In the case of convert_argument, we would return the same expression
back rather than error_mark_node after the error message about
trying to convert to an incomplete type. This causes issues in
the gimplfier trying to see if another conversion is needed.

The code here dates back to before the revision history too so
it might be the case it never noticed we should return an error_mark_node.

Bootstrapped and tested on x86_64-linux-gnu with no regressions.

        PR c/100532

gcc/c/ChangeLog:

        * c-typeck.cc (convert_argument): After erroring out
        about an incomplete type return error_mark_node.

gcc/testsuite/ChangeLog:

        * gcc.dg/pr100532-1.c: New test.
---
 gcc/c/c-typeck.cc                 | 2 +-
 gcc/testsuite/gcc.dg/pr100532-1.c | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/pr100532-1.c

diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 6e044b4afbc..8f8562936dc 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -3367,7 +3367,7 @@ convert_argument (location_t ploc, tree function, tree 
fundecl,
     {
       error_at (ploc, "type of formal parameter %d is incomplete",
                parmnum + 1);
-      return val;
+      return error_mark_node;
     }
 
   /* Optionally warn about conversions that differ from the default
diff --git a/gcc/testsuite/gcc.dg/pr100532-1.c 
b/gcc/testsuite/gcc.dg/pr100532-1.c
new file mode 100644
index 00000000000..81e37c60415
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr100532-1.c
@@ -0,0 +1,7 @@
+/* { dg-do compile } */
+/* PR c/100532 */
+
+typedef __SIZE_TYPE__ size_t;
+void *memcpy(void[], const void *, size_t); /* { dg-error "declaration of type 
name" } */
+void c(void) { memcpy(c, "a", 2); } /* { dg-error "type of formal parameter" } 
*/
+
-- 
2.34.1

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