
+  doAssert not compiles(nonexistant)
+  const a1 = false and nonexistant
+  doAssert not a1
+  const a2 = true and false
+  doAssert not a2
+  const a3 = true and true
+  doAssert a3
+
+  static:
+    const a4 = false and nonexistant
+    doAssert not a4
+    let a5 = false and nonexistant # still works, we're inside a static context
+    doAssert not a5