fix(flags): -fPIC is a property of the target format, not of the dialect

Windows CI, once the packer stopped swallowing the compiler's output:

    error: unsupported option '-fPIC' for target 'x86_64-pc-windows-msvc'

every MSVC-ABI shared build died in clang-scan-deps before compiling anything.
The comment beside the condition already said the right thing — "PE code is
position independent by design" — and the condition tested the DIALECT:
`!isMsvcDialect`. Windows' default toolchain is clang, which speaks the GNU
dialect while targeting the MSVC ABI, so the flag went out anyway.

Same shape as the shared-library guard this PR replaced: asking which COMPILER
when the question is which TARGET. It keys on the target format now.

Unreachable until this PR, because `kind = "shared"` was refused on that ABI —
the refusal was hiding an untested path, which is what refusals do. 257 pins the
absence on every Linux CI pass through mingw-cross rather than only on Windows:
the flag is equally meaningless for a PE target whichever compiler emits it, and
GCC merely ignores it where clang refuses.

Author: speak-agent

src/build/flags.cppm

@@ -21,6 +21,7 @@ import mcpp.platform.runtime_search;
 import mcpp.toolchain.clang;
 import mcpp.toolchain.detect;
 import mcpp.toolchain.dialect;
+import mcpp.toolchain.triple;
 import mcpp.toolchain.hostflags;
 import mcpp.toolchain.linkmodel;
 import mcpp.toolchain.model;
@@ -411,15 +412,31 @@ CompileFlags compute_flags(const BuildPlan& plan) {
 
     const bool isMsvcDialect = (d.id == "msvc");
 
-    // PIC? (GNU-only concept; PE code is position independent by design.)
+    // PIC is a GNU concept and a property of the TARGET FORMAT: PE code is
+    // position independent by design (base relocations), and clang rejects the
+    // flag outright — `unsupported option '-fPIC' for target
+    // 'x86_64-pc-windows-msvc'`.
+    //
+    // ⚠️ The condition used to be `!isMsvcDialect`, i.e. the DIALECT. Windows'
+    // default toolchain is clang, which speaks the GNU dialect while targeting
+    // the MSVC ABI, so `-fPIC` was emitted and every MSVC-ABI shared build died
+    // in clang-scan-deps before compiling anything. It was unreachable while
+    // `kind = "shared"` was refused on that ABI; allowing it is what surfaced
+    // this. Same shape as the shared-library guard itself: asking which
+    // COMPILER when the question is which TARGET.
+    const bool peTarget = [&] {
+        if (auto t = mcpp::toolchain::triple::parse(plan.toolchain.targetTriple))
+            return t->is_pe();
+        return bool(mcpp::platform::is_windows);
+    }();
     bool need_pic = false;
     for (auto& lu : plan.linkUnits) {
         if (lu.kind == LinkUnit::SharedLibrary) {
             need_pic = true;
             break;
         }
     }
-    std::string pic_flag = (need_pic && !isMsvcDialect) ? " -fPIC" : "";
+    std::string pic_flag = (need_pic && !isMsvcDialect && !peTarget) ? " -fPIC" : "";
 
     // Include dirs — this is the TYPED PATH channel (bare paths from the
     // manifest; the dialect prefix is applied here at emission), not the

tests/e2e/257_shared_library_pe.sh

@@ -66,6 +66,21 @@ imp="$(find target -name 'libmathkit.dll.a' | head -1)"
     echo "FAIL: no import library. The .dll alone is a library only mingw's ld"
     echo "      will link, so the package would be unusable everywhere else."
     exit 1; }
+# ⚠️ And no `-fPIC`. PE code is position independent by design, and clang
+# targeting the MSVC ABI REJECTS the flag — `unsupported option '-fPIC' for
+# target 'x86_64-pc-windows-msvc'` — killing the build in clang-scan-deps before
+# anything compiles. The condition used to be the DIALECT rather than the target,
+# and Windows' default toolchain is clang, which speaks the GNU dialect while
+# targeting MSVC. Asserted here rather than only on Windows because this runs on
+# every Linux CI pass through mingw-cross, and the flag is equally meaningless
+# for a PE target whichever compiler emits it.
+nj_pic="$(find target -name build.ninja | head -1)"
+grep -q '\-fPIC' "$nj_pic" && {
+    grep -n 'fPIC' "$nj_pic" | head -3
+    echo "FAIL: -fPIC on a PE target. It means nothing here, and clang targeting"
+    echo "      the MSVC ABI refuses it outright."
+    exit 1; }
+
 # It is a declared output of the link edge, not a side effect ninja knows nothing
 # about — otherwise the consumer that links it has no producer and ninja stops
 # with 'no known rule to make it'.