Fix sizeof(T) in typedesc templates called from generic type when clauses (#25374)

The `hasValuelessStatics` function in `semtypinst.nim` only checked for
`tyStatic`, missing `tyTypeDesc(tyGenericParam)`. This caused
`sizeof(T)` inside a typedesc template called from a generic type's
`when` clause to error with "'sizeof' requires '.importc' types to be
'.completeStruct'".

The fix adds a check for `tyTypeDesc` wrapping `tyGenericParam`,
recognizing it as an unresolved generic parameter that needs resolution
before evaluation.

Also documents the `completeStruct` pragma in the manual.

changelog.md

@@ -103,7 +103,15 @@ errors.
 
 ## Compiler changes
 
+- Fixed a bug where `sizeof(T)` inside a `typedesc` template called from a generic type's
+  `when` clause would error with "'sizeof' requires '.importc' types to be '.completeStruct'".
+  The issue was that `hasValuelessStatics` in `semtypinst.nim` didn't recognize
+  `tyTypeDesc(tyGenericParam)` as an unresolved generic parameter.
 
 ## Tool changes
 
 - Added `--stdinfile` flag to name of the file used when running program from stdin (defaults to `stdinfile.nim`)
+
+## Documentation changes
+
+- Added documentation for the `completeStruct` pragma in the manual.

compiler/semtypinst.nim

@@ -249,13 +249,24 @@ proc hasValuelessStatics(n: PNode): bool =
         a
     proc doThing(_: MyThing)
   ]#
+  result = false
   if n.safeLen == 0 and n.kind != nkEmpty: # Some empty nodes can get in here
-    n.typ == nil or n.typ.kind == tyStatic
+    if n.typ == nil:
+      result = true
+    elif n.typ.kind == tyStatic:
+      result = true
+    elif n.typ.kind == tyTypeDesc:
+      # Check if the base type is an unresolved generic parameter.
+      # This handles cases where a template containing sizeof(T) is called
+      # inside a generic object's when clause - the T needs to be resolved
+      # before we can evaluate the condition.
+      let base = n.typ.skipTypes({tyTypeDesc})
+      if base.kind == tyGenericParam:
+        result = true
   else:
     for x in n:
       if hasValuelessStatics(x):
         return true
-    false
 
 proc replaceTypeVarsN(cl: var TReplTypeVars, n: PNode; start=0; expectedType: PType = nil): PNode =
   if n == nil: return

doc/manual.md

@@ -7981,6 +7981,35 @@ underlying C `struct`:c: in a `sizeof` expression:
   ```
 
 
+CompleteStruct pragma
+---------------------
+The `completeStruct` pragma is a contract indicating that an `importc` type
+declaration contains all fields of the corresponding C type, allowing
+`sizeof`, `alignof`, and `offsetof` to be computed at compile-time.
+
+By default, `importc` types are assumed to be incomplete (their size is
+unknown at compile-time). Use `completeStruct` when you need compile-time
+size information and can guarantee the Nim definition matches the C layout:
+
+  ```Nim
+  type
+    InotifyEvent {.importc: "struct inotify_event", header: "<sys/inotify.h>",
+                   completeStruct.} = object
+      wd: cint
+      mask: uint32
+      cookie: uint32
+      len: uint32
+      # All fields must match the C struct exactly
+  ```
+
+If the Nim fields don't match the C struct, a static assertion will fail
+during C code generation.
+
+Without `completeStruct`, attempting to use `sizeof` on an `importc` type
+at compile-time will error with "'sizeof' requires '.importc' types to be
+'.completeStruct'".
+
+
 Compile pragma
 --------------
 The `compile` pragma can be used to compile and link a C/C++ source file

tests/generic/tgeneric_typedesc_sizeof.nim

@@ -0,0 +1,34 @@
+discard """
+  output: '''
+42
+'''
+"""
+
+# Regression test for semtypinst.nim hasValuelessStatics bug.
+#
+# Bug: hasValuelessStatics only checked for tyStatic, missing tyTypeDesc(tyGenericParam)
+# Fix: Added check for tyTypeDesc wrapping tyGenericParam in compiler/semtypinst.nim
+#
+# The bug triggers when:
+# 1. A generic type has a when clause calling a typedesc template with sizeof(T)
+# 2. A generic proc on that type is called, triggering instantiation
+# 3. The T in sizeof(T) becomes tyTypeDesc(tyGenericParam), which wasn't recognized as unresolved
+#
+# Error without fix: 'sizeof' requires '.importc' types to be '.completeStruct'
+
+template isSmall(T: typedesc): bool =
+  sizeof(T) <= 8
+
+type Foo[T] = object
+  when isSmall(T):
+    a: T
+  else:
+    b: ptr T
+
+proc bar[T](x: var Foo[T]) =
+  discard
+
+var x: Foo[int]
+x.a = 42
+x.bar()
+echo x.a