sem: correctly show errors from defer node body

Summary
=======

Errors in a defer node body are now correctly reported, no longer
producing misleading `'defer' takes a 'void' expression` messages.

Details
======

A`defer` with an error node body is no longer treated as an
expression and having the underlying error supressed. Instead the body
error is correctly reported. The `defer` AST is also correctly wrapped
in an error node.

A relatively comprehensive test sutie for `defer` statements was added,
along with an improvement to the void expression error message
requirement, which shows the `defer` body itself.

compiler/front/cli_reporter.nim

@@ -962,7 +962,7 @@ proc reportBody*(conf: ConfigRef, r: SemReport): string =
       result = "invalid context for 'bind' statement: " & render(r.ast)
 
     of rsemExpectedTypelessDeferBody:
-      result = "'defer' takes a 'void' expression"
+      result = "'defer' takes a 'void' expression, got: " & render(r.ast[0])
 
     of rsemUnexpectedToplevelDefer:
       result = "defer statement not supported at top level"

compiler/sem/semexprs.nim

@@ -3878,9 +3878,22 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
   of nkDefer:
     if c.currentScope == c.topLevelScope:
       localReport(c.config, n, reportSem rsemUnexpectedToplevelDefer)
-    n[0] = semExpr(c, n[0])
-    if not n[0].typ.isEmptyType and not implicitlyDiscardable(n[0]):
-      localReport(c.config, n, reportSem rsemExpectedTypelessDeferBody)
+    let res = semExpr(c, n[0])
+    case res.kind
+    of nkError:
+      result = copyNodeWithKids(n)
+      result[0] = res
+      result = c.config.wrapError(result)
+    else:
+      if res == n[0]: # no change
+        discard
+      else:
+        result = copyNodeWithKids(n)
+        result[0] = res
+
+      # TODO: convert to nkError
+      if not res.typ.isEmptyType and not implicitlyDiscardable(res):
+        localReport(c.config, n, reportSem rsemExpectedTypelessDeferBody)
   of nkMixinStmt: discard
   of nkBindStmt:
     if c.p != nil:

tests/lang_stmts/defer/tdefer.nim

@@ -0,0 +1,34 @@
+discard """
+  description: "Test for defer statements"
+"""
+
+block runs_at_the_end_of_scope:
+  proc foo(): int =
+    defer:
+      inc result
+    doAssert result == 0
+
+  doAssert foo() == 1
+
+block questionably_not_at_the_end_of_proc_scope:
+  proc foo(): int =
+    if true:
+      defer:
+        inc result
+      doAssert result == 0
+    doAssert result == 1
+    inc result
+    doAssert result == 2
+
+  doAssert foo() == 2
+
+block templates_do_not_create_scopes_defer_applies_to_the_surrounding:
+  template bar() =
+    defer:
+      inc result
+
+  proc foo(): int =
+    bar()
+    doAssert result == 0
+
+  doAssert foo() == 1

tests/lang_stmts/defer/tdefer_cannot_be_top_level.nim

@@ -0,0 +1,8 @@
+discard """
+  description: "Defer cannot be decalred at the top level"
+  errormsg: "defer statement not supported at top level"
+  line: 7
+"""
+
+defer:
+  discard

tests/lang_stmts/defer/tdefer_requires_stmt_1.nim

@@ -0,0 +1,9 @@
+discard """
+  description: "Defer body must be statement"
+  errormsg: "'defer' takes a 'void' expression"
+  line: 8
+"""
+
+block:
+  defer:
+    true # this needs to be used

tests/lang_stmts/defer/tdefer_requires_stmt_2.nim

@@ -0,0 +1,13 @@
+discard """
+  description: "Defer body must be a statement, without error"
+  errormsg: "type mismatch: got <bool, int literal(1)>"
+  line: 12
+"""
+
+## somewhat of a regression test to see what happens when the defer body has an
+## error, this was an issue encountered in CPS, where an error body was treated
+## as an expression.
+
+block:
+  defer:
+    true + 1