process non-language pragma nodes in generics (#24254)

fixes #18649, refs #24183

Same as in #24183 for templates, we now process pragma nodes in generics
so that macro symbols are captured and the pragma arguments are checked,
but ignoring language pragma keywords.

A difference is that we cannot process call nodes as is, we have to
process their children individually so that the early untyped
macro/template instantiation in generics does not kick in.

compiler/semgnrc.nim

@@ -616,7 +616,24 @@ proc semGenericStmt(c: PContext, n: PNode,
     let bodyFlags = if n.kind == nkTemplateDef: flags + {withinMixin} else: flags
     n[bodyPos] = semGenericStmtScope(c, body, bodyFlags, ctx)
     closeScope(c)
-  of nkPragma, nkPragmaExpr: discard
+  of nkPragmaExpr:
+    result[1] = semGenericStmt(c, n[1], flags, ctx)
+  of nkPragma:
+    for i in 0 ..< n.len:
+      let x = n[i]
+      let prag = whichPragma(x)
+      if x.kind in nkPragmaCallKinds:
+        # process each child individually to prevent untyped macros/templates
+        # from instantiating
+        # if pragma is language-level pragma, skip name node:
+        let start = ord(prag != wInvalid)
+        for j in start ..< x.len:
+          # treat as mixin context for user pragmas & macro args
+          x[j] = semGenericStmt(c, x[j], flags+{withinMixin}, ctx)
+      elif prag == wInvalid:
+        # only sem if not a language-level pragma 
+        # treat as mixin context for user pragmas & macro args
+        result[i] = semGenericStmt(c, x, flags+{withinMixin}, ctx)
   of nkExprColonExpr, nkExprEqExpr:
     checkMinSonsLen(n, 2, c.config)
     result[1] = semGenericStmt(c, n[1], flags, ctx)

tests/generics/mpragma1.nim

@@ -0,0 +1,3 @@
+macro aMacro*(u:untyped):untyped =
+  echo "in macro"
+  result = u

tests/generics/mpragma2.nim

@@ -0,0 +1,6 @@
+import mpragma1
+proc p*[T]() =
+  proc inner() {.aMacro.} =
+    discard
+  inner()
+  discard

tests/generics/tpragma.nim

@@ -0,0 +1,10 @@
+discard """
+  nimout: '''
+in macro
+'''
+"""
+
+# issue #18649
+
+import mpragma2
+p[string]()